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METALS & FABRICATION
ENG093
ARC WELDING 1
Basic Arc Welding Information Book
www.vetinfonet.dtwd.wa.gov.au
ENG093
Arc Welding 1
Basic Arc Welding Information Book
Learning Resource
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First published 2007
2nd edition January 2009
ISBN 978-0-7307-9807-1
© VET (WA) Ministerial Corporation 2007
This resource includes content from:
Metal Fabrication Volume 2, Part 1: Fabrication & Welding Theory, Department of Training and
Workforce Development
Metal Fabrication Volume 3, Part 1: Fabrication & Welding Theory, Training Publications
Engineering: Fa brication & Welding Theory, Technical Publications Trust
Engineering: Welding Theory 1, Training Publications
Welding Metallurgy, Training Publications
Engineering: Welding Technology Aluminium/Stainless Steel (Resource Book), Training Publications
Metals & Engineering Training Package: Manual Metal Arc Welding, Department of Training and
Workforce Development
Engineering: Welding 5 Certification Theory, Department of Training and Workforce Development
Metals & Engineering Training Package: Weld Using Gas Metal Arc Welding Process, Department of Training
and Workforce Development
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Various photographs reproduced with permission, from the 2006 edition of the BOC Industry Reference
Manual, BOC Australia Limited, Sydney.
This document duplicates Standards Australia Ltd copyrighted material. It is reproduced with permission from
SAI Global under Licence 0706-C102 to Department of Training and Workforce Development. All copies of the
material must be obtained from the Licensee. Not for resale, reproduction or distribution on whole or in part
without written permission from SAI Global: tel +61 2 8206 6355 or [email protected]
Metals& Fabrication
Arc Welding 1
Contents
Chapter 1
Arc welding safety .....................................................1
Chapter 2
Electricity and welding machines ..........................31
Chapter 3
Weld preparation and workmanship ......................57
Chapter 4
Air-arc gouging ........................................................73
Chapter 5
Manual metal arc welding (MMAW) ........................77
Chapter 6
Gas tungsten arc welding (GTAW) .......................101
Chapter 7
Gas metal arc welding (GMAW)............................121
Chapter 8
Flux-cored arc welding (FCAW) ...........................157
Chapter 9
Submerged arc and electro-slag welding
processes (SAW) ...................................................177
Appendix
Competency mapping
i
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Arc Welding 1
Chapter 1 – Arc welding safety
Introduction
To achieve safe working conditions in the metal fabrication and welding industry, all
personnel should be able to recognise the hazards which apply to their particular
occupation. Welding operators must also know the correct operating procedures for the
equipment.
An operator can be subjected to many safety hazards associated with the industry. As
with any other industrial worker, they may be injured through incorrect lifting practices,
falling or tripping, or incorrect use of hand tools and machines. The operator will also
encounter particular hazards associated with welding.
A clean, tidy workplace, free from combustible materials, is an essential requirement for
the safety of welding personnel.
Additionally, others working in the vicinity of welding operations are at risk from hazards
such as electrocution, fumes, radiation, burns or flying slag and noise. They too must
be protected if their health and safety is not to be put at risk.
In this chapter you will look at the following.
•
Types of hazards
ο
electric shock
ο
fumes
ο
radiation
ο
fire and explosion
ο
burns
•
Personal protective equipment (ppe)
•
The working environment
•
ο
confined spaces/hazardous locations
ο
cutting or welding in or near hazardous locations
ο
working on tanks and containers
First aid for operators
ο
basic objectives
ο
basic immediate first aid for some common operator injuries.
1
1
Chapter 1 – Arc welding safety
Types of hazards
The major hazards associated with arc welding are:
•
electric shock
Fig 1.1 – Electric shock
•
fumes
Fig 1.2 – Fumes
2
Metals& Fabrication
•
Arc Welding 1
radiation
Fig 1.3 – Radiation
•
fire and explosion
Fig 1.4. – Fire and explosion
•
burns.
Fig 1.5. – Burns
3
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Chapter 1 – Arc welding safety
Electric shock
Electrical principles and requirements for arc welding machines will be discussed
in depth in Chapter 2 – Electricity and welding machines, however at this stage it is
necessary to clarify some basic electrical terms.
Term
voltage (V)
Definition
the force which makes current flow
voltage is essentially electrical pressure
current (A)
the flow of electrons and is measured in amperes
open circuit voltage (OCV)
the voltage between welding terminals when
the machine is switched on but welding is not in
progress
resistance
the hindrance of a conductor to the passage
of current, ie a force which opposes the flow of
electricity
conductor
a material that permits the easy flow of electricity
insulator
a material that will not convey an electric current
Electric shock may only cause a minor tingling sensation or it may cause muscle
spasms, or paralysis and this may cause an operator to grip onto the source of
electricity. In the worst case scenario this may contribute to the welding operator’s
death.
In arc welding processes a number of potential electrocution sources can be identified.
The primary input lead is either 415 V or 240 V and should never be tampered with,
altered, or repaired except by a licensed electrician. The output circuit of an arc
welder is controlled at a ‘safe voltage’ but this safe voltage can also kill if given ideal
conditions. The ancillary circuits of most welders are also at a safe voltage of either
32 V or 24 V, although some machine manufacturers also use 110 V on older control
circuits.
4
Metals& Fabrication
Arc Welding 1
The prevention of electrical shock from welding equipment relies on three major
principles.
1. That the OCV of welding
machines is low enough to
prevent easy passage of
current through the body.
Although arc welding machines are capable of supplying
a wide range of current, they generally operate within
the following voltage range:
Arc voltage: 16–36 V
To maximise safety for the welding operator, OCV is
restricted to:
Maximum OCV for AC machines is 80 V
Maximum OCV for DC machines is 110 V
2. That electrical resistance in
the welding circuit is low. The
current will take the path of
least resistance.
Maintain insulation on leads and handpiece
Avoid moisture and use insulating gloves
3. That the current path is
confined to the welding circuit.
Don’t touch live parts
Don’t put body in current path
However, even with these limitations severe electric shock is still possible. The
following factors will influence the severity of electrical shock.
The amount of current passing through the body
Increased voltage will result in greater current flow. Even quite low voltages can be
dangerous where resistance is low.
The direction of the current flow
If the current path is via vital organs then the risk of serious injury is much greater.
The body resistance
The body is a semi-conductor and dry skin acts as an insulator that naturally resists the flow
of current. Moist skin in contact areas, and contact over large areas increases the chance of
electric shock.
The duration of the current flow
A longer exposure to electric current means a greater risk of consequence.
The state of health of the person receiving the shock
A healthy person will better resist the effects of electric shock. The phase of the heart cycle at
the instant the shock occurs will also influence the severity of electric shock.
5
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Chapter 1 – Arc welding safety
To avoid electric shock, the following practices are highly recommended.
Dry gloves in good repair should
be worn when handling equipment,
particularly when changing
electrodes.
BOC Limited © 2006
Fig 1.5.1
Footwear should be insulating, dry,
and in good condition.
BOC Limited © 2006
Fig 1.5.2
Welding equipment should be in
good repair and fully insulated.
BOC Limited © 2006
Fig 1.5.3
6
Metals& Fabrication
Work return
contact
Arc Welding 1
Work return contact points should
be close to the site of welding and
be carefully selected.
Current flow through the shortest pathway
Fig 1.5.4
All connections should be clean
and tight.
BOC Limited © 2006
Fig 1.5.5
Electrical supply circuits (primary)
should be kept as short as possible
and be serviced only by electrical
tradespersons.
Electrical supply
circuit (primary)
BOC Limited © 2006
Fig 1.5.6
7
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Chapter 1 – Arc welding safety
ON
OFF
Machines should be switched OFF
and unplugged when changing
leads or carrying out maintenance.
Fig 1.5.7
Confined space
not illustrated
Wooden
duckboard
Fig 1.5.8
8
Dry insulating material should be
used in confined spaces; wooden
boards or rubber mats are ideal.
Metals& Fabrication
Arc Welding 1
When working in a confined space
or in wet or moist conditions,
electrodes should not be left in the
holder and the power should be
isolated when electrodes are being
changed.
Photograph courtesy of HPM Industries Pty Ltd
Fig 1.5.9
Remember, sweating decreases
body resistance. Therefore, be
extra careful when welding under
hot conditions.
Fig 1.5.10
Fig 1.5 – To avoid electric shock (1–10)
9
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Chapter 1 – Arc welding safety
Fumes
Fumes are produced in all welding and cutting operations. They are a mixture of:
•
atmospheric gases
•
arc shielding gases
•
vaporised elements from the parent metal, metal coatings, or flux coated welding
consumables
•
airborne particles small enough to be inhaled.
Welding fumes are normally at levels low enough to pose no great health risk.
However, when fume concentration is excessive the operator will be deprived of the
oxygen needed to maintain good health. Fumes of highly toxic metals, even in low
concentrations, may also cause health problems with respect to the upper respiratory
tract, lungs, blood, liver, kidneys and central nervous system.
Certain substances found in welding fumes are recognised as being particularly
dangerous, even in very low concentrations. Welding operators should be aware of the
dangers associated with metals such as beryllium, cadmium, zinc and lead.
Table 1.1 gives an indication of the toxicity of some of the metals more commonly
encountered by welding operators.
Metal
beryllium
Effect
A highly toxic, quick acting
poison. A carcinogen
Typical fume
source
copper based bearing
alloys
Ventilation
glove box, fresh air
supply
casting alloys
cadmium
chromium
A highly toxic carcinogen.
Causes heart, lung, kidney
damage
silver brazing alloys
A carcinogen. Causes lung
and skin disease, nasal
irritation
chromium alloys
surface coatings
glove box, fresh air
supply
local exhaust
stainless steel
electroplating
lead
Causes fatigue, nerve and
kidney damage and high
blood pressure
copper base castings
local exhaust
lead based paints
free machining steels
copper
An irritant to nose and
throat. Causes metal fume
fever
copper alloys and
castings
local exhaust
nickel
Causes skin and respiratory
irritation and kidney
damage. A carcinogen
nickel alloys
local exhaust
Causes metal fume fever
surface coatings
zinc
stainless steel
local exhaust
bronze and brass
aluminium
Causes irritation to nose and
throat and chronic bronchitis
plates, castings
extrusions
Table 1.1 – Toxicity of some common metals
10
local exhaust
Metals& Fabrication
Arc Welding 1
Any toxic material that is used in a workshop must be accompanied by a material
safety data sheet (MSDS) and these should be held in a secure but accessible location.
A sample MSDS for chromium, which is a common alloying material, is shown on the
next page.
11
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Chapter 1 – Arc welding safety
Material safety data sheet (MSDS) for CHROMIUM
1
PRODUCT IDENTIFICATION
PRODUCT NAME:
CHROMIUM
FORMULA:
CR
FORMULA WT:
52.00
CAS NO.:
7440-47-3
NIOSH/RTECS NO.:
CB4200000
PRODUCT CODES:
4961
EFFECTIVE:
09/10/86
REVISION
#03
PRECAUTIONARY LABELLING:
BAKER SAF-T-DATA™ SYSTEM
HEALTH
- 0 NONE
FLAMMABILITY
- 0 NONE
REACTIVITY
- 0 NONE
CONTACT
- 0 NONE
HAZARD RATINGS ARE 0 TO 4
(0 = NO HAZARD; 4 = EXTREME HAZARD).
LABORATORY PROTECTIVE EQUIPMENT
SAFETY GLASSES; LAB COAT
PRECAUTIONARY LABEL STATEMENTS
DURING USE AVOID CONTACT WITH EYES, SKIN, CLOTHING. WASH THOROUGHLY
AFTERHANDLING. WHEN NOT IN USE KEEP IN TIGHTLY CLOSED CONTAINER.
SAF-T-DATA™ STORAGE COLOR CODE: ORANGE (GENERAL STORAGE)
2
12
HAZARDOUS COMPONENTS
COMPONENT
%
CAS NO.
CHROMIUM
90-100
7440-47-3
Metals& Fabrication
3
Arc Welding 1
PHYSICAL DATA
BOILING POINT: 2200 °C (3992 °F)
VAPOUR PRESSURE (MM HG):
N/A
MELTING POINT: 1900 °C (3452 °F)
VAPOUR DENSITY (AIR=1):
N/A
SPECIFIC GRAVITY:
(H2O=1)
7.14
EVAPORATION RATE:
(BUTYL ACETATE=1)
N/A
SOLUBILITY (H2O):
% VOLATILES BY VOLUME:
NEGLIGIBLE (LESS THAN 0.1%)
0
APPEARANCE & ODOUR:
STEEL GREY TO SILVER PELLETS.
4
FIRE AND EXPLOSION HAZARD DATA
FLASH POINT (CLOSED CUP) N/A
FLAMMABLE LIMITS:
UPPER - N/A %
LOWER - N/A %
FIRE EXTINGUISHING MEDIA
USE WATER SPRAY, ALCOHOL FOAM, DRY CHEMICAL OR CARBON DIOXIDE.
SPECIAL FIRE-FIGHTING PROCEDURES
FIREFIGHTERS SHOULD WEAR PROPER PROTECTIVE EQUIPMENT AND SELFCONTAINED BREATHING APPARATUS WITH FULL FACEPIECE OPERATED IN POSITIVE
PRESSURE MODE. MOVE CONTAINERS FROM FIRE AREA IF IT CAN BE DONE WITHOUT
RISK. USE WATER TO KEEP FIRE-EXPOSED CONTAINERS COOL.
UNUSUAL FIRE & EXPLOSION HAZARDS
CAN BE AN EXPLOSION HAZARD, ESPECIALLY WHEN HEATED.
5
HEALTH HAZARD DATA
NOTE: WHILE THE SPECIFIC COMPOUNDS CANNOT BE IDENTIFIED, THERE IS
EVIDENCE THAT CERTAIN CHROMIUM COMPOUNDS CAUSE CANCER IN HUMANS
AND EXPERIMENTAL ANIMALS. CHROMIUM IS WIDELY DISTRIBUTED IN AIR, WATER,
SOIL AND FOOD. TRIVALENT CHROMIUM MAY BE AN ESSENTIAL TRACE INGREDIENT
IN THE HUMAN DIET. ALL CHROMIUM COMPOUNDS ARE REGULATED BY THE EPA,
BUT NO SPECIFIC DATA IS AVAILABLE TO LINK TRIVALENT CHROMIUM TO CANCER.
PRUDENT JUDGEMENT DICTATES THAT EXPOSURE SHOULD BE MINIMISED AS MUCH
AS POSSIBLE.
(SEE IARC MONOGRAPH ON EVALUATION OF CARCINOGENIC RISK OF CHEMICALS
TO HUMANS, VOLUME 23 LYON, FRANCE IARC, 1980, PP. 205-323).
THRESHOLD LIMIT VALUE (TLV/TWA):
0.5 MG/M3 (PPM)
PERMISSIBLE EXPOSURE LIMIT (PEL):
1 MG/M3 (PPM)
CARCINOGENICITY:
NTP: YES
IARC: YES
Z LIST: NO
OSHA REG: NO
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Chapter 1 – Arc welding safety
EFFECTS OF OVER EXPOSURE
CONTACT WITH SKIN OR EYES MAY CAUSE SEVERE IRRITATION OR BURNS.
DUST MAY ULCERATE MUCOUS MEMBRANES. EXCESSIVE INHALATION OF DUST
IS IRRITATING AND MAY BE SEVERELY DAMAGING TO RESPIRATORY PASSAGES
AND/OR LUNGS. INGESTION MAY RESULT IN SEVERE INTESTINAL IRRITATION WITH
BURNS TO MOUTH.
NOTE: PRODUCT IS A SOLID MASS; HOWEVER, WARNINGS ARE BASED ON
INHALATION DUST, MIST OR FUME EMISSIONS THAT ARE POSSIBLE DURING
MANUFACTURING OR CHEMICAL REACTIONS.
TARGET ORGANS
RESPIRATORY SYSTEM
MEDICAL CONDITIONS GENERALLY AGGRAVATED BY EXPOSURE NONE IDENTIFIED
ROUTES OF ENTRY
INGESTION, INHALATION
EMERGENCY AND FIRST AID PROCEDURES
INGESTION: IF SWALLOWED AND THE PERSON IS CONSCIOUS, IMMEDIATELY GIVE
LARGE AMOUNTS OF WATER. GET MEDICAL ATTENTION.
INHALATION: IF A PERSON BREATHES IN LARGE AMOUNTS, MOVE THE EXPOSED
PERSON TO FRESH AIR. GET MEDICAL ATTENTION.
EYE CONTACT: IMMEDIATELY FLUSH WITH PLENTY OF WATER FOR AT LEAST 15
MINUTES. GET MEDICAL ATTENTION.
SKIN CONTACT: IMMEDIATELY WASH WITH PLENTY OF SOAP AND WATER FOR AT
LEAST 15 MINUTES.
6
REACTIVITY DATA
STABILITY:
STABLE
HAZARDOUS POLYMERISATION:
WILL NOT OCCUR
CONDITIONS TO AVOID:
FLAME
INCOMPATIBLES:
CARBONATES, STRONG BASES,
MINERAL ACIDS
7
SPILL AND DISPOSAL PROCEDURES
STEPS TO BE TAKEN IN THE EVENT OF A SPILL OR DISCHARGE
WEAR SUITABLE PROTECTIVE CLOTHING. CAREFULLY SWEEP UP AND REMOVE.
DISPOSAL PROCEDURE
DISPOSE IN ACCORDANCE WITH ALL APPLICABLE FEDERAL, STATE, AND LOCAL
ENVIRONMENTAL REGULATIONS.
EPA HAZARDOUS WASTE NUMBER:
14
D007 (EP TOXIC WASTE)
Metals& Fabrication
8
Arc Welding 1
PROTECTIVE EQUIPMENT
VENTILATION:
USE ADEQUATE GENERAL OR LOCAL EXHAUST VENTILATION
TO KEEP FUME OR DUST LEVELS AS LOW AS POSSIBLE.
RESPIRATORY PROTECTION: A RESPIRATOR WITH DUST/MIST FILTER IS
RECOMMENDED. IF AIRBORNE CONCENTRATION EXCEEDS TLV, A SELF-CONTAINED
BREATHING APPARATUS IS ADVISED.
EYE/SKIN PROTECTION: SAFETY GLASSES WITH SIDE SHIELDS, PROPER GLOVES
ARE RECOMMENDED.
9
STORAGE AND HANDLING PRECAUTIONS
SAF-T-DATA™ STORAGE COLOR CODE: ORANGE (GENERAL STORAGE)
SPECIAL PRECAUTIONS
KEEP CONTAINER TIGHTLY CLOSED. SUITABLE FOR ANY GENERAL CHEMICAL
STORAGE AREA.
10
TRANSPORTATION DATA AND ADDITIONAL INFORMATION
DOMESTIC (DOT)
PROPER SHIPPING NAME
CHROMIUM
HAZARD CLASS
ORM-E
LABELS
NONE
REPORTABLE QUANTITY
1 LBS
INTERNATIONAL (IMO)
PROPER SHIPPING NAME
CHEMICALS, NOS (NON-REGULATED)
Reference: West Virginia Toxics Release Inventory Database Search //gis.wvdep.org/tri/cheminfo/msds452.txt
15
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Chapter 1 – Arc welding safety
Control of fumes
To ensure that the concentration of fumes and exposure to fumes is within safe limits,
various controls can be applied.
Substitution
Where practicable, a less dangerous material, consumable, process
or procedure can be substituted.
Limiting the period of
exposure
Limiting the time any one operator is exposed to excessive fume
concentration is not the most desirable method, but in some cases
may be the only practical solution.
Work methods
Good housekeeping and work practices can avoid the unnecessary
generation and exposure to fumes. For example, removing surface
contaminants from parent material prior to welding or cutting. It
should be noted that certain degreasing agents decompose under
heat and ultraviolet radiation to give off toxic fumes.
Ventilation
This is the most common method of control and can be achieved by
various means.
Types of ventilation
•
Natural ventilation – in the greater majority of workshops and open sites, the
natural flow of air through open plan layouts and natural breeze is sufficient to
disperse fume concentrations.
•
General exhaust ventilation – this method is often used where the workshop does
not have adequate natural ventilation. Fumes rise and are dispersed into the
atmosphere, generally through ceiling exhaust fans.
•
Local exhaust ventilation – this method collects fumes at its source and directs
them away from the work area. The suction inlet should be as close as possible
to the source. There are various types of local exhaust systems, each offering
certain advantages and suited to certain applications.
•
Local dispersion ventilation – in some cases, suitable ventilation can be obtained
locally by fans which deflect and disperse the fumes away from the operator.
Fig 1.6 – Ventilation types for welding
16
Metals& Fabrication
Arc Welding 1
Personal respiratory protection
In special situations where general or local ventilation systems are not effective or
convenient in reducing fume levels, you may use personal respiratory protection by
one of the following methods. These comply with Australian Standards® AS/NZS 1716
Respiratory protective devices.
•
Hose mask respiration method, which is a full-face piece fitted with a length
of relatively large bore air hose drawing from a clean source by the normal
breathing action of the wearer.
•
Airline respiration which may comprise a full-face piece, half-face piece, hood or
helmet type. Clean air is supplied at a suitable pressure from a remote source.
•
Self-contained breathing apparatus using a cylinder of compressed air. This
equipment is not dependant on an air compressor which may be subject to failure
and is recommended for use in confined spaces.
•
Dust respirator which may consist of a full-face, or half-face mask fitted with the
correct filter cartridge.
These provide protection only against fume particles and not against gases.
Personal: Airline
Personal: Filter
Fig 1.7 – Local dispersion ventilation
Radiation
Types of radiation
Three types of radiation are emitted by arc welding processes: visible, infra-red and
ultraviolet. The first two types are also emitted from flames in gas welding and cutting.
•
Visible radiation – exposure to high intensity visible radiation may result in
‘dazzle’ with temporary loss of vision. There is no long-term or permanent
damage to the eyes.
•
Infra-red radiation – this radiation acts in the same manner as exposure to heat,
producing burns. Permanent damage is unlikely, but the heat adds to discomfort.
Infra-red radiation can damage the unprotected internal structure of the eye,
such as the iris, the lens and the retina. In severe cases of repeated exposure to
luminous infra-red, eye cataracts can be produced.
•
Ultraviolet radiation – ultraviolet is the most common and powerful radiation
hazard in welding. This radiation attacks the eyes and exposed skin.
17
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Chapter 1 – Arc welding safety
Brief exposure can produce an inflammation of the cornea of the eye resulting in a
condition known as ‘arc eye’ or ‘welding flash’. The symptoms of arc eye do not appear
until several hours after the exposure (similar to sunburn). Pain, watering of eyes, and
photophobia (intolerance to light) occur. These symptoms may last several days in
severe cases, but generally subside leaving no permanent or residual damage.
Prolonged exposure to ultraviolet light can cause permanent damage to eyes and skin
in the form of impaired vision, cataracts and skin cancer.
The amount of ultraviolet radiation emitted from the arc depends on several factors (the
welding process, the type of electrode, the amperage, and the arc length).
High current density welding processes such as the tungsten inert gas and gas metal
arc processes, in particular, emit powerful ultraviolet radiation.
It is most important to realise that all three radiation types can be reflected from
shiny surfaces – such as the underside of galvanised roofs, plates, or painted
screens.
Protection from radiation
Personal protection
Protection is needed for both the eyes and skin. For arc welding, a suitable welding
helmet or face shield, fitted with the recommended filter for the job in hand, is necessary.
Photograph © 2007 JupiterImages Corporation
Fig 1.8 – Welding helmet
18
Metals& Fabrication
Arc Welding 1
Recommended filters for manual metal arc welding (MMAW) are given in table 1.2.
Recommended filters for MMAW
Amperage
Shade No.
Up to 100
8
100−200
10
200−300
11
400−500
12
Over 400
13
Based on AS/NZS 1338.1: 1992 – Table A2 (www.saiglobal.com)
Table 1.2 – Recommended filters for MMAW
For more detailed and current information refer to AS/NZS 1338.1:1992 for electric
welding.
Higher current density/open arc processes such as GTAW or GMAW require darker
shade lenses for the same current.
For gas welding and cutting, the use of protective goggles fitted with the recommended
shade 5 filter, is essential. Clear safety spectacles give only limited protection from
stray radiation, however spectacles fitted with lenses not less than 2 mm thick
incorporating a shade filter of up to 2.5 are highly desirable to give protection from
stray arc welding flashes. In order to protect the skin from radiation it is essential that
suitable clothing is worn to cover all areas which could be exposed. Woollen materials
have much greater resistance to ultraviolet radiation than synthetic and plain cotton
materials which can rapidly deteriorate or rot when exposed to strong ultraviolet
radiation. Leather aprons, sleeves, jackets and gloves are usually required in welding
processes where strong radiation is emitted.
Where reflection is likely, for example in welding on highly reflective metals such as
aluminium or stainless steel, protection for the eyes and skin against indirect radiation
is required.
Protection of others from radiation
Adequate protection should be provided for all personnel within 12 metres of an open
arc or gas flame. Suitable screens (either fixed or portable) are desirable. These
screens and surrounding walls or partitions should have a matt finish and dull colours
in order to reduce reflection.
Personnel working near welders should wear safety spectacles complying with the
requirements of Australian Standards® AS/NZS 1336:1997 Recommended practices
for occupational eye protection.
19
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Chapter 1 – Arc welding safety
Fire and explosion
Oxy-acetylene flame cutting and welding operations are a major cause of industrial
explosions and fires. Each year losses amounting to several million dollars and loss
of life or severe injury result from fire caused by welding and cutting operations. The
safety requirements depend largely upon the processes being used and the location
of the work being carried out. Protection against fire and explosion should comply
with statutory regulations covering prevention and comply with the requirements
of Australian Standards® AS 1674.1 Safety in welding and allied processes – Fire
precautions.
Sources of fire
The temperature of the arc (or flame) of welding and cutting is sufficient to cause the
combustion of many materials. Solid materials such as wood, wood-based products,
paper, synthetic materials, liquid materials such as paint and oil, and grease-soaked
materials, have a low ignition temperature and will readily ignite through direct contact
with the arc or flame. Ignition will also occur by contact with metal offcuts, electrode
stubs and spatter. Such materials should be cleared away from any welding or cutting
areas, as hot particles lodged amongst them may initially produce smouldering and
then fire.
When considering the area affected by cutting and welding sparks, account should
be taken of the process and the job situation. Cutting and gouging can produce high
speed particles travelling long distances (up to 10 metres in the flat), and hot particles
falling from a high workstation will travel further than normal, as illustrated in Fig 1.9.
7m
5m
2.5m
To 20m
3.5m - 5.9m
4.2m - 7.8m
5.8m - 10.2m
Illustration reproduced with the permission of Welding Technology Institute of Australia (WTIA)
Fig 1.9 – Typical travel distances for hot or molten metal
particles in cutting or welding
Causes of explosion
The risk of explosion is always present when welding or cutting. These processes may
project hot sparks into an atmosphere containing flammable gases, liquids or solids.
Non-volatile oils or solids which do not produce flammable gases at atmospheric
temperatures may produce flammable or explosive gases when exposed to heat from
welding or cutting. Drums, fuel tanks and other containers pose a particular hazard to
the safety of the welding operator, and no cutting or welding should be carried out until
all precautions have been carried out and the job has been made safe.
20
Metals& Fabrication
Arc Welding 1
Fire and explosions become greater hazards in situations where flammable gases and
liquids are present. In the ordinary workshop, operators should be aware of normal fire
precautions.
Prevention of fire and explosion
•
Maintain clean and tidy work areas, free from accumulations of combustible
materials.
•
Check that work introduced for cutting or welding does not constitute a fire or
explosion hazard.
•
Ensure that screens, aids, and building fittings are not constructed from
flammable materials.
•
Ensure that personal clothing is sound and made from suitable materials.
•
Store flammable substances and gases in a safe area or separate building.
•
Be aware of fire extinguisher locations and how to operate each type.
•
Avoid oxygen enrichment of clothing or work space, which may be caused by
leaking oxygen valves.
Burns
Because welding and cutting is associated with intense heat, the operator is always in
danger of receiving painful burns.
Burns are classified in terms of their extent and depth. The extent of a burn is
described by calculating the burned area as a percentage of total skin area. The depth
of a burn is described by degree:
•
superficial burn – produces reddening of the skin (first degree)
•
intermediate burn – produces blistering (second degree)
•
severe burn – extends below the surface of the skin and causes injury to
underlying tissues (third degree).
In welding and cutting operations, burns can result from:
•
ultraviolet and infra-red radiation
•
contact with slag, sparks and hot particles
•
contact with hot work or heat radiated from work
•
electrical leakage, in particular, leakage from high-frequency devices
•
fire and explosions.
Protection from burns
•
Use tongs to handle hot metal.
•
Make provision for disposal of hot metal and electrode stubs.
•
Wear all the necessary protective clothing.
•
Protective clothing must be non-flammable, and free from oil, grease, tears, and
fraying.
21
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Chapter 1 – Arc welding safety
Personal protective equipment (PPE)
Arc welding, like most welding processes, requires operators to protect themselves
from the radiated heat and rays associated with the process.
Perhaps the most efficient way of doing this is by the wearing of protective clothing.
The use of all protective clothing is dictated by the nature of the work and the comfort
of the operator.
Ideally, clothing for the operator should consist of:
•
long-sleeved cotton shirt
•
sleeves rolled down and buttoned
•
strong trousers without cuffs
•
strong leather shoes or work boots
•
aprons
•
gloves
•
spats (leather)
•
caps
•
leather capes or jackets.
Fig 1.10 – Dressed for safety
22
Metals& Fabrication
Arc Welding 1
Always take care to check clothing for frayed edges, torn areas and open pockets
where sparks can lodge and start burning. Work clothes should also be free of oil or
grease. This may be difficult in some workshops, but a spare pair of clean overalls
could be left at work specifically for welding operations.
The working environment
There are work situations which present increased hazards to the health and safety of
the welding operator. These are:
•
confined spaces
•
hazardous locations
•
working on tanks and containers.
Confined spaces/hazardous locations
Working in confined spaces usually entails difficult access and cramped conditions.
The workplace is often poorly ventilated, and the welder is often completely surrounded
by a conductor which forms part of the welding circuit. Under these circumstances the
welding operator is at increased risk from:
•
a build-up of fumes
•
electric shock.
The possibility of a build-up of dangerous fumes whilst welding in a confined space
must be allowed for and adequate ventilation be provided through:
•
exhaust fans
•
an additional supplementary air supply.
The possibility of an electric shock is greater because the operator can easily make
contact with the job, and awkward and enclosed workplaces often lead to higher levels
of perspiration.
The operator should keep themselves as dry as possible and use the necessary
protective clothing to prevent electrocution. Additionally:
•
an all-insulated electrode holder should be used
•
high-frequency attachments should not be used
•
portable electric lamps exceeding 32 V supply should not be used. Electronic
leakage breakers (ELB) devices are acceptable.
Provision must be made, close to the work area, for the power to be switched off by an
assistant when:
•
the welder is not prepared for welding
•
the electrode is being changed
•
the operator leaves the job.
23
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Chapter 1 – Arc welding safety
Confined space regulations
The following regulations are specified as mandatory when working in a confined
space.
•
Adequate ventilation must be provided.
•
A lifeline must be attached.
•
A semi-skilled operator who is trained in rescue and resuscitation must be
stationed at the manhole to monitor the work space at all times; to adjust
oxy-acetylene gear and the welding machine, whilst continually observing the
operator.
•
All leads and hoses are to be kept clear of the floor, dampness and falling metal
sparks. Circular vessels must be prevented from rolling.
•
General tidiness and care is essential, equipment should not be allowed to
contact hot work or sharp objects.
•
Oxy-flame cutting equipment should not be left inside the confined space when
not in use, and it should always be lit by the assistant outside and then passed to
the operator inside.
•
Oxygen should never be used for dusting down or any purpose other than for the
oxy-flame.
Local exhaust
ventilation
Mats for thermal and
electrical insulation (high
frequency requires
special insulation)
To exhaust
ventilation
system
Fully insulated
electrode holder
Observer
Main switch
within reach of
observer
Dry wooden
form work
Work
terminal
Dog and
wedge
Electrode
terminal
Fig 1.11 – Precautions for welding in confined spaces
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Metals& Fabrication
Arc Welding 1
Hazardous locations
Although many workplaces may be described as hazardous, a hazardous location is
defined as: ‘An area where flammable dust, fibres or gases may be present so as to
pose a fire or explosion hazard’.
Hazardous locations may be classified into four main groups typically, as follows.
1.
In locations where flammable liquids are manufactured, used, handled or stored,
or where vapours may be present.
eg
refineries, fuel stores
dry cleaning plants
spray painting premises
varnish and paint manufacturing plants.
2.
In locations where combustible dust is thrown into suspension in the air and
quantities may be sufficient to produce explosive mixtures.
eg
sections of flour mills
grain elevators
cocoa and coal pulverising plants
iron ore or aluminium plants
metal grinding plants
charcoal grinding plants.
3.
In locations where easily ignitable fibres are produced, handled, used or stored.
eg
cotton or cotton seed mills
wood working plants
sections of clothing factories.
4.
In any location or part of a ship.
Cutting or welding in or near hazardous locations
If at all possible, work should be removed from the hazardous locations and carried out
to a safe location.
Cutting or welding in or adjacent to hazardous locations should not take place until the
following conditions have been established.
•
A hot work permit has been obtained.
•
Authorisation has been obtained from the responsible officer.
•
It is impractical to move the work to a safe area.
•
The production of any hazardous or explosive substance has ceased or been
excluded from the work area.
•
The location has been tested and found to be free from flammable substances.
25
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Chapter 1 – Arc welding safety
In general terms the operator’s responsibility with respect to hazardous locations can
be expressed as follows.
•
Always examine work areas for possible hazards.
•
Seek authorisation before proceeding with cutting or welding whenever any doubt
exists.
•
Work must be carried out in accordance with the provisions of the hot work
permit.
•
Always examine the possibility of removing the work to a safe area.
•
Be vigilant in the provision and maintenance of any safety screens, doors or
barriers required to ensure safety.
•
Be vigilant in the possible entrapment or catching of any sparks, offcuts or
electrode butts as provided for in the safety arrangements.
•
Always check behind walls, partitions, bulkheads etc, to ensure safety in
adjoining areas.
•
Take precautions when working in the vicinity of storage batteries as they are
liable to explosion, particularly whilst charging.
A fire watch must be maintained for a minimum of one hour after any cutting and
welding operations have ceased.
Working on tanks and containers
Responsibility for work
When working on tanks or containers the operator should display the same caution
as when working in hazardous locations. If there is any possibility that the container
may have held petrol, oil or any volatile liquid, special precautions are necessary.
Sight and smell are not reliable indicators of the presence of flammable gases as
some substances may release them when heated. Doubtful cases should be referred
to a qualified person for testing, and subsequent work carried out by an experienced
operator under supervision.
Recommended practice
Where steam is available, this may be used to remove materials which are easily
volatile. Washing with strong soda solution or detergents will remove heavier oils.
Chlorinated hydrocarbon solvents must not be used for cleaning prior to welding.
Even after thorough cleansing, the container should, whenever possible, be filled with
water before any cutting or welding operation is performed. In most cases it should be
possible to place the container in such a position that it can be filled with water to within
a short distance from the point where cutting or welding is to be done.
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Metals& Fabrication
Arc Welding 1
In doing this, however, care should be taken to make sure that there is a vent or
opening to provide for the release of heated air from the container. Where it is not
possible to fill the container with water, carbon dioxide or nitrogen may be used for
added protection. If possible, periodic examination of the air contents of the vessel
should be made by means of a detector of combustible gases, where such an
instrument is available.
Pressure regulator
Small free space with opening
1 - 2cm
Open pipe
Welding
point
Welding
point
1 - 2cm
Container
Carbon dioxide
cylinder
Open pipe
Air exit
Container
Full of water
Full of water
Entry of carbon dioxide
Illustration reproduced with the permission of Welding Technology Institute of Australia (WTIA)
Fig 1.12 – Preparation of tanks for welding
First aid for operators
Basic objectives
In the event that a person is injured or suddenly becomes ill, efficient first aid should
be carried out as quickly as possible, preferably by trained first aiders. Taken before
medical help is available and often at a critical stage, this action can save lives, reduce
the severity or worsening of the injury and limit discomfort.
Essential emergency action
This involves:
•
ensuring that there is no danger to the patient or rescuer
•
getting the casualty out of any danger zone, without endangering anyone
•
giving first aid to the casualty.
For all but minor injury, arrange for medical assistance. If there is little risk in moving
the casualty, arrange for transport and for care during transport to a doctor, hospital or
nurse. If there is any risk of further injury posed by moving the patient, he/she should
not be moved, and qualified medical help should be sought.
27
27
Chapter 1 – Arc welding safety
Basic immediate first aid for some common operator injuries
Welding flash (arc eye)
This is an eye injury caused by exposure to ultraviolet rays.
•
In mild cases, add eye-drops and shade the eyes.
•
In severe cases, loosely pad both eyes (cold may help) and get the casualty
quickly to a doctor.
Hot particles in the eyes
•
Cover BOTH eyes of the victim and take the patient as quickly as possible to a
doctor.
•
In the case of chemical burns (eg from acids, alkalis or similar liquids) remove the
chemicals from the eyes by washing at once with large amounts of running water
and flush continuously for up to 20 minutes.
•
Urgently seek medical advice.
Burns (to hand/s and body)
Minor burns
Minor burns should be immediately cooled under cold running water, then covered with
sterile dressing material. Avoid the use of ointment or powder as these may interfere
with any subsequent medical treatment.
Major burns
Since urgent action is essential, cool the area with running water quickly and over a
long period to remove any residual heat, and get the casualty to hospital as quickly as
possible. Keep the casualty covered with a light blanket or other suitable material. Care
must be taken to ensure that dressings, blankets, etc will not stick to the wound.
Electric shocks
Electric shock usually does not kill at once, but may stun the victim and stop his or her
breathing. Delay in rescue and resuscitation may be fatal.
In the event of electric shock, immediately switch off the electricity where practicable,
and then pull or push the patient clear. If the electricity cannot be switched off
immediately, remember that the patient is electrified until released and take precautions
against receiving a shock yourself. The patient must be pulled or pushed away from
the conductor using any type of DRY insulating material, such as wood, rope, clothing,
rubber or plastic. DO NOT USE METAL OR ANYTHING MOIST. In some cases it may
be easier to remove the conductor from the patient. Where necessary take care that
the patient does not sustain injury by falling.
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Metals& Fabrication
Arc Welding 1
Resuscitation
After rescue, if the patient is not breathing, commence artificial respiration immediately
and CONTINUE WITHOUT INTERRUPTION, FOR HOURS IF NECESSARY. When
assistance is available, send for a doctor and an ambulance.
Artificial respiration and cardiac massage
The need for artificial respiration is evident if the patient is not breathing, and it must
begin immediately. At the same time a check on the patient’s carotid pulse will
establish the need for cardiac massage. If no pulse can be felt, cardiac massage
should proceed together with artificial respiration. The techniques employed are
described in detail in Australian Standards® AS 1674.2 Safety in welding and allied
processes – electrical, and also in the Trust publication module – ‘Resuscitation’ and
other first aid manuals.
Severe bleeding
Apply direct pressure to the wound by placing a large dressing over the wound and
holding it in position with a firm bandage. If the dressing becomes saturated with blood,
do not remove it but apply another. This will aid clotting. In an emergency, if a dressing
is not readily available, firmly press the sides of the wound together with the fingers or
hand. Elevate the injured part to decrease the blood flow to the wound. Seek medical
attention immediately.
Fractures
Do not move the patient, but immobilise the person and the fractured limb by use of
pillows, blankets or other suitable materials. Bleeding should be controlled if present,
and the patient kept warm until qualified medical help arrives.
Inhalation of toxic or intensively irritating gas or fumes
Remove the patient out of the danger zone at once and into fresh air. Place them in
a warm comfortable position. The patient should be taken to, or seen by, a doctor as
quickly as possible. Where possible, the gas or fumes involved should be identified to
assist the doctor with treatment.
Commonly referred to safety standards
Australian Standard®
•
AS 1674.1
Safety in welding and allied processes – Fire precautions.
•
AS 1674.2
Safety in welding and allied processes – Electrical.
•
AS/NZS 1716
Respiratory protective devices.
•
AS/NZS 1336
Recommended practices for occupational eye protection.
•
AS/NZS 1337
Eye protectors for industrial applications.
•
AS/NZS 1338.1 Filters for eye protectors against radiation generated in welding
and allied operations.
•
AS/NZS 1338.2 Filters for eye protectors – Filters for protection against
ultraviolet radiation.
•
AS/NZS 1338.3 Filters for eye protectors – Filters for protection against infra-red
radiation.
29
29
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Arc Welding 1
Chapter 2 –
Electricity and welding machines
Introduction
All arc welding processes have a few basic requirements for their operation. They must
have a safe voltage available that is sufficient for the operator to get the arc started and
be maintained. They also require sufficient amperage to provide the heat for melting of
the parent metal and filler material.
Arc welding processes have been a popular and widely applied method of welding for
many years. They offer sound and reliable weld, simple operation and low capital cost.
In this chapter you will look at the following.
•
•
Arc welding process overview
ο
manual metal arc welding (MMAW)
ο
gas metal arc welding (GMAW)
ο
flux cored arc welding (FCAW)
ο
submerged arc welding (SAW)
ο
gas tungsten arc welding (GTAW)
Electricity and welding machines
ο
welding current
ο
current types
ο
welding machines
ο
amperage control
ο
machine characteristics
ο
selecting a welding power source.
At the end of this chapter, you will complete an activity.
Arc welding process overview
There are a lot of arc welding processes used in the metal fabrication and welding
industries. Some of these are commonly used and others are used in specialist
applications. This section introduces some of the most commonly used arc welding
processes; which are:
•
manual metal arc welding (MMAW)
•
gas metal arc welding (GMAW)
•
flux cored arc welding (FCAW)
•
submerged arc welding (SAW)
•
gas tungsten arc welding (GTAW).
31
Chapter 2 – Electricity and welding machines
Manual metal arc welding (MMAW)
MMAW is a welding process that creates an electric arc between a hand held,
flux-coated, consumable filler wire and the work piece. The arc heat melts the parent
metal and filler wire. The flux coating breaks down in the arc to produce a gaseous
shield that excludes atmospheric gases from the weld zone. The flux coating also
provides a de-oxidising action and forms a slag on the cooling weld.
The MMAW welding process needs a suitable and constant current power source (AC
or DC), a handpiece, a work clamp, leads and flux-covered consumable electrodes.
Direction of travel
Electrode flux
Electrode
Arc column
OR
Arc plasma
Solidified slag
Shielding gases
Weld metal
Parent metal
Fig 2.1 – MMAW process
MMAW equipment is cheap and simple to use and people with very little training and
practice are able to use the MMAW process to make reliable welds. A skilled operator
can use the MMAW process to weld practically any material in virtually any situation.
Typical uses for the MMAW process include:
32
•
light/heavy/fabrication
•
general engineering
•
site work
•
repairs.
Metals& Fabrication
Arc Welding 1
Gas metal arc welding (GMAW)
GMAW is a welding process that creates an electric arc between an automatically
fed wire electrode and the work piece. The arc heat melts the parent metal and filler
wire. A supply of shielding gas that excludes atmospheric gases from the weld zone is
introduced around the arc.
The GMAW welding process needs a suitable and constant voltage power source (DC),
a wire feed unit, consumable wire electrodes, a shielding gas supply, a welding gun, a
work clamp and leads.
Direction of
travel
Wire guide and contact tip
Gas nozzle
Solidified
weld metal
Shielding gas
Work
Arc
Molten weld metal
Fig 2.2 – GMAW process
GMAW equipment is not as cheap as MMAW and requires some skill to set up
properly. A good operator can use the GMAW process to weld most materials in most
welding positions.
Typical uses for the GMAW process include:
•
light/heavy fabrication
•
general engineering
•
most materials and thicknesses.
33
Chapter 2 – Electricity and welding machines
Flux-cored arc welding (FCAW)
FCAW is a welding process that creates an electric arc between an automatically fed,
hollow wire filled with flux and the work piece. The arc heat melts the parent metal, filler
wire and flux. The flux may also add additional material or elements and breaks down
in the arc to produce a gaseous shield that excludes atmospheric gases from the weld
zone. An optional supply of shielding gas may be introduced around the arc.
Thread
protector
Direction of
travel
Current carrying
contact tip
Molten slag
Powered metal, vapour
or gas forming materials,
deoxidisers and scavengers
Solidified
slag
Arc shield composed of
vaporised and slag forming
compounds protects metal
transfer through arc
Solidified
weld metal
Molten
weld metal
Metal droplets covered
with thin slag coating,
forming molten puddle
Fig 2.3 – FCAW process
The FCAW welding process needs a suitable and constant voltage power source (DC),
wire feed unit, consumable flux-filled electrodes, an optional shielding gas supply, a
welding gun, a work clamp and leads.
FCAW equipment is generally more robust than GMAW plant and requires some skill to
set up properly. The process may be self shielding or gas shielded.
Typical uses for the FCAW process include:
•
heavy fabrication
•
general engineering.
FCAW has a better deposition rate and fusion than GMAW.
34
Metals& Fabrication
Arc Welding 1
Submerged arc welding (SAW)
SAW is a welding process that creates an electric arc between an automatically fed
wire electrode and the work piece. The arc heat melts the parent metal and filler wire.
A supply of flux material is introduced around the arc to contribute to the welding
operation, exclude atmospheric gases from the weld zone and form a slag over the
cooling weld.
The SAW welding process needs a heavy duty power source (DC), a wire feed unit,
wire electrodes, flux and a flux delivery system, a power head unit, a welding gun, a
work clamp and leads.
Molten flux
From flux
hopper
Electrode
Slag
Direction of travel
Granular
flux blanket
Solidified weld
metal
Molten welded
metal
Arc path
Base metal
Fig 2.4 – SAW process
SAW equipment is heavy and specialised and generally produces high quality welds on
steel products.
Typical uses for the SAW process include:
•
heavy/fabrication
•
flat butt and fillet welds only.
35
Chapter 2 – Electricity and welding machines
Gas tungsten arc welding (GTAW)
GTAW is a welding process that creates an electric arc between a non-consumable
tungsten electrode and the work piece. The arc heat melts the parent metal. A supply
of inert shielding that excludes atmospheric gases from the weld zone gas is introduced
around the arc. Filler wire may be introduced into the weld area.
The GTAW welding process needs a suitable and constant current power source
(AC or DC), tungsten electrodes, an inert shielding gas supply, a welding torch,
a work clamp and leads.
Gas cup.
Either ceramic or
Water-cooled metal
ACHF, DCSP
or DCRP
power supply
Direction of
travel
Tungsten Electrode
virtually
(non-consumable)
Welds made with
or without addition
of filler metal
Many joint designs
in all positions
can be welded
easily
Weld surface
smooth & clean
Work
return lead
No flux
required
Base metal
can be any
commercial
metal
Weld possesses good
quality. Little or no loss
of alloying elements
Inert gas
shields electrode
and weld puddle.
Highly
concentrated heat
Fig 2.5 – GTAW process
The GTAW equipment process requires some skill to set up properly and a good
operator can produce high quality welds on nearly all materials.
Typical uses for the GTAW process include:
•
light fabrication
•
general engineering
•
root runs, pipe work.
GTAW welds most materials and thicknesses (carbon steel, aluminium, stainless
steel etc).
36
Metals& Fabrication
Arc Welding 1
There are other welding processes used in the metal fabrication industry that are not
covered in this text. These are:
•
electric resistance welding (ERW)
•
electroslag welding (ESW)
•
laser welding (LW)
•
fuel gas welding (FGW).
Fuel gas welding is covered in greater depth in gas welding theory.
Electricity and welding machines
All welding processes depend on three main requirements for their operation.
1.
A heat or energy source – needed for fusion.
2.
Atmospheric shielding – to prevent oxygen and nitrogen in the atmosphere from
contaminating the weld.
3.
Filler metal – to provide the required weld build-up.
These factors are looked at more closely in later chapters dealing with the various arc
welding processes.
Welding current
To be suitable for welding, the current used must meet the following requirements.
•
There must be sufficient amperage to provide the heat for fusion.
•
The voltage must be high enough to initiate the arc, but low enough to ensure the
safety of the welding operator.
•
There must be a suitable means of current control.
Mains supply is unsuitable for use as the welding current as the voltage is too high and
the amperage too low. Mains supply must be ‘transformed’ to make it suitable for use
in welding. Alternatively, the welding current can be produced from a dedicated welding
generator or alternator.
Current types
Electric current may be either:
•
alternating current (AC)
•
direct current (DC).
Term
Definition
alternating current (AC)
an electric current that reverses direction in a circuit
at regular intervals
direct current (DC)
an electric current flowing in one direction only
37
Chapter 2 – Electricity and welding machines
Alternating current (AC)
Alternating current is produced by an alternator – AC is usually taken from the mains
supply which operates at 50 cycles/sec.
•
There is a period of current flow from positive to negative followed by a period of
current flow from negative to positive.
•
The flow changes direction 50 times every second.
•
The voltage falls to zero 100 times/sec (therefore the arc is broken and
re-established 100 times every second.
Due to the even periods of current flow with AC:
•
the heat is distributed evenly at the electrode and work piece
•
there is no choice of polarity.
Term
Definition
(positive or negative) polarity in this instance refers
to the welding terminals being positive (+ve) or
negative (–ve)
polarity
Direct current (DC)
DC may be produced in the following ways:
•
by chemical reaction as produced in a storage battery
•
by a generator driven by a rotational shaft
•
by converting AC by means of a rectifier or inverter.
Direct current exhibits the following characteristics:
_
DC flows continuously in one direction at the preset voltage
•
in DC the current always flows from negative to positive
•
with DC the flow of electrons striking the positive pole (+ve) generates two thirds
of the heat from the arc at the positive pole.
1/50 Sec.
1Hz
+
V
O
L
T
S
•
+
0
TIME
V
O
L
T
S
0
_
Fig 2.6 – Typical AC and DC output curves as seen in an oscilloscope
38
TIME
Metals& Fabrication
Arc Welding 1
Polarity
Polarity refers to the way in which the electrode lead is connected to the DC welding
power source. When welding with positive polarity the electrode lead is connected to
the positive terminal of the welding machine. When welding with negative polarity the
electrode lead is connected to the negative terminal.
Changing polarity with DC does not change the direction of current flow. Current still
flows from the negative to the positive pole. Changing polarity, however, alters the point
at which the greater portion of heat is generated in the welding circuit. For example:
•
most of the heat is generated at the electrode with +ve polarity (electrode
connected to positive)
•
most of the heat is generated at the work piece with –ve polarity (electrode
connected to negative).
There is no polarity with AC welding circuits. As current flow is equal between both
the positive and negative poles, the heat is distributed between both the positive and
negative poles, and between the electrode and the work piece.
Arc blow
Arc blow is a problem peculiar to DC circuits. Arc blow is the effect of electromagnetic
forces within the circuit which deflect the metal droplets as they flow across the arc
gap. As the current within the circuit increases, the magnetising effect increases
accordingly. Consequently, arc blow is more severe at higher amperages, particularly
above 300 A.
Term
arc blow
Definition
a deflection of the arc by electromagnetic forces in
the welding circuit caused by the flow of the welding
current
arc blow occurs in DC circuits only
Some of the methods used to control or minimise the effects of arc blow are:
•
change to AC
•
change polarity
•
change the position of the work return lead
•
use two work return connections
•
change the direction of welding
•
wrap the work return lead around the job
•
reduce the amperage.
39
Chapter 2 – Electricity and welding machines
AC versus DC
AC and DC welding circuits each have their own advantages and disadvantages.
AC welding machines are cheap, and though they lack portability they are simple and
trouble-free in their design and operation.
DC generator machines are generally portable and offer better control of welding
conditions, but are more expensive to buy and maintain.
Alternating current (AC)
Direct current (DC)
portability
these machines generally consist of
static step-down transformers and
are considered as stationary
most modern types have features
that allow portability (especially the
self-contained types)
power supply
the use of these machines is
restricted to the location of the
nearest alternating current power
point
petrol or diesel engine driven
machines can be used in any
location
efficiency
70–90 per cent electrically efficient
40–60 per cent efficient but some
modern types compare with
alternating current efficiency
polarity
no polarity
choice of polarity
arc blow
unaffected
arc blow occurs even in normal
currents and is difficult to control
above 300 amperes
maintenance
as there are no moving parts to be
considered, maintenance costs are
low
revolving and wearing parts add to
maintenance
initial costs
cheaper plant as less construction
is involved
more costly due to generator and
motor construction
electrodes
restricted to use of electrodes that
are suitable for alternating current
only
suitable for all types of electrodes
running cost
cheaper running costs due to the
use of an installed power supply
added costs due to the use
of electric motors or internal
combustion engines
voltage control
constant open circuit voltage
the open-circuit voltage can be
varied by the operator
arc length
limited arc length
greater tolerance in arc length
due to the characteristics of the
machine
Table 2.1 – Comparison of AC and DC machines
40
Metals& Fabrication
Arc Welding 1
Welding machines
There are various types of welding machines available to accommodate the wide range
of welding processes and applications that comprise ‘welding’.
Welding machines range from small ‘hobby type’ machines putting out as little as 100
amps to large industrial types with outputs in the thousands of amps.
To ensure the safety of the welding operator, the open circuit voltage (OCV) of welding
machines is restricted by regulations to:
•
AC – maximum OCV 80 volts
•
DC – maximum OCV 110 volts.
Term
Definition
open circuit voltage (OCV)
the voltage between welding terminals on the
machine when it is switched on but welding is not in
progress
AC machines
When an AC mains supply is available, it is possible to use a step-down transformer to
reduce the supply voltage of 415 V to a safe OCV of around 70–80 volts. At the same
time current is increased so as to provide sufficient heat for welding.
Mains supply
(P)
PRIMARY
(P)
(S)
SECONDARY
(S)
Fig 2.7 – Step-down transformer
The step-down welding transformer consists of a laminated, soft iron core carrying two
coils which are not electrically connected. The first is connected to the supply (primary).
Voltage applied across the first coil will produce, by induction, a voltage in the second
coil. The value of this secondary (induced) voltage will be proportional to the ratio
between the turns in the two coils. If each coil has an equal number of turns, equal
voltage will appear at the secondary connections. If, however, a transformer has 400
turns in the primary coil, and 50 turns in the secondary coil, then a primary voltage of
400 V will induce 50 V to appear at the secondary connections.
41
Chapter 2 – Electricity and welding machines
The power into the welding machine is calculated by multiplying the volts by the amps
and is expressed as volt-amps (VA). This figure is generally quite large and is usually
divided by 1000 and expressed as kilovolt amperes (kVA).
Power IN = volts x amps
400 V x 50 A = 20 000 VA
Since transformers have very low losses, we can consider here that the total power
put into the machine must equal the power output. Therefore, in this machine which is
theoretically 100% efficient:
Power OUT = 20 000 VA
The output voltage is determined by the ratio of the windings of the transformer.
Therefore in this case the output voltage will be equal to 50 volts.
If power = amps x volts, it can be seen that:
amps = power
volts
Therefore, in our welding machine the output current is equal to:
20 000 = 400 amps
50
The output current of 400 amps at 50 volts would now be suitable for welding,
particularly if some form of current control were added.
We considered a transformer that was theoretically 100% efficient, however in practice
this would not be the case. Let us say that our transformer was only 90% efficient. This
means that we would have a power loss of 10 per cent.
Since the voltage is determined by the ratio of the windings of the machine and is
therefore fixed, the power loss would be in the form of reduced output amperage.
Therefore our output amperage would be:
400 A x 90% = 360 A
DC machines
Direct current for welding may be obtained from a generator set, a transformer rectifier
unit, or from an inverter.
Generators
A welding generator basically consists of an armature carrying a number of windings,
which rotate in a magnetic field produced by electromagnets (field coils). The passage
of the armature through this field induces a voltage through the windings. The current is
collected by carbon brushes running on a copper commutator at one end of the rotating
armature, and current will flow when the circuit is made. The armature is rotated by an
electric motor connected to an AC supply or by a diesel or petrol engine.
42
Metals& Fabrication
Term
Arc Welding 1
Definition
armature
a coil in which voltage is induced by motion through
a magnetic field
commutator
a device used to enable a rotating coil to produce
DC current
Electrode positive
GENERATOR
2
3
heat
Work
1
3
heat
Electron flow -VE to +VE
Fig 2.8 – Direct current circuit with electrode positive
Welding generators are constructed to produce high current flow at comparatively
low voltages, which are suitable for welding. The current produced by the generator
should be steady and the voltage must not fluctuate during welding. A steady current
is maintained by compensation coils or reactors to absorb current fluctuations and
produce a more stable arc.
Generators can be driven using either petrol or diesel engines. These machines offer
the advantage of being portable and are popular for site work where line power is
unavailable. Some generators also provide auxiliary power which is useful for power
tools and lighting.
Most modern portable power supplies utilise a highly efficient, high frequency alternator
and electronics to provide both AC and/or DC current at constant voltage or constant
current type outputs, suitable for use in a wide range of welding operations.
Rectifiers
A rectifier is a device which permits current flow in one direction only and can therefore
be used to convert AC to DC. They can be supplied as an individual unit, but most
often are incorporated into the welding power source. The rectifier consists of metal
plates coated with a selenium compound, or of silicon diodes – each having the special
property of allowing the current to flow in one direction only. This means that when an
alternating voltage is applied, only the positive half-cycles are effective. This ‘half wave
rectification’ is undesirable and uneconomical, so the rectifier units are arranged in the
form of a bridge to achieve ‘full wave rectification’.
43
Chapter 2 – Electricity and welding machines
Transformer/rectifiers
Where both AC and/or DC welding current is desirable, eg for gas tungsten arc welding
(GTAW) or when DC is required from mains supply, eg for gas metal arc welding,
(GMAW) a transformer/rectifier is commonly selected. GMAW machines usually
provide DC output. Molten metal arc welding (MMAW) and GTAW machines usually
provide both AC and DC output. By means of a switch or by changing leads, the welder
can select either positive or negative polarity on the DC output.
Most GTAW machines are equipped with a high-frequency oscillator which provides
a high-frequency spark to enable the arc to be started without the electrode making
contact with the work. The high-frequency spark may be used simply to start the arc
when using DC or may be continuous to re-establish and maintain a steady arc with AC.
Half wave rectification
Full wave rectification
+
+
0
0
_
_
Current regulator
Full wave rectifier
Diode rectifiers
AC
supply
primary
Secondary
Fig 2.9 – Single phase transformer with bridge (full wave) rectifier
Inverters
Inverters are fast taking over from other types of welding machines. These machines
are able to provide AC and smooth stable DC output at high-efficiency levels, and
feature lightweight construction.
The machines operate on either 240 V or 415 V AC input current and immediately
rectify this to DC using a series of high temperature diodes. This DC current is stored in
filter capacitors and then converted to an oscillated AC output in an oscillator stage at
a much higher frequency than the input supply. This high-voltage/high-frequency signal
is then fed into a high-efficiency transformer primary coil and high-frequency AC current
is produced in the secondary coil. The frequency can be anything from 5 kHz upward,
depending on the design and type of output required.
Due to the high-frequency AC generated by the oscillator, the weight of transformers
can be reduced dramatically because there is no magnetic loss or heat loss through
the windings, and much greater transformer efficiencies can be achieved.
44
Metals& Fabrication
Arc Welding 1
Now that a high-frequency, low voltage, but high current power supply has been
created it can be used as high-frequency AC welding power for MMAW or GTAW of
aluminium. Alternatively the AC can then be rectified into DC current again and passed
through a second filtering system to produce a very smooth current flow. Welding
machines with an output frequency of around 5 kHz demonstrate a characteristic
whistling sound during welding.
Inverter welding machines have very good electronic controls that can regulate the
voltage and current. They are also very efficient and highly portable due to their
reduced weight.
The inverter cycle
1.
Mains current rectified to DC and stored.
2.
DC is oscillated to a high frequency.
3.
High-frequency/high voltage AC is transformed to low voltage AC.
4.
AC rectified to produce DC.
5.
DC filtered to a smooth current.
Power switches
50Hz/AC
1
DC
Input rectifier
2
DC
Output rectifier
3
AC
5 kHz
4
AC
Transformer
5
Output DC
Filter
Fig 2.10 – Inverter cycle
Amperage control
If a welding operator draws current directly from a transformer, with no form of current
control, the welding current is fixed and will only be limited by the resistance of the arc,
the welding leads and the transformer’s characteristics. The current may be excessive
and there would be no means for the operator to select the correct setting for the job.
Some method of current control is required if a machine is to be practical to use. Five
common types of current control devices are described below.
Term
Definition
resistance
a resistance inhibits the flow of electrons along a
conductor. The greater the resistance the greater
the voltage drop. Electrical resistance always
causes heat to be produced.
voltage drop
the difference in electrical pressure (volts) between
two points in a circuit, caused by resistance
opposing the flow of current, ie excessively long
welding cables, loose connections, damaged
cables, arc length too long
45
Chapter 2 – Electricity and welding machines
Movable coil
This consists of a special arrangement in the transformer that allows the distance
between the primary and secondary windings to be varied, enabling the amount of
current induced in the secondary coil to be altered.
Amperage is usually selected by winding a hand wheel or shifting a lever. This action
moves the primary coil in the machine either closer to or away from the secondary
coil, which is usually mounted on the machine base. The closer the two coils are
together, the greater the magnetic force between them, and consequently the higher
the amperage. As the coils are moved further apart, efficiency is lowered resulting in
reduced current output.
Iron core
80
Primary
coil
(movable)
Lead
screw
Minimum output
E
Secondary
coil
(fixed)
0
100
I
200
Base
Iron core
80
Primary
coil
(movable)
Maximum output
Lead
screw
E
Secondary
coil
(fixed)
0
Base
Fig 2.11 – Movable coil AC transformer
46
100
I
200
Metals& Fabrication
Arc Welding 1
Resistance
Electrical resistance in a circuit opposes the flow of current. By varying the resistance
in the welding circuit the amperage can be controlled. This is usually done by
passing the current through one variable resistance coil, or a series of coils with fixed
resistance. Resistance is inefficient where high currents are used, as large amounts of
heat are generated.
Resistance
coils
Transformer
50
50
30
20
Primary
power in
Primary
coils
10
5
Secondary
coils
Machine terminal
Machine terminal
Work piece
Fig 2.12 – Fixed resistance coils
With machines as in Fig 2.12 above, amperage is selected by pushing in buttons
to make contact with the appropriate resistance coil. Each coil allows only a certain
amperage to flow through it. The more coils selected, the greater the amperage.
47
Chapter 2 – Electricity and welding machines
Moving core choke
This consists of a coil of wire or copper strip heavy enough to carry the welding current,
which is wound around an iron core. This induces a counter-voltage which chokes back
on the current flow. By adjusting the amount of iron within the coil, the flow of current
can be controlled. The further the core is pushed into the coil, the greater the choking
effect and consequently less amperage flow.
A strong magnetic field is also generated and this will tend to draw the core into the
coil, so a locking device is necessary.
Transformer
Machine terminal
Machine terminal
Primary
coils
Secondary
coils
Work piece
Fig 2.13 – Moving core choke transformer
Reactors
Various forms of reactor are used to control welding current. By saturating the
laminated iron core of the reactor with the magnetic flux of direct current, the available
output alternating current is reduced. In DC circuits the current passes through the
reactor prior to being converted to DC in the rectifier section of the power source.
AC
DC
0
AC
Line (AC)
Control
Fig 2.14 – Saturable reactor
48
Metals& Fabrication
Arc Welding 1
Silicon controlled rectifier (SCR)
Modern welding machines use silicon controlled rectifier (SCR) devices to provide a
‘one knob’ output control system. The SCR circuit is fitted into the transformer output
circuit and is an electronic device that can be switched on and off at various points
in the AC cycle. When this is coupled with a feedback circuit the output voltage and
current can be easily controlled. These machines can provide AC and/or DC current
choice and may also offer constant current or constant voltage type output from the
same machine.
Control
knob
+DC
AC
AC
-DC
Fig 2.15 – Silicon control rectifier (SCR)
49
Chapter 2 – Electricity and welding machines
Machine characteristics
Further to classifying welding machines as AC or DC, welding machines are also
classified according to their characteristic output curve. Machines are classified as
either:
•
constant current (CC) – also known as drooping characteristic
•
constant voltage (CV) – also known as constant potential of flat characteristic.
The machine characteristic is often referred to as the slope of the machine, as it can be
seen that the output curve slopes downward.
a) 70
60
V 50
O
L 40
T 30
S
20
10
0
b)
V 45
O
L 30
T
S 15
100
200
AMPS
300
0
100
200 300
AMPS
400
Fig 2.16 – Volt/amp curves (a) constant current machines (CC)
and (b) constant voltage machines (CV)
The curves on the above graphs represent the power output of each type of machine.
They show voltage output at a given load amperage. It can be seen from the output
curves that a change in arc voltage produces a change in the output amperage of the
power source.
Term
arc voltage
Definition
the voltage across the arc between the electrode
and parent metal. This will vary depending on the
length of the arc, type of electrode, etc
Some welding processes (such as GMAW) are intolerant of changes in arc voltage.
Others (such as MMAW or GTAW) are intolerant of fluctuations in amperage. The arc
length (electrical stick-out with GMAW) varies by natural movement of the operator’s
hand during welding, causing resistance across the arc to vary. An increase of arc
length would cause an increase in arc voltage and a decrease in amperage. Shortening
the arc length would have the opposite effect. These fluctuations in amperage
and voltage are controlled by manufacturing machines which have the desired
characteristic curve.
50
Metals& Fabrication
Arc Welding 1
Constant current machines
Constant current machines translate fluctuations in arc length to changes in arc voltage
and permit little change in the output amperage. This is desirable in hand held welding
processes such as MMAW or GTAW, where changes in arc voltage have little effect on
welding, but fluctuations in amperage would make it difficult for the welding operator to
control the welding process.
Constant voltage machines
Constant voltage machines are designed to hold the arc voltage steady, and allow the
amperage to fluctuate with minor variations in arc or stick-out length. In power feed
processes such as GMAW, arc conditions are greatly affected by even small changes
in arc voltage. Therefore changes which would naturally occur in stick-out length (with
the movement of the operator’s hand) are translated to fluctuations in amperage whilst
holding the arc voltage constant. It should be noted that small variations in stick-out
length will produce relatively large changes in amperage.
Variable slope machines
Some machines allow adjustment of the open circuit voltage, however not only does
the OCV change, but the slope (current response) of the machine changes also.
It can be seen in Fig 2.17 that when the maximum OCV is selected, the machine has
an output curve associated with a constant current machine. When the minimum OCV
is selected, a different output curve results. This slope is infinitely variable between the
maximum and the minimum OCV. This type of machine is ideal for applications such
as pipe welding as it gives the operator the ability to control amperage by means of
adjusting the arc length.
Maximum OCV
V
O
L
T
A
G
E
/
V
O
L
T
S
a
50
Minimum 0CV
b
125 A
27 V
32
Arc voltage
22
100
Current amps
200
15
Fig 2.17 – Typical volt-ampere curves possible with a variable voltage power source
The steep curve (a) allows minimum current change
The flatter curve (b) permits the welder to control
current by changing the length of the arc
51
Chapter 2 – Electricity and welding machines
Duty cycle
An essential factor in the performance of any welding machine is the machine duty
cycle.
The duty cycle is the percentage over a five-minute period that the machine can
operate at the rated output amperage.
It is important to realise that:
•
the duty cycle rating may not be at the maximum current output of the
machine
•
semi-automatic and fully automatic processes may require that the machine be
rated at or near 100%
•
if the current required is higher than the amperage at which the machine is rated,
the operating time will have to be reduced
•
welding at an amperage lower than the amperage at which the machine is rated
will enable the operating time to be increased
•
simply reading the maximum output current on the dial of a welding machine is
not a reliable indicator of the machine’s performance capability.
Example: A welding machine is rated at 60% duty cycle at 300 A on the front label.
The maximum amperage output of this machine is 350 A.
At 100% duty cycle the allowable amperage would be 232 A.
Selecting a welding power source
The choice of machine depends on three major factors:
•
the type of work the machine is required to do
•
the operating conditions – field or site work, shop work, available power
•
the type of welding required, with regard to specific operating conditions, for the
least cost.
There are six basic machine types commonly available:
52
•
AC transformers
•
transformer rectifiers
•
inverters
•
motor generators
•
independently driven generators
•
engine driven generators.
Metals& Fabrication
Arc Welding 1
AC transformers
BOC Limited © 2006
Fig 2.18 – AC transformer
These stationary machines require mains current to operate. They are cheap
to buy and maintain and electrically efficient, but offer limited control of the welding
current and restricted electrode choice.
Transformer rectifiers
BOC Limited © 2006
Fig 2.19 – Transformer rectifier
These stationary machines provide AC/DC welding current from an AC main by means
of a rectifier. They offer quiet, efficient operation with virtually no moving parts. These
machines are commonly used for GTAW and GMAW.
53
Chapter 2 – Electricity and welding machines
Inverters
6
7
Operating Controls
1 Negative (–) dinse connection
5
2 Positive (+) dinse connection
4
3 Overload protection indicator
3
8
4 Welding current regulator
5 Main power switch and signal light
6 Carry strap
2
7 process
7 Selector switch for welding
8 Machine body
1
9 Work clamp and cable
10 Electrode holder and cable
10
9
BOC Limited © 2006
Fig 2.20 – Inverter
Inverters also require mains primary current. Compared to other machines of similar
current capacity they are compact, lightweight and provide a smooth DC output. They
are commonly used as MMAW, GMAW and GTAW machines.
AC motor generators
BOC Limited © 2006
Fig 2.21 – AC motor generator
An AC electric motor and a DC generator are built on a common shaft. The AC motor
turns the shaft and direct current is produced in the generator section, and output to the
welding terminals. These machines offer smooth current with a choice of polarity and
OCV. Small machines (typically to 300 A) are commonly used for MMAW and larger
machines are commonly used to provide current for SAW.
54
Metals& Fabrication
Arc Welding 1
Independently driven generator
These machines are normally purchased where a power take off (PTO) is available,
such as those on a truck, tractor or 4WD. A welding current is then available wherever
the host vehicle can go. Often this type of machine has a power pack built in to provide
power for other devices such as lights, drills and grinders.
Fig 2.22 – Independent driven generator
Engine-driven generator
These machines are DC generators or AC alternators with electronic control coupled
to a diesel or petrol internal combustion engine. They are extremely portable, and
are commonly used for site construction work. These machines are equipped with
governors to maintain constant engine speed and idling devices to reduce engine
speed when welding is not in progress. Most machines are water-cooled, but machines
with air-cooled engines are available for light duty use. Initial maintenance costs of
these machines is high. Diesel engines cost more than petrol engines, but are more
economical to run and maintain.
BOC Limited © 2006
Fig 2.23 – Engine-driven alternator
55
Chapter 2 – Electricity and welding machines
Activity
In the workshop:
•
note the various types of welding machines
•
identify the type of current they produce, the OCV and the machine duty cycle
from the front label
•
identify the current control method used by each machine
•
detail the different applications for each welding machine.
Your lecturer can assist you with this activity.
56
Metals& Fabrication
Arc Welding 1
Chapter 3 –
Weld preparation and workmanship
Introduction
There are many types of joints and preparations used in welding, with the correct
selection based on a wide range of factors. Some of these are:
•
welding process being used
•
thickness of the material
•
required strength of the weld
•
accessibility – from each side
•
positioning of the parts.
In this chapter you will look at the following.
•
•
Selection of joint type
ο
fillet welds
ο
butt welds
Points to remember
ο
welding terms and positions.
At the end of the chapter, you will complete an activity.
Selection of joint type
The type of joint depends on three factors:
•
intensity of loading – butt welds are better able to transfer stress, however, when
forces are essentially static, as in buildings for example, fillet welds are usually
the preferred type
•
ease of welding – fillet welds are generally easier to make and require less
operator skill
•
cost – fillet welds are generally cheaper to produce as the cost of weld
preparation for butt welds is often considerable.
Welds may be one of four basic types:
•
fillet weld
•
butt weld
•
pad weld (surfacing)
•
plug and slot weld.
These welds may also be combined to produce compound welds.
Pad and plug welds and slot welds are not commonly used in general fabrication and
will not be considered in this text.
57
57
Chapter 3 – Weld preparation and workmanship
Fillet welds
A fillet weld is a weld approximately triangular in cross-section, lying external to the
planes of the parts being joined.
Parts
Description
1. parent metal
the parts to be joined
2. root
where the parts to be joined are in the closest
proximity
3. face
the exposed surface of the weld
4. toe
where the weld face meets the parent metal
5. depth of fusion
the degree to which the weld penetrates the
parent metal
6. leg length
the distance from the root to the toe
7. actual throat thickness
the distance from the root to the weld face
measured through the centre of the weld
8. design throat thickness
the distance from the root to the hypotenuse
of a triangle lying wholly within the weld (used
for design calculations)
9. reinforcement
the distance between the design throat
thickness and the actual throat thickness
Penetration
Reinforcement
Design throat
thickness
Toe
Root
Leg length
Parent material
Fig 3.2 (a) – Parts of a fillet weld
Fillet weld configuration
The weld configuration relates to the relationship of the plates to be joined. The
joint types may be tee fillet, outside corner or lap and these may be made in various
positions, eg flat, vertical.
Fig 3.2 (b) – Fillet weld configurations
58
Metals& Fabrication
Arc Welding 1
Lap joints
The minimum overlap for parts carrying stress is five times the thickness of the thinner
part joined. Both ends of the lap require welding.
Min. 5t
t
Fig 3.3 – Lap joint overlap
eg
Minimum lap of 2 x 6 mm plates
5 x 6 mm = 30 mm
Minimum lap of 5 mm plate lapped onto 8 mm plate
5 x 5 mm = 25 mm
Fillet weld profile
Three fillet profiles are possible. A perfect fillet weld would be the right size and equal
leg, have an even front face and described as mitre-shaped. An over-welded fillet weld
or a weld that has excess reinforcement and abrupt transition at the toes that may
cause stress concentration, is described as convex. A weld that has a reduced throat
distance compared to a leg size is described as concave (this may cause centerline
cracking or weld failure).
Convex
Mitre
Concave
Fig 3.4 – Fillet weld profile
Ideally, fillet welds should be slightly convex. It should be noted that concave fillet
welds require longer leg lengths to meet the requirements of nominal size.
59
59
Chapter 3 – Weld preparation and workmanship
Fillet weld size
The amount of fillet weld required to obtain the necessary strength may be specified in
one of two ways.
1.
Nominal size – the length of the leg of a triangle which can be inscribed wholly
within the cross-section of the weld and the throat thickness, which must be 0.7
of the leg length. Where a gap exists in the root of the joint, a reduction in the
nominal size may be made.
Fig 3.5 – Nominal size
Where the amount of weld required is specified on an engineering drawing by nominal
size, the length of weld of the required size will be stated.
eg 200 mm of 6 mm fillet
2.
Effective area – the amount of weld required may also be expressed in terms
of effective area. The effective area of a weld is the effective length multiplied by
the design throat thickness (DTT). The effective length (EL) is the length of the
weld which is on the specified size.
DTT
EL
Fig 3.6 – Weld effective area
60
Metals& Fabrication
Arc Welding 1
Example 1
What is the effective area of 400 mm of 8 mm fillet weld?
DTT
effective area
=
0.7 x nominal size
=
0.7 x 8 mm
=
5.6 mm
=
effective length x DTT
=
400 x 5.6
=
2240 x mm2
Example 2
A lifting lug requires 1600 mm2 of fillet weld to provide the necessary strength. What
length of 10 mm2 fillet weld is required?
DTT
=
0.7 x nominal size
=
0.7 x 10 mm
=
7.0 mm
effective area
=
effective length x DTT
effective length
=
effective area ÷ DTT
=
1600 ÷ 7.0
=
228 mm
Therefore, the required length of 10 mm fillet is 228 mm.
Use of the effective area method allows the fabricator flexibility in the welding process,
for example:
If an effective area of 2000 mm2 were specified:
200 mm of 10 mm DTT fillet =
2000 mm2
400 mm of 5 mm DTT fillet
2000 mm2
=
End returns
Welds terminating at the ends or sides of parts of members should, where possible,
be returned around the corners for a distance of not less than twice the nominal size
of the weld, to help prevent cracking. The weld carried around the corner is not taken
into account for purposes of strength calculations. This weld part is counted as the
allowance for start and finish of the weld.
2 x nominal size
Fig 3.7 – End returns
61
61
Chapter 3 – Weld preparation and workmanship
Intermittent fillet welds
There are many applications where the required strength can be achieved without
the need for a continuously welded joint. Where this is the case it is common to
use intermittent fillet welds. There are two types of intermittent fillet welds: chain or
staggered.
Staggered
Chain
Fig 3.8 – Intermittent fillet welds
Any section of intermittent fillet welding shall have an effective length of not less than
four times the weld size with a minimum length of 40 mm.
The clear spacing between the effective lengths of each weld carrying stress shall not
exceed the following number of times the thickness of the thinner part joined.
•
16 times for compression.
•
24 times for tension, and in no case be more than 300 mm.
Chain intermittent welding is preferred to staggered intermittent welding.
Where staggered intermittent welding is used, the welds on each side of the parts
joined shall be continued to the end of the part.
Butt welds
Butt welds are used to join metal products such as sheet, plate, rolled and pressed
sections. This type of joint has the advantage of having high strength without changing
the profile of the structure. Butt welds are better able to transfer stress than fillet welds
and are preferred for live or cyclic loading.
Industrial uses for butt welds include:
62
•
boiler and pressure vessel construction
•
ship building
•
earth moving equipment
•
aircraft and submarines.
Metals& Fabrication
Arc Welding 1
Butt weld terminology
The terminology that applies to the parts of a fillet weld applies equally to butt welds,
with the major difference being design throat thickness, which in a full penetration butt
weld is equal to the plate thickness.
The terms concerned with the preparation for butt welds require explanation at this
stage.
Graphic
Terminology
Description
weld root
the portion of the weld
where the parts to be
joined are in the closest
proximity to each other
root face
that portion of the
prepared edge of a part
to be joined by a butt
weld that has not been
bevelled. This unbevelled
section will support the
first run of weld metal
deposited in the groove
root gap
the separation between
parts to be joined by a
butt weld. The gap is for
the purpose of ensuring,
as far as possible,
complete fusion or
penetration through the
full thickness of metal
angle of bevel
the angle of the prepared
edge of a component
bevelled for welding
included angle
the angle between
the fusion faces of
components prepared for
welding
Weld root
Weld root
Root face
Root gap
Angle of
bevel
Included
angle
63
63
Chapter 3 – Weld preparation and workmanship
throat thickness
the distance from the
root to the weld face
measured through the
centre of the weld
design throat
thickness
in a full penetration butt
weld the design throat
thickness is equal to the
thickness of the thinner
part joined
reinforcement
reinforcement in a butt
weld is the term given to
the metal lying outside
of the planes of the parts
being joined
Throat
thickness
Design
throat
thickness
Reinforcement
Reinforcement
Fig 3.9 – Butt welds
Preparation of plate edges for butt welds
In most cases, especially when joining metal of considerable thickness, it is difficult to
produce satisfactory butt welds unless the edges to be joined are adequately prepared.
Sheet metal and thin plate may be welded without preparation (up to 5 mm thick),
but for metal over 6 mm in thickness the edges must be prepared in such a way as to
provide a ‘V’ or ‘U’ shaped groove in which the weld metal is deposited, so allowing
complete fusion or penetration through the full thickness of the metal.
Failure to properly prepare the edges may lead to the production of faulty welds, as
the correct manipulation of the electrode may be impeded and/or the desired degree of
penetration may not be achieved.
Plates which have been cut by shearing should have all burrs and irregularities
removed before welding.
Plates prepared for welding by oxy-flame cutting techniques should have an even
surface, free from notches or grooves. For this reason machine flame-cut surfaces
are preferred to hand flame-cut surfaces. Imperfections on bevelled edges may be
removed by filing or grinding. Preheating may be required when oxy-flame cutting weld
preparation on hardened steel, particularly if thick.
‘U’ and ‘J’ preparations may be carried out by means of oxy-flame gouging, but usually
such forms of bevelling are prepared by machining the parts.
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Metals& Fabrication
Arc Welding 1
Weld preparation is commonly applied by:
•
shearing
•
grinding
•
machining
•
oxy-flame or plasma cutting
•
arc or oxy-flame gouging.
Butt welds can be either a complete penetration butt weld where fusion exists
through the full thickness of the joint, or an incomplete penetration butt weld where
the depth of the weld is less than the thickness of the plates joined.
At this stage it is only intended to discuss complete penetration butt welds, and even
here the types of butt welds referred to will be the more common types. Additional
information can be gained by referring to Australian Standard® AS/NZS 1554 Structural
steel welding.
Types of butt welds
Butt welds are made between the edges of abutting plates and are generally described
according to the way these edges are prepared. The edge preparation chosen for
a particular type of joint must generally ensure that complete penetration can be
achieved with minimum weld metal and effort, while bearing in mind other relevant
factors such as:
•
the accessibility of the joint to be welded – whether it can be welded from both
sides of the joint or only one
•
the position of the joint to be welded, ie vertical, horizontal, flat.
The type of butt weld selected for a particular job is usually the one which is easiest
and cheapest to make when all other factors have been considered.
Edge preparation and specification
The various types of edge preparation in common use for the welding of steels are as
follows.
•
Closed square butt joint – The edges are not prepared and are fitted together
without a gap. This preparation is suitable for steel up to 3 mm thick and is
welded from both sides.
t = 3max.
Fig 3.10 – Closed square butt
65
65
Chapter 3 – Weld preparation and workmanship
•
Open square butt joint – The edges are not prepared but are separated slightly
to allow fusion through the full thickness of the plate. The gap is equal to half the
plate thickness, to within 1.5 mm. Suitable for steel up to 6 mm in thickness, but
must be welded from both sides.
t = 6max.
G=
t
2
Fig 3.11 – Open square butt
•
Open square butt joint with permanent backing material – This type of joint is
used when welding plates up to 6 mm thick, where welding is possible from one
side only. The gap is equal to the plate thickness. Complete fusion of the weld
into the backing material must be obtained.
G=t
t = 6max.
Fig 3.12 – Open square butt with backing material
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Metals& Fabrication
•
Arc Welding 1
Single vee butt joint – Used on steel up to 12 mm thick and on metal of greater
thickness, where access from both sides is difficult. Where possible the back
of the first run must be cleaned out and the job completed by deposition of a
backing run.
60 - 70
RF = 3max.
G = 2 - 5mm
Fig 3.13 – Single vee butt
•
Single bevel butt joint – Applications for single bevel butt joints are as for single
‘V’ joints described previously.
45
RF = 3 max.
G = 2 - 5mm
Fig 3.14 – Single bevel butt
67
67
Chapter 3 – Weld preparation and workmanship
•
Double vee butt joint – Used on plate 12 mm and over when welding can be
applied from both sides. It allows a reduction in weld metal compared to a single
‘V’ preparation on the same thickness of steel. This type of preparation also tends
to minimise distortion as the weld contraction is equal on each side of the joint.
Not economical on steel over 50 mm thick.
70
RF = 3 max.
G = 2 - 5mm
Fig 3.15 – Double vee butt
•
Double bevel butt joint – Applications for double bevel butt joints are as for double
‘V’ joints described previously.
45
RF = 3 max.
G = 2 - 5mm
Fig 3.16 – Double bevel butt
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Metals& Fabrication
Arc Welding 1
Backing runs
Backing material is used to support the root run of a butt weld, or to provide a sound
weld through the full plate thickness, when access is possible from one side only.
To help reduce weld deposition rates complete penetration butt welds are often welded
from both sides. The back of the first root run should be gouged and/or ground to clean
metal to ensure complete penetration of the other side run.
Backing material
Permanent backing material is known as a backing strip. Temporary backing material is
known as a backing bar.
Backing strips are fused into the weld and should:
•
be no less than 3 mm thick and be of sufficient size to ensure they do not burn
through
•
have weldability not less than that of the parent metal
•
fit as close as possible with a maximum gap between the parent metal and the
backing strip of 1.5 mm.
Points to remember
•
Test welds should be carried out to ensure the suitability of amperage/root face/
gap combinations. Frequent tacks and a consistent gap should be used.
•
For economy, an electrode of the largest possible size should be employed and
where possible welding carried out in the downhand position.
•
Small variations in gap or root face dimensions can significantly effect penetration
and fusion in the root of a joint. Accuracy and consistency of weld preparation
and fit-up is essential.
69
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Chapter 3 – Weld preparation and workmanship
Welding terms and positions
Abbreviation
Weld type
a
pad weld
b
lap weld
c
corner weld
d
plug weld slot weld
e
single vee butt weld
f
double vee butt weld
g
intermittent fillet welds
h
fillet weld
Edge
weld
Flat
position
Vertical
position
Horizontal
position
a
f
e
g
Overhead
position
d
b
h
c
Fig 3.17 – Weld types
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Metals& Fabrication
Terms
1.F
Throat of weld vertical
Weld description
1.F
flat fillet
2.F
horizontal fillet
3.F
vertical fillet
4.F
overhead fillet
2.F
Vertical plate
Axis of weld vertical
Axis of weld
horizontal
Horizontal
plate
Axis of
weld
horizontal
3.F
Arc Welding 1
4.F
Axis of weld
horizontal
Horizontal plate
Vertical
plate
Vertical plate
Fig 3.18 (a) – Fillet weld terms
71
71
Chapter 3 – Weld preparation and workmanship
Terms
1G
Weld description
1.G
flat butt weld
2.G
horizontal butt
3.G
vertical butt
4.G
overhead butt
5.G
fixed pipe axis horizontal
6.G
fixed pipe axis 45 degrees
3G
2G
Pipe shall be
rolled while
welding
Plates and
axis of pipe
horizontal
Plates and
axis of pipe
vertical
Plates vertical
axis of weld vertical
5G
Test
position flat
Test position
horizontal
4G
Plates
horizontal
6G
Pipe shall not be turned
or rolled while welding
45
Fig 3.18 (b) – Butt weld terms
•
The flat position is also referred to as the downhand position.
•
The welding position ‘VERTICAL’ can be ‘VERTICAL UP’ and ‘VERTICAL
DOWN’ and hence application codes make no distinction between the two.
•
A fillet weld where one plate is in the flat position and one plate is in the vertical
position, is commonly referred to as an H/V (horizontal/vertical) fillet.
Activity
Refer to Australian Standard® AS/NZS 1554 Structural steel welding to further
research the various weld preparations that are available for use by the code. Note
how the specifications required for the weld preparations are altered to suit the various
welding processes and for various positions.
72
Metals& Fabrication
Arc Welding 1
Chapter 4 – Air-arc gouging
Introduction
Air-arc gouging has similar applications as those for oxy-flame gouging but the process
differs in its equipment and method.
In this chapter you will look at the following.
•
Air-arc gouging process
•
Air-arc gouging applications
•
Equipment
•
ο
power source
ο
electrode holder
ο
air supply
ο
electrode materials
Air-arc gouging technique.
At the end of the chapter, you will complete an activity.
Air-arc gouging process
The air-arc gouging process removes metal by melting it with the heat of an electric arc
and directing a jet of compressed air to clear away the molten metal. As the process
does not depend on oxidation it may be used for materials which do not oxy-gouge,
such as non-ferrous metals. Further advantages over oxy-flame gouging include faster
operation and a reduced heat-affected zone with less distortion. The advantages are
offset (to some extent) by reduced portability and the need to guard against increased
fumes and long streams of hot sparks. Care needs to be taken that carbon is not in the
prepared area.
Air-arc gouging applications
The applications of air-arc gouging are the same as discussed under oxy-flame
gouging; namely, removing welds and preparing edges. Precise control of the groove is
possible and increased speeds are provided by the air-arc process. As already noted,
materials which do not flame gouge can be successfully gouged by this process.
Additionally air-arc gouging is ideally suited to alloy steels, quenched and tempered
steels, and other metals where the high heat input of flame gouging may prove
damaging.
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Chapter 4 – Air-arc gouging
Equipment
Electrode holder
The gripping jaw of the holder is fitted with a self-aligning rotating head. When the air
valve in the holder is opened, jets of compressed air are emitted parallel to the axis of
the carbon electrode. The self-aligning rotating head permits the torch to be used in
any position and ensures that the air stream is always directed to converge at the arc.
Max 150mm
Min 75mm
Air jets
Air valve
Air flows
Photograph reproduced with permission Lincoln Electric Co (Aust) Pty Ltd
Fig 4.1 – Electrode holder
Air supply
Compressed air may be supplied by shop air, a local compressor or from cylinders,
usually at a pressure of 560 kPa. The air supply hose must have a bore of not less
than 6 mm and be free from restrictions. Although the actual pressure is not critical, it
is important that sufficient air is supplied to ensure a clean and slag free cut or gouged
surface.
Electrode materials
Electrodes are made of a blended mixture of carbon and graphite, bonded together
and enveloped in a thin layer of copper. The copper coating aids electrical conductivity
through the electrode and acts as a stiffener to the carbon, increasing its working life
and reducing radiated heat.
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Metals& Fabrication
Term
electrode
Arc Welding 1
Definition
an electrically conductive structure which transfers
electrons
Electrodes are available in a range of sizes from 4 mm to 12 mm and suit both DC
and AC. The choice will depend on the job application, ie the amount of metal to be
removed, and the equipment available.
A hollow design electrode is also available which delivers the compressed air down the
centre of the electrode.
Air-arc gouging technique
The current setting should conform to the manufacturer’s guide on the electrode
packets, and be sufficient to obtain a smooth and continuous forward movement, but
without overheating and rapid burning of the electrode. Compressed air pressure of at
least 500 kPa is necessary to clear molten metal from the groove. In addition, the flow
of air will tend to cool the electrode and increase its life.
The electrode is gripped in the holder jaws with a forward projection of 75 mm to
150 mm from the air jets. Note that the air jets are underneath the electrode.
Gouging is commenced by turning on the air flow and touch-starting the arc. Once the
groove is started and reaches the correct size, a smooth forward movement is made
along the line of gouge. Groove size is determined by electrode size, speed of travel,
current and electrode angle. The angle between electrode and work is usually 20°,
increasing the angle will deepen the groove. Widening of the groove may be achieved
by sideways movement of the electrode.
A recent design innovation is an apparatus that uses a hollow steel electrode that
consumes oxygen. The gouging action is started by initiating the oxygen flow and then
creating an arc between the electrode and a striking pad, while they are connected to a
suitable battery.
Care must be taken to use the appropriate protective practices and protective
equipment as there is a greater risk of eye and body injury from the intense arc
radiation created by this process. Proper precautions also need to be taken against the
excessive noise created.
75
Chapter 4 – Air-arc gouging
Activity
Setting up for air-arc gouging
In the workshop:
•
identify safe working practices and protective equipment in the workshop
•
select an appropriate power source and parts
•
set machine variables
•
obtain correct electrodes
•
test air supply.
With your lecturer’s assistance, produce a test gouge on a practice piece of material.
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Arc Welding 1
Chapter 5 –
Manual metal arc welding (MMAW)
Introduction
Manual Metal Arc Welding (MMAW) is one the earliest of the arc welding processes,
but has remained popular despite the introduction of newer and more sophisticated
processes. Indeed, this lack of sophistication is one of the major attractions of the
process.
In this chapter you will look at the following.
•
The process
ο
applications
ο
advantages
ο
limitations
•
Equipment
•
MMAW variables
ο
amperage
ο
angle of approach
ο
angle of travel
•
MMAW faults
•
MMAW electrodes
ο
functions of flux coating
ο
types of coating
ο
care of electrodes
ο
electrode deterioration
ο
storage of electrodes
ο
redrying of electrodes
ο
electrode classification.
At the end of the chapter, you will complete an activity.
77
Chapter 5 – Manual metal arc welding (MMAW)
The process
The MMAW process operates as follows.
A low voltage, high amperage current flows to create an arc between the tip of the
electrode and the work piece. This generates the heat for welding and causes the work
piece and the tip of the electrode to melt.
Term
amperage
Definition
a measurement of the amount of electrical current
The flux coating on the electrode decomposes (burns) due to the intense heat of the
arc and generates a gaseous shield, which protects the weld pool and surrounding hot
metal from the atmosphere.
The electrode melts off and is transferred across the arc in the form of droplets. The
molten metal provided by the electrode adds to the molten parent metal and they
become the weld metal when solidified.
Molten electrode flux which is transferred across the arc acts as a scavenger, picking
up impurities from the surface of the parent metal. The slag which forms covers the
weld pool, solidifies, and protects the hot weld metal as it cools.
The flux ingredients provide arc ionisation (the air gap between the tip of the electrode
becomes electrically conductive), enabling the use of alternating current.
Direction of
travel
Electrode flux
Arc column
or
Arc plasma
Solidified slag
Electrode
Shielding gases
Weld metal
Parent metal
Fig 5.1 – Arc welding
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Metals& Fabrication
Arc Welding 1
Applications of the process
Many welding operators have grown up using the MMAW process. This familiarity
and the fact that it is simple to set up and use, makes it first choice for selection in
many instances. The low cost of equipment makes the process accessible to most
people, and MMAW has no special requirements such as external gas shielding or
high-frequency arc initiation. Sound welds are easily produced and the process doesn’t
tend to result in weld defects such as lack of fusion which is common in GMAW.
MMAW is widely used for:
•
structural work
•
pressure vessels
•
piping
•
maintenance welding
•
site construction
•
general fabrication.
Advantages of the process
The advantages of the MMAW is its versatility and the availability of a wide range
of consumables. Set-up time is short, making the process ideal for small jobs, short
production runs, and where the welding is carried out on site.
MMAW offers the following advantages over other welding processes:
•
low capital cost for equipment
•
versatility across a wide range of applications
•
simple, reliable equipment
•
low maintenance of equipment
•
ideal for site work
•
wide operator appeal
•
sound, reliable welds.
Limitations of the process
Although faster than some welding processes, MMAW has lower deposition rates than
many of the newer welding processes that use a higher current. The process has a
low operator duty cycle, with the operator spending a lot of time changing electrodes
and chipping slag. These two factors combine to limit the application of this process,
especially if high production rates are required.
79
Chapter 5 – Manual metal arc welding (MMAW)
Equipment
Equipment for manual metal arc welding consists of the following.
•
Power source – usually a constant-current type output transformer or transformer
rectifier is used, although various other types of power sources, such as
generators or inverters can also be used. The function of the power source is to
supply welding current with sufficient amperage to provide the necessary heat, at
a voltage which is safe to use.
•
Electrode handpiece and lead – to carry current to the arc via the electrode.
•
Work return lead – connects the work piece to the power source thereby
completing the welding circuit. (A closed circuit is necessary for current flow).
Transformer
Electrode
terminal
Work
terminal
Electrode
Electrode
holder
Work return
lead
Electrode
lead
Arc
Work
Fig 5.2 – Manual metal arc welding equipment
MMAW variables
The major variables of the MMAW process are:
•
amperage
•
arc length
•
travel speed
•
angle of approach
•
angle of travel.
Arc voltage is not considered to be a variable in the MMAW process as this is
essentially dependent on the electrode flux type and only varies from around
21–25 volts.
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Metals& Fabrication
Arc Welding 1
Amperage
An increase in amperage will:
•
increase the heat of the welding arc
•
increase fusion and penetration
•
give a higher deposition rate
•
increase arc force
•
enable easier arc starting
•
give a more fluid weld pool
•
increase spatter
•
increase emission of ultra violet radiation.
Correct current
Current too low
Rounded
bead
Low penetration
Trapped
slag
Current too high
Flattened
bead
Excessive
spatter
Undercut
Fig 5.3 – Effects of amperage
81
Chapter 5 – Manual metal arc welding (MMAW)
A decrease in amperage will have the opposite effect.
As the size and thickness of the metal to be welded increases, so the heat required for
fusion increases, necessitating higher amperages. Also the higher the heat input, the
slower the cooling rate of the weld zone. Slow cooling rates are generally desirable
when welding most metals.
A simple equation is: VOLTAGE x AMPERAGE = HEAT
Angle of approach
Another simple rule for welders is that the metal goes were you point the electrode.
Following that rule, it can be seen in the fillet weld example below that to get an even
weld build up, the electrode must be pointed evenly at both plates, ie 45° and the
welding arc is ‘directional’, that is metal transfer is essentially along the line of the
electrode.
Fig 5.4 – Angle of approach
Unless attention is given to the angle of approach, defects such as slag inclusions, lack
of fusion and penetration, and unacceptable weld contours may result.
Angle of travel
The angle of travel is established essentially as a means of keeping the molten weld
pool behind the arc, and preventing the slag from catching up to the electrode and
causing slag inclusions. Although the angle of travel is commonly set at 60–70° many
factors such as amperage, electrode type, and travel speed will determine the actual
angle used. It should be noted however that the angle of travel used should be the
minimum required for slag control, as laying the electrode too flat causes problems
such as poor appearance, excessive spatter, reduced penetration, and a narrow,
convex bead shape.
Direction of travel
Fig 5.5 – Angle of travel
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Metals& Fabrication
Term
slag inclusions
Arc Welding 1
Definition
imperfections of the weld metal or surface caused
by slag (impurities in the molten mix)
Manual metal arc welding faults
The defects commonly encountered in MMAW are:
•
weld cracking
•
porosity
•
slag inclusions
•
lack of fusion
•
insufficient or excessive penetration
•
contour faults
•
undercut
•
excessive spatter
•
stray arcing.
Cracking
Cracking is considered to be a serious weld fault and rarely is any amount of cracking
tolerated.
Cracks may be described depending on how, when and where they occur, eg
longitudinal, transverse, crater, centre line, hot, cold, toe and underbead. Cracks may
occur in either the parent metal, usually as fusion or heat affected zone cracks, or in
the weld metal.
Hot cracking − Usually occurs in metals that are hot, short and/or have high rates of
thermal expansion. Hot cracking most commonly occurs in the weld metal with the
most common examples being longitudinal and crater cracks.
Cold cracking − Most commonly occurs in the base metal adjacent to the fusion zone.
The most common example of this is underbead cracking in hardenable steels.
Crater cracks − These come from hot shrinkage. The crater solidifies around all sides
toward the centre, leading to a high concentration of stress at the centre of the crater.
If the metal lacks ductility, or the hollow crater cannot accommodate the shrinkage,
cracking may result. Crater cracks may, under stress, spread from the crater and lead
to failure of the weldment.
Cracking in MMAW welds on mild steel is generally not a major problem.
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Chapter 5 – Manual metal arc welding (MMAW)
Porosity
Porosity in MMAW welds can be the result of welding on a parent metal that is
susceptible to this condition (such as steel that contains high amounts of dissolved
gases or sulphur). Porosity may also be caused by welding on dirty material or material
contaminated with moisture, oil, paint or grease. The electrode may have been
contaminated, or too much current or too long an arc length used.
Slag inclusions
Slag inclusions in MMAW can occur at the weld root; between weld runs, or on the weld
surface. They generally occur in MMAW as a result of low amperage, poor electrode
manipulation, working on dirty or contaminated metal, or incorrect joint preparation.
Lack of fusion/lack of root penetration
With MMAW, lack of fusion or lack of root penetration is commonly caused by
low amperage, working on dirty or contaminated material or using the wrong joint
configuration, electrode angle or travel rate.
Excessive penetration
Excess weld metal protruding through the root of a butt weld may occur in MMAW
because of incorrect joint preparation, wrong electrode choice, excessive amperage or
incorrect variables.
Contour defects
Contour defects may be in the form of insufficient or excessive leg size, overroll or
overlap, excessive convexity or concavity of the bead, or simply a rough, uneven
appearance.
These are mainly caused by the operator but by using the correct electrode, amperage,
travel speed and electrode angle adjustments, many of these problems can be fixed.
Undercut
Undercut in MMAW is defined as a groove or channel in the parent metal, occurring
continuously or intermittently along the toes or edge of a weld.
Undercut is a common problem in MMAW and can be caused by excessive amperage,
too long an arc length, wrong electrode angles, or wrong travel rate.
Excessive spatter
Although some spatter is a normal part of MMAW, excessive spatter is ugly and difficult
to remove. Some electrode types produce more spatter than others, but generally
excessive spatter is caused by high amperage or too long an arc length.
Stray arcing
Defined as damage to the parent metal resulting from the accidental striking of an arc
away from the weld.
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Metals& Fabrication
Arc Welding 1
Even though stray arcing is not a major problem associated with the MMAW of mild
steel, it is good practice to take precautions against accidental arcing of the electrode
anywhere other than in the weld zone.
Stray arcing can lead to serious weld failure in a material that is crack sensitive, or is
going to be put in a stressed situation.
Manual metal arc welding (MMAW) electrodes
Wire
O
Core wire
Electrode
coating
(flux)
Fig 5.6 – Electrode construction
The manual metal arc welding electrode consists of a core of wire surrounded by a flux
coating. The wire is generally of similar composition to the metal to be welded. The
flux is applied to the wire by the process of extrusion. For welding carbon and low alloy
steels (the metals most commonly fabricated using the MMAW process) electrodes will
have one of four flux types, either:
•
cellulose type coating
•
rutile type coating
•
hydrogen controlled coating (low hydrogen)
•
iron powder type coating.
The flux coatings (from which the electrode types take their name) account for the
major differences between electrode types.
The ingredients of the flux coating are carefully controlled so as to give desirable
running characteristics and weld metal properties.
85
Chapter 5 – Manual metal arc welding (MMAW)
Among these desirable running characteristics are:
•
arc stability
•
ease of striking
•
elimination of porosity
•
minimum spatter
•
elimination of noxious fumes and odours
•
a tough durable coating
•
control of penetration
•
high deposition rates
•
desirable physical and mechanical weld metal properties.
The aforementioned list is by no means exhaustive and many characteristics are
incompatible, eg deep penetration and minimum spatter. Therefore when choosing an
electrode for use, compromises must be made.
The choice of an electrode for a particular application depends upon:
•
the composition of the parent metal
•
the size and thickness of the parent metal
•
the mechanical properties required of the weld metal
•
the physical properties required of the weld metal
•
the welding position
•
the amount of penetration required
•
the amount of spatter allowable
•
available welding current
•
deposition rate required
•
appearance
•
cost
•
slag detachability
•
weld contour and size
•
fluidity of the slag
•
operator appeal.
Functions of the flux coating
In the early days of arc welding, bare wire electrodes were used. The results obtained
from these electrodes left much to be desired. Over the years, electrodes have
improved and flux coatings have evolved to the stage where the deposited weld metal,
in many cases, has better metallurgical properties than the parent metal.
86
Metals& Fabrication
Arc Welding 1
The flux coating of the electrode has many functions. It:
•
provides a gaseous shield to protect the weld from atmospheric contamination
•
provides arc ionisation. This gives a stable arc and enables the use of AC
•
controls the chemical composition and properties of the deposited weld metal
•
controls the deposition rate
•
controls spatter
•
influences the degree of penetration
•
provides slag which performs the following functions:
ο
forms a protective cover over the weld metal to prevent the formation of
oxides while the weld metal is cooling
ο
acts as a scavenger to remove oxides and impurities from the weld
ο
helps to produce the correct bead shape and improve weld appearance
ο
slows the cooling rate of the weld metal
ο
enables ‘positional’ welding.
Some of the ingredients used in flux are:
•
wood pulp (cellulose), titanium dioxide (rutile), limestone, fluorspar, silica, and
feldspar – for producing slag and shielding gas
•
ferro-manganese and ferro-silicon – used as deoxidisers
•
potassium and sodium silicates, used as binders
•
clays and gums – used as binders
•
ferro-chromium, ferro-molybdenum and nickel powder – for alloying
•
iron powder and iron oxide – to increase deposition.
Electrode coating types
Electrodes by coating types
cellulose
(wood pulp base)
rutile
(titanium dioxide)
basic
(calcium carbonate)
rutile plus
iron oxide
rutile plus
iron powder
As shown above, there are five basic electrode coating types used to make electrodes
for the welding of carbon and low alloy steels.
These coating types are then arranged into four basic electrode types or groups, which
are:
•
cellulose
•
rutile
•
hydrogen-controlled
•
iron powder.
87
Chapter 5 – Manual metal arc welding (MMAW)
The general characteristics for each of these groups are as follows.
Cellulose
These electrodes contain a high percentage of alpha flock (wood flour) and 3–7%
moisture in the coating. This provides the fiery, deep-penetrating arc characteristic of
cellulose electrodes. Cellulose electrodes run on low amperages compared to rutile
electrodes (approximately 15%) and the thin, fluid slag does not completely cover the
finished weld deposit. High spatter levels are produced and the weld appearance is
characterised by coarse, uneven ripples.
Application – used for the first (root) run on pipes and plates, welding in the vertical
position (particularly vertical down) and wherever deep penetration is required.
Storage conditions – should contain 3–7% moisture for best results. Do not store in
electrode ovens.
Rutile
These electrodes contain a high proportion of titanium-dioxide and are known as the
general purpose group of electrodes. They are used for general welding of low carbon
steels, and are suitable for use in all welding positions.
Rutile electrodes have a smooth running and stable arc, low to moderate spatter
levels and moderate penetration. Most of them operate on AC or DC and have good
appearance and easy slag detachability.
Rutile electrodes may also have small amounts of iron oxide added to them to give
them a fiery more penetrating arc.
Application – used for general purpose welding on most joint types and weld positions.
Storage conditions – rutile electrodes have no special storage requirements. Storage in
a warm dry place is sufficient.
Electrode diameter
E4312
E4313
E4314
2.6 mm
50−90
50−90
60−110
3.25 mm
90−40
90−140
95−150
4.0 mm
130−190
130−190
140−200
Table 4.1 – Approximate amperages for rutile electrodes
Hydrogen-controlled (low hydrogen)
These electrodes have coatings of calcium carbonate and are designed to produce low
hydrogen levels in the deposited weld metal as a means of minimising cracking in the
heat affected zone. They are characterised by a globular transfer of metal across the
arc, low penetration as a means of minimising weld metal dilution, and fluid slag.
88
Metals& Fabrication
Term
globular transfer
Arc Welding 1
Definition
a type of metal transfer in which molten filler metal
is transferred across the arc in large droplets
Application – hydrogen-controlled electrodes are used for welding high strength steels
and produce tough, ductile weld metal with tensile strengths in excess of 490 N/mm2.
Amperages used are similar to rutile electrodes but they require a minimum of 60 OCV.
Storage conditions – should contain less than 0.2% moisture. They are supplied in
sealed packets or cans to prevent absorption of moisture from the atmosphere. Upon
opening, the electrodes should be transferred to an electrode oven and conditioned at
300 °C for at least one hour before use. Once they have been conditioned (all moisture
is driven off) they should be kept at a minimum temperature of 100 °C. They should be
used ‘hot’ from the oven and not allowed to cool.
Iron powder/iron oxide
These electrodes have coatings which contain a high percentage of iron in the form of
iron powder and/or iron oxide. They are characterised by high deposition rates, smooth
arcs, low spatter, good appearance and excellent slag detachability. The heavy flux
coating necessitates higher amperages be used as compared to other electrode types.
Application – electrodes containing iron powder in the flux coating are commonly
used for structural welding of low carbon steels and are suitable for welding in the flat
position only.
Care and storage of electrodes
The condition of electrodes can seriously affect the quality of the welded joint,
particularly when dealing with alloy and high strength steels.
Types of electrode deterioration
The condition of flux-coated electrodes may deteriorate due to:
•
excessive absorption or loss of moisture
•
mechanical breakage of coverings
•
formation of surface deposits
•
contamination.
Excessive absorption or loss of moisture
During the manufacturing process the coated electrodes are dried to a predetermined
moisture level, giving the optimum welding characteristics for that particular electrode.
Hydrogen-controlled electrodes require the minimum of moisture; on the other hand,
cellulose electrodes require up to 7% moisture.
89
Chapter 5 – Manual metal arc welding (MMAW)
The absorption of excessive moisture by the coating, either from the atmosphere,
condensation, or from other sources, can cause:
•
weld metal porosity
•
excessive spatter
•
arc instability
•
poor weld contour
•
undercut
•
difficulty with slag removal
•
blistering of the flux coating, especially with cellulose types
•
increased risk of lamellar tearing
•
increased risk of hydrogen induced cracking.
Term
lamellar tearing
Definition
damage or tearing of layers
Mechanical breakage of coverings
Coated electrodes are reasonably robust but the covering can be damaged by rough
careless handling or by excessive bending. Loss of covering leads to erratic arcing,
and inadequate protection of the molten weld metal. For this reason, it is good practice
to discard electrodes with mechanically damaged coverings.
Formation of surface deposits
Electrodes that have been kept for long periods of time in non-ideal storage conditions,
usually form a white powdery deposit on the flux coating. This deposit is produced by
a chemical reaction between the carbon dioxide in the atmosphere and the sodium
silicate of the flux binder. This reaction forms crystals of sodium carbonate and silica
powder. If there are heavy deposits on the covering it is possible that rusting of the
core wire has occurred, which may lead to hydrogen-induced cracking. Heavy surface
deposits indicate that redrying of the electrodes is required.
Contamination
The coating of electrodes can become contaminated by oil, grease, paint and other
fluids through bad handling or storage practices. Some contaminants, such as paint,
may introduce undesirable material into the weld and others may interfere with the
welding process. Oil, for instance, is also a source of hydrogen and may lead to
hydrogen-induced cracking.
Recommended practices
Deterioration of the types described above can be prevented or sometimes corrected
by adopting good practices in packaging, handling, transport and storage.
90
Metals& Fabrication
Arc Welding 1
Storage of electrodes
Electrodes are supplied in sealed packets or cans to prevent absorption of moisture.
They should be stored in a moisture-free environment that has a fairly even
temperature. Electrode packets, cans, and bulk packs should not be opened until
required for use.
Once the electrode container is opened the following procedure should be adopted.
•
Mild steel electrodes should be stored in a warm dry place.
•
Cellulose electrodes must not be stored in an electrode oven.
•
Hydrogen controlled electrodes should be conditioned and stored in an electrode
oven at a minimum temperature of 100 °C.
When retrieving electrodes from storage they should be used in order of receipt. This
method will ensure that electrodes do not remain in storage for any length of time.
Redrying of electrodes
Redrying of electrodes when their moisture content exceeds the recommended range
should be carried out in accordance with the manufacturer’s specifications. The
manufacturer provides guidance in this area.
•
Electrodes (other than hydrogen controlled) that are affected by excessive
moisture content can be redried at 120 °C for approximately one hour.
•
Hydrogen controlled electrodes that are affected by excessive moisture content
can be redried at 400 °C for half to one hour’s duration. If facilities to carry out
this procedure are not available then drying for a minimum of one hour at 250 °C
will do for most applications. Note: the redrying or reconditioning of hydrogen
controlled electrodes is not recommended for critical welds.
Electrode classification
MMAW electrodes are classified under the Australian Standard® AS/NZS 4855
Covered electrodes for manual metal arc welding of non-alloy and fine grain steels
– Classification.
This standard deals with the manufacture, testing, marking and packaging, and
classification of covered electrodes for manual metal arc welding.
The classification system of the code provides a mechanism for identification of the
various electrodes; their description, characteristics and applications.
91
Chapter 5 – Manual metal arc welding (MMAW)
Australian Standard® AS/NZS 4855
The Classification system consists of the following, as per ISO 2560B.
E 49 16
ο
Letter – denoting electrode
ο
First two digits – represent
approximately one-tenth the minimum
tensile strength of the deposited weld
metal (shown in megapascals) in two
groupings nominally referred to as
ο
Second two digits – indicate the flux
type, the welding position(s) in which
the electrode is capable of making
satisfactory welds and the welding
current to be used
E43XX and E49XX
(43 = 430 N/mm2, 49 = 490 N/mm2)
Fig 5.7 – Electrode classification to AS/NZS 4855
EXX10 and EXX11 electrodes (E4310, E4311, E4910)
Electrodes of EXX10 and EXX11 classification have thin coatings which contain at least
15% cellulose and up to 30% titania as rutile or titanium white.
Cellulose electrodes operate with a forceful, deeply penetrating spray type arc
with fairly high spatter. As a result of the decomposition of the cellulose material, a
voluminous gas shield is formed around the arc region, protecting the weld metal from
atmospheric contamination. The slag is very fluid, thin, friable and easily removed when
cold, but may not appear to completely cover the deposit.
Table 4.2 shows a brief summary of the electrode types covered in Australian
Standard® AS/NZS 4855.
92
Metals& Fabrication
Symbol
Type of covering
Arc Welding 1
Welding
a
positions
Type of
current
03
Rutile basic
Allb
a.c. and d.c. (±)
10
Cellulosic
All
d.c. (+)
11
Cellulosic
All
a.c. and d.c. (+)
12
Rutile
Allb
a.c. and d.c. (−)
13
Rutile
Allb
a.c. and d.c. (±)
14
Rutile + iron powder
Allb
a.c. and d.c. (±)
15
Basic
Allb
d.c. (+)
16
Basic
Allb
a.c. and d.c. (+)
18
Basic + iron powder
Allb
a.c. and d.c. (+)
19
Ilmenite
Allb
a.c. and d.c. (±)
20
Iron oxide
PA, PB
a.c. and d.c. (−)
24
Rutile + iron powder
PA, PB
a.c. and d.c. (±)
27
Iron oxide +iron
powder
PA, PB
a.c. and d.c. (−)
28
Basic + iron powder
PA, PB, PC
a.c. and d.c. (+)
40
Not specified
48
Basic
99
As described by
manufacturer
manufacturer’s recommendations
All
a.c. and d.c. (+)
As specified by
manufacturer
As specified by
manufacturer
NOTE: A description of the characteristics of each of the types of covering is given in
annex C.
a
Positions are defined in ISO 6947. PA = flat, PB = horizontal vertical fillet, PC =
horizontal, PG = vertical down.
b
All positions may or may not include vertical down welding. This shall be specified in
the manufacturer’s trade literature.
Table 4.2 – Welding position, current and covering type
Term
friable
Definition
easily broken into small fragments or reduced to a
powder
93
Chapter 5 – Manual metal arc welding (MMAW)
These electrodes are readily used in all positions and are suitable for all types of
welding on low-carbon steel. Special applications recommended for these electrodes
involve changes in welding position during the running of the electrode (eg pipe
welding in situ). Sizes larger than 5 mm are not generally used in all positions.
For optimum performance, the coating of these electrodes must contain 3–7%
moisture. Operating characteristics will be adversely affected if excessive drying
occurs.
Owing to the burn out of the coating and high spatter loss, maximum current values are
limited. However, with current values near the maximum these electrodes can be used
for deep penetrating welds in the flat and horizontal positions (eg square butt joints).
EXX11 electrodes can be used with AC or DC current. EXX10 electrodes can only be
used with DC. When operating on DC current, positive polarity is preferred.
EXX12 electrodes (E4312)
EXX12 electrodes have thin coatings containing a high proportion of titania as rutile,
titanium white or ilmenite.
These electrodes are designed to operate from AC or DC power sources. Electrode
negative is the preferred polarity with DC current. The arc is usually stable at low open
circuit voltages.
The electrodes have a fairly viscous, full-covering slag that is easily removed when
cold, except perhaps from the first run of a deep vee. The arc is quiet, medium
penetrating and with low spatter.
These electrodes are recommended for general purpose use with structural
fabrications and sheet steels. Due to the viscosity of the slag, some of these electrodes
are suitable for vertical down welding.
EXX13 electrodes (E4313)
EXX13 electrodes have thin coatings containing a high proportion of titania as rutile,
titanium white or ilmenite with the addition of basic materials to increase the fluidity of
the slag.
These electrodes demonstrate the same arc characteristics as EXX12 electrodes and
can be operated from AC or DC power sources. Electrode negative is the preferred
polarity with DC current.
Due to the fluid slag that the EXX13 electrode produces (more fluid than other types of
rutile electrodes) the EXX13 electrodes are more suitable for welding in the vertical up
or overhead positions, and are unsuitable for welding vertical down.
EXX14 electrodes (E4314)
EXX14 electrodes have medium-thick coatings containing a high proportion of titania
white or ilmenite, and sufficient iron powder to give metal recovery rates 105–130% of
the mass of the core wire melted.
The slag is fairly viscous, full covering and easily removed when cool. It is sometimes
self-releasing. The arc is medium penetrating and with low spatter.
These electrodes are successfully operated from AC or DC power sources including
those with low open circuit voltages. Electrode negative is the preferred polarity when
welding with DC current.
94
Metals& Fabrication
Arc Welding 1
Due to the medium-thick coating containing iron powder, operating characteristics
are improved allowing touch welding to be carried out. Slag is not excessive and
these types of electrodes are recommended for use in general shop and structural
fabrication.
EXX15 and EXX16 electrodes (E4915, E4916)
EXX15 and EXX16 electrode classifications have coatings containing a high proportion
of basic material such as limestone, and fluorides such as fluorspar. The coating
ingredients are specially selected for low-hydrogen content and during manufacture the
electrodes are baked at high temperatures to remove moisture.
EXX15 electrodes are designed to operate from DC power sources only. EXX16 can be
operated satisfactorily on AC or DC, with electrode positive being the preferred polarity.
The arc is quiet, medium to low penetrating with globular transfer of metal from the
electrode to the weld pool and produces moderate spatter. The slag is very fluid, full
covering and easily removed when cool.
These electrodes are particularly recommended for steels affected by underbead
cracking. The virtual elimination of hydrogen from the arc atmosphere reduces the
possibility of the defect occurring in difficult to weld steels such as medium and high
carbon steels and low-alloy high tensile steels. Tough, ductile welds are produced with
these electrodes and by keeping the hydrogen content low, preheat and post-heat
temperatures can be reduced. Other uses include the welding of highly restrained joints
in heavy sections, as the tendency for weld metal cracking is reduced, and the welding
of free machining (high sulphur content) steels, as well as malleable cast iron.
It is recommended that as short an arc as possible be maintained in all positions
of welding to prevent porosity, and that the electrode be used in a properly dried
condition.
Term
underbead cracking
Definition
cracking in the heat affected zone, not extending to
the surface of the base material
EXX18 electrodes (E4918)
EXX18 electrodes have medium-thick coatings containing a high proportion of basic
material such as limestone, fluorides such as fluorspar and sufficient iron powder
to give metal recovery rates of 105–130% of the mass of the core wire melted.
Manufacture of these electrodes is very similar to that of the EXX15 and EXX16
electrodes ensuring low hydrogen content.
Deposition rates are higher than with EXX15 and EXX16, owing to the iron
powder content, and the extra thickness of the coating allows a higher current per
corresponding core wire diameter to be used.
They are suitable for use with AC or DC, with electrode positive being the preferred
polarity.
95
Chapter 5 – Manual metal arc welding (MMAW)
EXX19 electrodes
Electrodes of EXX19 classification have coatings based on the mineral ilmenite and
consequently have an arc action and slag characteristics between the EXX1 2/13
titania types and the EXX20 iron-manganese oxide type.
The electrodes are characterised by a rather fluid slag. They provide deeper
penetration than the EXX13 group and excellent radiographic quality weld metal. They
are designed for use on AC or DC electrode negative or positive, and are suitable
for multi-pass welding steel up to 25 mm thick. Stable arc and good operational
characteristics provide smooth even beads in all positions including the vertical
(using the upward progression only). The weld metal has excellent ductility and crack
resistance with good impact properties.
EXX20 electrodes (E4920)
These electrodes have medium-thick coatings containing a high proportion of oxides
and/or silicates of iron and manganese.
Using either AC or DC power sources, a spray type arc is produced with medium
to deep penetration according to the current being used. The slag is voluminous,
completely covers the deposit and is honeycombed on the underside. The slag is easily
removed, even from the first run of a deep groove.
These electrodes are principally used for horizontal fillet and flat butt welds in heavy
carbon steel plate where good penetration is required.
EXX24 electrodes (E4924)
EXX24 electrodes have thick coatings containing a high proportion of titania as rutile,
titanium white or ilmenite, and sufficient iron powder to give metal recovery rates in
excess of 130% of the mass of the core wire melted.
Using AC or DC power sources (negative polarity preferred), these electrodes operate
with a low to medium penetrating smooth spray type arc with very low spatter. The slag
is fluid, full covering and dense, and when cool is self-releasing or easily removed.
Owing to the high iron powder content and increased coating thickness, high currents
are required.
These electrodes are recommended for the high-speed welding of low carbon steel in
the flat and horizontal positions. Touch welding technique is normally used.
EXX27 electrodes (E4927)
EXX27 electrodes have thick coatings containing a high proportion of oxides and/or
silicates or iron and manganese, and sufficient iron powder to give metal recovery rates
in excess of 130% of the mass of core wire melted.
They are similar to the EXX20 electrodes but contain iron powder to increase
deposition rates. They demonstrate similar arc characteristics and can be used with AC
or DC current. Electrode negative is the preferred polarity.
Recommended usage of these electrodes is in the flat and horizontal fillet positions and
they are particularly applicable to high speed welding of low-carbon steel where good
penetration and ease of deslagging are required. Touch welding techniques are usually
employed.
96
Metals& Fabrication
Arc Welding 1
EXX28 electrodes (E4928)
EXX28 electrodes have thick coatings containing a high proportion of basic material
such as limestone, fluorides such as fluorspar, and sufficient iron powder to give metal
recovery rates in excess of 130% of the mass of core wire melted.
They operate with a medium penetrating spray type arc and low spatter. The slag is
fluid, full covering and easily removed. Power sources can be either AC or DC, with
electrode positive being preferred.
These electrodes are restricted to use in flat and horizontal positions, and are generally
used where large amounts of low-hydrogen weld metal in heavy sections is required.
Touch welding techniques are usually used.
As with all hydrogen controlled electrodes, it is important to maintain a close arc to
reduce the possibility of porosity, and that the electrodes used are properly dried.
EXX48 electrodes
EXX48 electrodes demonstrate the same usability, composition, and design
characteristics as EXX18. In addition these electrodes are specially designed for
vertical down welding. Some electrodes of this type are designed to provide a flat to
slightly concave, fully-loaded penetration bead without undercut on single vee welding,
such as in piping and pipelines.
EXX99 electrodes
The coating and running characteristics of electrodes in this classification are such that
one or more features prevent their classification in any of the preceding classes.
97
Chapter 5 – Manual metal arc welding (MMAW)
An example of an electrode of this electrode of this classification is shown below.
E 55 16 - N 3
ο
Letter – denoting electrode
ο
Numeral – indication of mass
percentage of alloy addition
ο
First two digits – represent
approximately one-tenth the minimum
tensile strength of the deposited weld
metal (shown in N/mm2)
ο
Suffix letter – denotes alloy content
This number shows a high tensile
strength.
Importantly, the end of this classification contains the following.
•
A suffix – the suffix consists of a letter (or letters) followed by a numeral. The
letters denote the type of weld metal which the electrode deposits.
eg: N3 = Ni
mass % 1, 5
•
98
Numerals – the numerals are used to give an indication of the percentage of
alloy addition. As the percentage increases, so does the magnitude of the suffix
numeral. The numerals range from one through five.
Metals& Fabrication
Arc Welding 1
Activity
In the workshop:
•
identify safe working practices and protective equipment in the workshop
•
identify a suitable MMAW welding plant
•
with the mains power switch OFF, set up the MMAW plant (start by checking the
electrical cable connections to the work lead and electrode handpiece)
•
fit a 3.2 mm general purpose electrode into the electrode holder and adjust the
amperage to suit
•
with your lecturer’s assistance, strike an arc between the electrode and scrap
work piece
•
with your lecturer’s assistance, produce a weld on a practice piece of material.
Further information and understanding of the covered electrode classification
system can be obtained by consulting Australian Standard® AS/NZS 4855 Welding
consumables – Covered electrodes for manual metal arc welding of non-alloy and fine
grain steels – Classification.
99
100
Metals& Fabrication
Arc Welding 1
Chapter 6 –
Gas tungsten arc welding (GTAW)
Introduction
Gas tungsten arc welding (GTAW) has increased in popularity because of the relative
ease with which it can be applied to difficult weld materials (notably aluminium and
stainless steel), and the increased use of these materials in various industries. Thin
metals, out of position work, and automatic applications are well within the scope of
the GTAW process, and it is in these areas that it excels. Welds produced are of high
quality in terms of both soundness and appearance.
In this chapter you will look at the following.
•
•
•
The process
ο
applications
ο
advantages
ο
limitations
Equipment
ο
power source
ο
shielding gases
ο
welding torches
ο
gas nozzles
ο
electrodes
Gas tungsten arc welding techniques
ο
starting the arc
ο
arc wander
ο
butt welds
ο
fillet welds
ο
pipe welding
ο
weld backup.
At the end of the chapter, you will complete an activity.
The process
The GTAW process operates as follows.
The gas tungsten arc process employs an electric arc created between a hand-held
non-consumable tungsten electrode and the work piece, to melt the parent metal and
provide the heat required for fusion. Under normal conditions the tungsten electrode
does not melt and become part of the weld. A separate inert gas shield is introduced
around the arc zone to exclude the atmosphere and its undesirable effects. Additional
weld metal may or may not be required, and is added by dipping compatible filler rods
into the weld pool.
101
Chapter 6 – Gas tungsten arc welding (GTAW)
ACHF, DCSP
or DCRP
power supply
Gas cup.
Either ceramic or
water-cooled metal
l
ve
no
ctio
Tungsten electrode
(virtually
non-consumable)
e
Dir
a
f tr
Welds made with
or without addition
of filler metal
Many joint designs
in all positions
can be welded
easily
Weld surface
smooth & clean
No flux
required
Base metal
can be any
commercial
metal
Weld possesses good
quality, little or no loss
of alloying elements
Inert gas
shields electrode
and weld puddle.
Highly
concentrated heat
Work return
lead
Fig 6.1 – Gas tungsten arc welding
Applications
GTAW has a wide range of applications, particularly its use on ferrous materials (such
as plain carbon, carbon-manganese and alloy steels), and non-ferrous metals (such
as aluminium and its alloys) and also copper and copper-based alloys (such as brass
and bronze). It can be used to weld a wide variety of metal thicknesses in all types of
applications, including:
102
•
general engineering applications
•
transport industries
•
sheet metal industries
•
marine and transport industries
•
boiler and pipe welding.
Metals& Fabrication
Arc Welding 1
Advantages
Some of the greatest advantages of the GTAW process are as follows.
•
Its open arc, which means the weld zone is highly visible to the operator and thus
gives greater control of the weld pool and fusion zone.
•
The arc heat is highly concentrated and there is virtually no sparks, spatter or
fumes.
•
The process operates in an inert atmosphere and therefore does not produce any
adverse effects on the weld or weld area.
•
High quality welds with a good visual appearance can be produced easily.
•
That no flux is required, therefore no slag is produced, saving time required for
post-weld clean-up.
•
That the process has a wide range of applications (nearly all the ferrous and nonferrous materials, together with some of the more exotic materials such as nickel
and titanium).
Limitations
Some limitations of the GTAW process are as follows.
•
Equipment is relatively expensive and to make full use of the process a high
degree of skill is required from the operator.
•
The process is not suitable for use on dirty material and does not like a windy
environment.
•
It is not really suitable for thicker sections or high productivity work, although it
can be mechanised to improve quality and efficiency.
•
It has more intense arc radiation and fume safety hazards, dependent on the
material being welded.
Equipment
Power source
Gas tungsten arc welding power sources can be obtained to operate on domestic or
industrial mains supply voltages. Most industrial machines operate on a 440 volt supply
and provide current in the range 200–500 amperes with a 60 per cent duty cycle.
Any AC or DC manual metal arc welding machine (constant current) can be used to
supply the current for GTAW. It is important, however, that the machine has a good
current control for low amperages in order to maintain a steady arc when welding
thin material. When using DC for welding, a high-frequency unit is desirable but not
essential. With AC a high-frequency unit is definitely required. This will be discussed
later. The ideal power source for GTAW is one that has been specially designed for the
process (Fig 6.2).
103
Chapter 6 – Gas tungsten arc welding (GTAW)
These welding machines are typically transformer rectifiers or inverters which supply
both AC and DC and have a high-frequency unit incorporated in them. They usually
have other controls peculiar to the GTAW process, such as the following.
•
Remote current control – usually foot operated which enables the welder to alter
the amperage whilst welding.
•
Soft start switch – which reduces the current when starting the arc. This is an
advantage when welding aluminium or magnesium.
•
High-frequency spark intensity control – which is useful when welding aluminium
and magnesium.
•
Pre-gas timer – to allow the gas to flow before the arc is started and a post gas
timer which allow the gas (and water if used) to flow for a set time after the arc
is extinguished. This prevents atmospheric contamination of the weld pool and
assists in tungsten electrode cooling.
A better GTAW power source will provide the ability to control upslope and down slope
on the main current as well as a pulsing option.
Cooling water
supply
Gas
supply
Power source
Torch
Filler
metal
Water
drain
Base
metal
Gas
Work
lead
Foot pedal
(optional)
Electrode lead
Fig 6.2 – Equipment used in gas tungsten arc welding
Current type is important in GTAW and the choice depends mainly on the metal to be
welded and its thickness, which in turn decides the current level required.
Choice of current
With GTAW the operator has the choice of three types of welding current:
104
•
DC (-)
electrode negative
•
DC (+)
electrode positive
•
AC (hf)
AC with superimposed high-frequency.
Metals& Fabrication
Arc Welding 1
DC electrode negative
In the GTAW process two-thirds of the heat generated at the arc occurs at the positive
terminal and one-third of the heat at the negative terminal. Therefore it is beneficial,
whenever possible, to connect the tungsten electrode to the negative terminal since
higher amperages can be used without the tungsten becoming overheated. Also,
because most of the heat is concentrated in the parent metal, deeper penetration is
obtained.
With electrode negative the flow of electrons is from the tungsten electrode to the
parent metal (from negative to positive). The shielding gas, as it passes through the
arc, becomes electrically charged (ionised) and the ions of gas which are positively
charged are attracted to the negative electrode (Fig 6.3). No cleaning action occurs
with this polarity; it is only needed when welding metals with a high melting-point
surface oxide. Electrode negative is preferred for most of the common fabrication
metals (except aluminium). GTAW with electrode negative produces deep penetration
because it concentrates the heat in the joint area.
DC welding
power supply
1
3 heat at
electrode
Surface
oxide
Electrons
Gas ions
Deep
penetration
2 Heat at work
3
Fig 6.3 – Direct current electrode negative
DC electrode positive
With electrode positive, the gas ions are still positively charged but are now attracted
to the negative parent metal. They bombard the plate surface causing any oxide on
the plate surface to be chipped away, exposing bare metal which is easily melted. This
cleaning action is most useful when metals with a high melting-point surface oxide
have to be welded, eg aluminium, magnesium and titanium.
With electrode positive, the bulk of the heat is now concentrated at the tungsten
electrode which can become overheated unless a sufficiently large electrode diameter
is used. The penetration is wide and shallow and the arc tends to be erratic due to the
large electrode and relatively low amperage being used (Fig 6.4). Generally, electrode
positive is not recommended for GTAW.
GTAW with electrode positive produces good cleaning action as the argon ions flowing
towards the work strike with sufficient force to break up oxides on the surface of the
material. Since the electrons flowing towards the electrode cause a heating effect at
the electrode, weld penetration is shallow.
105
Chapter 6 – Gas tungsten arc welding (GTAW)
DC welding
power supply
2
3 heat at
electrode
Surface
oxide
Electrons
Gas ions
Shallow
penetration
1 Heat at work
3
Fig 6.4 – Direct current electrode positive
Alternating current
The ideal type of welding current for metals with a high melting-point surface oxide,
is one which gives good cleaning action during the electrode positive cycle and
deep penetration, cooler electrode of the electrode negative cycle. AC is actually a
combination of electrode negative and electrode positive. One half of the cycle is
negative and the other half positive (Fig 6.5). The heat is equally distributed at the
electrode and work piece.
1 cycle
DC electrode positive
zero line
DC electrode negative
Fig 6.5 – AC cycle
Unfortunately, the strong surface oxide on metals (such as aluminium) prevents the full
flow of current in the positive polarity direction of the cycle, causing the arc to become
unstable. Also, as the cycle passes through the zero voltage point, the arc goes out
and must re-ignite. To prevent instability or complete loss of the arc, a continuous highfrequency spark is required. The high-frequency current is able to
jump the gap between the electrode and the parent metal during the period of arc
shut down, and penetrate the oxide film to form a path for the welding current to
follow. Continuous high-frequency is needed with AC, so this type of current is usually
identified as AC(HF) (Fig 6.6).
106
Metals& Fabrication
Arc Welding 1
AC welding
power supply
1
2 heat at
electrode
Electrode
work
1 Heat at work
2
Fig 6.6 – Alternating current high-frequency
The electrode diameter required for a given amperage will vary depending on the
current type being used. A 1.6 mm tungsten electrode on DC electrode negative will
carry the same current as a 2.4 mm tungsten electrode on AC (hf), while a 3.0 mm
electrode would be required to carry the same current on DC positive.
Electrode cooling is provided by the torch through the copper collet, gas diffuser and
torch body.
Direct current electrode negative is the most common type of current used for welding
materials such as mild steel, stainless steel and alloy steels. Direct current electrode
negative is used also to obtain narrow, deep penetrating welds.
Direct current electrode positive may be applied to welding very thin aluminium and
magnesium parts, but is not commonly used because a large diameter electrode is
required to carry low current and the arc may be unstable.
Alternating current, with a superimposed high-frequency current, is most commonly
used for aluminium and magnesium as it combines good oxide clearing when the
electrode is positive, with good penetration when the electrode is negative.
Pulsed current
Pulsed current is also available on some GTAW equipment. The welding current is set
to fluctuate between a high fusion current level and a low solidification or background
current level; both of which are adjustable, as is the time for which each current level
is effective. The number of pulses can be varied from ten per second down to one per
second.
Pulsed current, which may be AC or DC, is particularly useful for welding very thin
materials, providing good penetration during the high cycle with cooling of the molten
pool during the low cycle. In effect, pulsed current produces a series of spot welds,
penetration is good, distortion is minimised, and control is improved for difficult welding
situations involving thin materials and positional welds.
107
High
pulse
time
Low
pulse
time
Cycle time
Per cent
weld
current
C
U
R
R
E
N
T
Welding current
Chapter 6 – Gas tungsten arc welding (GTAW)
TIME
TIME
Fig 6.7 – Pulsed current
Shielding gases
Generally an inert gas is used as the shielding medium to protect the weld zone from
contamination by the atmosphere. Argon is the inert gas most commonly used in
Australia. It is preferred to helium because of its lower cost and its general suitability
for a wide variety of metals. Argon is an electron carrier and also exhibits better oxide
removal characteristics than helium and aids the welding operation, as heat input to the
weld puddle is less affected by variations in arc length. On the other hand, helium as
an insulator gas provides higher arc voltages and greater heat input which increases
penetration and travel speeds.
Term
Definition
argon
a colourless, odourless inert gas
inert
having only a limited ability to react chemically;
chemically inactive
Mixtures of the two gases, in some special applications, and also other brews
(or combinations of gases) will prove advantageous, particularly in mechanised
applications. Those seeking further information should contact the local supplier.
Fixed pressure reduction regulators are used to supply gas to the torch, together with
a flow meter to give a precise indication of the gas flow rate being used. Gas flows are
adjusted between five and 14 litres per minute to suit the particular application.
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Arc Welding 1
Welding torches
Handheld GTAW welding torches may be air cooled for low to medium amperage
applications (these are also gas cooled by the gas supply). Water cooled torches
are required for industrial operations involving higher amperages and longer welding
periods. The electrode is held by a collet in a collet body/gas diffuser which allows the
removal and setting of the electrode in relation to the torch nozzle, or gas shroud.
Projection of the electrode should not be excessive as it may touch the work and
contaminate the electrode. Minimum projection of the electrode, normally 2 mm to
5 mm, will provide good welding conditions and satisfactory gas coverage of the
electrode and work. The collet is tightened by screwing in the torch back cap which
also provides insulation for the electrode.
Term
Definition
collet
grips electrode and passes electric current to
tungsten electrode
gas diffuser
fits into torch and distributes gas flow evenly (also
retains electrode collet)
Nozzle
Insulating
ring
Collet
Back
cap
Collet
body/
gas diffuser
Handle
BOC Limited © 2006
Fig 6.8 – Welding torch parts
109
Chapter 6 – Gas tungsten arc welding (GTAW)
A control for gas flow is located on the torch and this may incorporate a current on/off
switch. Some equipment allows gas to flow for short periods before the arc is struck
(pre-purge) and after welding current is switched off (post-purge), which provides gas
coverage of the electrode and work as they cool.
Term
Definition
pre-purge
flow of gas before the arc starts to ensure shielding
gas is around arc zone and to facilitate arc starting
post-purge
flow of gas after the arc is stopped to allow time for
tungsten electrode and weld area to cool
Gas nozzles
Gas nozzles or gas cups are used to protect the tungsten electrode from the
atmospheric gases and to deliver the shielding gas to the weld area. They may be
made from cheap alumina-type material, ceramic material or even metal or fused silica
(glass). The gas cup is available in a variety of sizes ranging from 8 mm to 20 mm.
The general rule for the gas cup size is four to six times the electrode diameter. This
may be altered however depending on the joint type and material being welded. For
example, an outside corner weld may require a larger gas cup size to give more
shielding, while an inside corner can be achieved easily with a small gas cup because
the gas will be trapped in the corner. Typically, aluminium or stainless steel may also
need a one size larger cup to give better gas coverage.
Electrodes
Different types of tungsten electrodes are available and provide a comprehensive
range for specific applications. Tungsten electrodes are identified by their tip colour.
This colour tip should be preserved, as identification of a tungsten electrode that has
lost its code can be difficult.
Pure tungsten electrodes (green tip)
Tungsten has the highest melting point of all metals – typically 3400 °C for pure
tungsten. These electrodes are recommended chiefly for use with balanced wave
alternating current power sources on the welding of aluminium, where other electrode
types are not generally used due to their emission characteristics. When used with
standard power sources, pure tungsten electrodes provide good stability with direct
current and high-frequency, stabilised alternating current with argon, helium or a
mixture of both as a shielding gas.
Pure tungsten electrodes have a lower current carrying capacity and poorer arc
starting characteristics than other electrodes, but have a reasonably good resistance to
contamination and maintain a clean balled end (which is preferred for aluminium and
magnesium welding). They are a general purpose electrode for less critical work.
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Metals& Fabrication
Arc Welding 1
2% Thoriated tungsten electrodes (red tip)
These electrodes contain one to two per cent thorium as an alloy and this gives the
electrode a greater ability to resist transfer across the arc and thus help to maintain
the sharpened point (when used chiefly for direct current electrode negative). This
is because they offer increased life, compared with pure tungsten type, due to their
higher electron emission. They have better arc starting, particularly at low open circuit
voltages, and good arc stability. The thoriated tungsten range of electrodes have a
higher current carrying capacity and greater resistance to weld pool contamination.
Thoriated tungsten electrodes are generally used when DC electrode negative is
selected for welding of ferrous materials and alloys such as mild steel, alloy steel and
stainless steel. They may be used on high-frequency, stabilised alternating current
work, but there can be difficulty maintaining the satisfactory balled end required for
good arc stability (when welding aluminium and magnesium). This condition frequently
produces arc wander and tungsten emission, resulting in contamination of the weld
metal.
0.8% Zirconiated tungsten electrodes (white tip)
These electrodes are treated with zirconium and are preferred for applications where
tungsten contamination of the weld metal must be minimised. They are recommended
for use with high-frequency, stabilised alternating current for the welding of aluminium
and magnesium due to the fact that they retain a clean balled end during welding and
have a high resistance to contamination. They have a longer life and higher current
carrying capacity than that of pure tungsten electrodes.
In recent studies related to health issues for welding operators, the thoriated and
zirconiated type electrodes have been found to produce a slight amount of radiation
when they are ground up. For this reason they should be used only when special
precautions are used. Due to this problem, new types of electrodes for GTAW have
been developed.
Ceriated tungsten electrodes (orange tip) and lanthanated tungsten electrodes
(grey tip)
These are relatively new types of non-radioactive alloy tungsten electrodes. They
can be used in situations where either thoriated or zirconiated tungsten electrodes
would normally be used. The tip may be ground to a point when using DC(–) or to a
ball if AC(hf) is to be used and they demonstrate good welding characteristics in all
applications.
The only drawback is that these electrodes are more expensive to buy than the
thoriated or zirconiated electrodes. However, if an operator has been instructed to use
the GTAW process on a particular job but does not know what type of metal or alloy
the item is made from, ceriated or lanthanated tungsten electrodes should be selected.
This will ensure that a sound weld can be produced no matter what welding current is
required.
111
Chapter 6 – Gas tungsten arc welding (GTAW)
The chart below sets out general recommendations for choosing operating conditions.
Electrode
diameter
Gas cup size
AC(hf)
DC(–)
0.5 mm
6 mm
5−15
5−20
1.0 mm
6 mm
15−40
15−70
1.2 mm
6 mm
20−60
40−90
1.6 mm
6 mm or 10 mm
20−90
65−120
2.4 mm
10 mm
60−160
140−250
3 mm
12 mm
120−220
250−380
5 mm
15 mm
160−340
300−550
6 mm
15 mm
280−470
500−700
Table 6.1 – Typical operating conditions for tungsten electrodes
Before assembling the electrode in the torch, one end should be prepared to suit the
type of welding current being used. For DC(-) it should be ground to a taper with the
nose section having an approximately 30° included angle; do not grind to a sharp point
but leave approximately ⅓ of the electrode diameter unground as a sharp point can be
lost from the electrode into the weld pool during welding. For AC welding, grind with a
chamfer to provide rapid formation of the balled end necessary for AC welding.
DC – VE
(a)
ACHF
(b)
Fig 6.9 – Electrode tip preparation (a) and (b)
Gas tungsten arc welding techniques
Starting the arc
After the gas flow is established and providing high-frequency current (hf) is used to
initiate the arc, the electrode does not have to touch the work piece or starting block
to effect arc initiation. The superimposed high-frequency current jumps the gap
between the electrode and the work piece or starting block and thus establishes a
path for the welding current to flow. On some machines there is facility for a flow of
gas before welding starts (pre-purge) and the current rate and rise time can also be
adjusted (up slope).
112
Metals& Fabrication
Term
Arc Welding 1
Definition
up slope
welding current rate and rise time can be adjusted
from a base or start current to allow for heating up
of the weld area
down slope
welding current rate and fall time can be adjusted
from main current to allow for crater fill and cooling
down of the weld area
50 mm
3 mm
Work
Fig 6.10 – Initiating the arc
When DC is employed without high-frequency, it will be necessary for the electrode to
make actual contact with a starting block. At the moment of contact and when the arc is
struck, the electrode should be raised 3 mm above the starting block to the work piece.
The torch is then moved quickly towards the work area.
To stop an arc, the current should be switched off and the torch held over the cooling
weld to provide a protective gas shield whilst the electrode and work are cooling. Some
care will be necessary, particularly with high quality work and in pipe preparations when
breaking the arc. In some instances it can be advisable to run off on to a tab or up the
side of the pipe preparation when completing a pass.
Equipment controls are available so that:
•
the operator may be able to adjust a gas flow and time (post-purge) to prevent
the risk of contamination
•
the current may gradually be decreased (down slope) at the end of a weld
pass, thus allowing the crater to be filled, instead of being finished in a concave
contour.
113
Chapter 6 – Gas tungsten arc welding (GTAW)
Arc wander
Occasionally the point from which the arc leaves the electrode can move and waver
without any apparent reason. This is termed ‘arc wander’ and is generally attributed to
one of the following causes:
•
low-electrode current density (too large an electrode for the current being
employed)
•
contamination of the electrode
•
magnetic effects.
In AC welding, a ball-ended electrode is used and, when the current density of the
electrode is at a sufficiently high level, the entire end of the electrode will be in a molten
state and completely covered by the arc. When too low a current density is used only
a small area of the electrode becomes molten, resulting in an unstable arc which has
poor directional characteristics and is difficult for the operator to control. Too high a
current density results in excessive melting of the electrode end.
Arc wander in GTAW can be reduced by careful selection of the electrode diameter. It
is much less serious in DC welding, due to the fact that a tapered point is ground on
the electrode.
Electrode contamination can be caused by excessive amperage or careless striking
of the arc. It may be preferable to use a piece of copper for starting purposes. Carbon
blocks are not recommended because of carbon pickup producing arc instability.
Contamination may also result from allowing the electrode to enter the molten pool or
from being touched by the filler rod. When contamination does occur, the only course of
action is to remove the electrode and either replace or clean it by grinding or breaking
off the contaminated end.
Magnetic effects are not frequently encountered and one remedy is to re-position the
work return clamp.
Butt welds
After the arc has been struck, the torch should be positioned at about 70° to the work
piece. The starting point of the work is first preheated by moving the torch in small
circles until a molten pool is formed (see Fig 6.11).
70° approx.
Work
Fig 6.11 – Formation of weld pool
114
Metals& Fabrication
Arc Welding 1
The end of the electrode should be held approximately 3 mm above the work piece.
When the puddle becomes bright and fluid, move the torch slowly and steadily along
the joint at a speed that will produce a bead of uniform width. No oscillating or other
torch movement is required, other than a steady forward motion.
Direction
of
travel
15° max
45° - 90°
25mm
Fig 6.12 – Positioning of filler rod
When filler metal is required to provide adequate reinforcement, the filler rod is held
at about 15° to the work and about 25 mm away from the starting point. Fig 6.12
illustrates torch and filler rod angle. First preheat and develop the puddle as described.
When the puddle becomes bright and fluid, move the arc to the rear of the puddle
and add filler metal by quickly touching the rod to the leading edge of the puddle. As
soon as the puddle is again bright, repeat the same procedure. Care should be taken
to ensure the filler rod end does not leave the protection of the gas shroud during the
welding process.
The rate of forward speed and amount of filler metal added will depend on the desired
width and re-inforcement of the weld bead. Fig 6.13 illustrates the filler rod movement.
Direction
of
travel
45°
45° - 90°
45°
15°
Fig 6.13 – Method of adding filler rod
115
Chapter 6 – Gas tungsten arc welding (GTAW)
Fillet welds
The torch should be held at approximately 45° to 90° to the work piece with the
electrode bisecting the angle between the joint members. All fillet welds require the
addition of a filler rod to provide the necessary build-up, with the filler rod being added
to the weld pool in a similar manner as described in butt welds. After establishing the
arc, the weld pool should be developed on both parts of the work piece by using an
oscillating movement similar to that used for butt welding, before the addition of filler
metal is applied. In awkward corners it may be desirable to extend the electrode to
provide better visibility and complete root fusion. Fig 6.14 illustrates torch and filler rod
relationship to the work piece.
Direction
of
travel
45°
45° - 90°
45°
15°
Fig 6.14 – Fillet joint torch and filler rod relationship
Pipe welding
The GTAW process is commonly used for pipe welding. High-quality welds with uniform
penetration may be readily made in such metals as mild and low alloy steels, stainless
steels, and aluminium and copper. The welds may be root passes in heavy pipe or
completely welded joints with root, filler and finishing passes.
In GTAW pipe welding you can gauge the success of the process by observing the
weld puddle. The shape of the puddle and its size clearly indicates the degree of
penetration inside the pipe. By manipulating the torch properly, the weld puddle can be
controlled at all times, so that it has the correct shape for the pipe joint being welded.
Thus smooth, fully penetrated, porosity free welds can be produced.
Argon is recommended as a backing gas for pipe welding, since it is most effective in
preventing oxidation of the back side of the weld.
The argon backing may be confined to the weld areas by paper baffles, by completely
filling the pipe or by the use of a removable backing device. Joint designs include `V’
and `U’ groove preparations for horizontal and vertical applications.
Consumable inserts (consumable backing rings) are available which will produce the
higher weld quality and the strongest inside weld reinforcements. The inserts fit into a
special `U’ groove preparation.
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Metals& Fabrication
Arc Welding 1
The joint preparation requires close joint tolerances, and fitting the insert into the joint
is time consuming, but since the composition of the insert may be selected to vary the
composition of the weld, the weld results may be superior. These inserts are available
in mild steel, low alloy steel, and stainless steel.
15° - 20°
Fig 6.15 – Relative position of torch and filler rod to pipe
5
3
Start
Stop
2
4
1
Fig 6.16 – Recommended welding sequence for pipe in the horizontal fixed position
The relative position of the torch and filler rod to the pipe is illustrated in Fig 6.15,
whilst the recommended welding sequence for pipe in the horizontal fixed position is
illustrated in Fig 6.16.
117
Chapter 6 – Gas tungsten arc welding (GTAW)
Weld backup
On many GTA welding applications, the joint should be backed up, particularly on light
gauge material. Backing is generally used to protect the underside of the weld from
atmospheric contamination, that may result in possible weld porosity or poor surface
appearance. In addition to protecting the underside of the weld, backup prevents the
weld puddle from dropping through by drawing away from the work piece some of the
heat generated by the intense arc, and can also physically support the weld puddle. A
weld can be backed up by:
•
metal backing bars
•
introducing an inert gas atmosphere on the weld underside
•
a combination of both methods.
Weld backing bars may be of a temporary or permanent type. The temporary type
does not form part of the welded joint and can be copper, stainless steel, mild steel
etc, depending on the material to be welded, and may be removed on completion of
the weld. The permanent type is usually of the same composition as the material to
be welded and becomes part of the welded joint as illustrated in Fig 6.17. They are
generally used where access does not allow the removal of the temporary type.
Work
Permanent
backing bar
Fig 6.17 – Permanent backing bar
A type of temporary backing bar commonly used is shown in Fig 6.18 where the
surface is cut or machined out directly below the joint. A bar of this type will protect the
bottom of the weld from excessive contamination by the atmosphere, as well as draw
heat away from the weld zone.
118
Metals& Fabrication
Hold down bars
or clamping
fingers
Work
Work
Temporary
backing bar
Arc Welding 1
Relief
groove
Backing bar
Channel to allow
free flow
of underbead
Hold down bars
or clamping
fingers
Work
Backing bar
Insert
Channel to
support molten
underbead
Fig 6.18 – Temporary backing bar
On applications where the final weld composition must conform to extremely rigid
specifications, extra care must be taken to exclude all atmospheric oxygen from the
weld underside. The use of temporary backing bars that will trap gas on the under side
can achieve this. A supply of inert gas can also be offered up to the under side.
Nitrogen may be used for stainless steel. Argon should be used for aluminium,
magnesium and other metals that oxidise readily or react with nitrogen at high
temperatures.
119
Chapter 6 – Gas tungsten arc welding (GTAW)
Activity
In the workshop:
•
identify safe working practices and protective equipment in the workshop
•
identify a suitable GTAW plant
•
with the mains power switched OFF, set up the GTAW plant (start by checking
the electrical cable connections to the work lead and electrode handpiece, then
the gas connections from the cylinder through to the handpiece)
•
select the necessary collet holder/gas diffuser and collet to suit a 2.4 mm
thoriated tungsten electrode
•
prepare the electrode by sharpening and fit it into the welding torch
•
fit a 10 mm diameter gas cup to the torch
•
adjust the power source current type and polarity to produce direct current
electrode negative DC(-)
•
have your lecturer check the equipment and settings and help you to adjust the
argon gas flow rate to nine to ten litres per minute with the power source turned
on
•
with your lecturer’s assistance, produce a weld on a practice piece of material.
Variable
Description
current (amps)
adjustable range
current type
AC, ACHF, DC-, DC+
pulse
adjustable pulse wave settings
AC wave control
adjustable wave control
voltage control
weld voltage control
Fig 6.19 – Chart showing variables that can be set on quality GTAW plant
120
Metals& Fabrication
Arc Welding 1
Chapter 7 –
Gas metal arc welding (GMAW)
Introduction
Since its introduction in the 1940s, gas metal arc welding (GMAW) has become the
most popular welding process in Australian industry. It is particularly suited to a wide
range of light and general fabrication applications. Gas metal arc welding is a
semi-automatic process where the wire is fed into the weld pool. This produces higher
deposition rates and greater efficiency over the manual metal arc welding process.
In this chapter you will look at the following.
•
•
•
•
•
GMAW principles
ο
advantages
ο
limitations
Safety in gas metal arc welding
ο
darker welding filters
ο
body protection
ο
ventilation
Equipment
ο
power source
ο
wire feed unit
ο
gun cable assembly
ο
gas supply system
ο
interconnecting cables
ο
wire feed systems
ο
drive rollers
ο
wire conduit (liner)
ο
contact tip
Metal transfer
ο
dip transfer
ο
globular transfer
ο
spray transfer
ο
pulsed current
Classification of consumables
121
Chapter 7 – Gas metal arc welding (GMAW)
ο
•
•
•
solid wire electrodes classification system
Gas metal arc welding variables
ο
wire speed/amperage
ο
arc voltage
ο
travel speed
ο
electrical stick-out
ο
torch angle
ο
angle of travel
Shielding gases
ο
carbon dioxide
ο
argon
ο
gas mixtures
ο
flow rates
Other machine controls
ο
spot timer
ο
burnback control
ο
spool brake
•
Joint design for gas metal arc welding
•
Gas metal arc welding defects
•
ο
porosity
ο
lack of fusion
ο
lack of root penetration
ο
excessive penetration
ο
contour defects
ο
undercut
ο
cracking
ο
stray arcing
ο
excessive spatter
Trouble shooting/equipment malfunction.
At the end of the chapter, you will complete an activity.
(GMAW) principles
Gas metal arc welding is an arc welding process where the necessary heat for fusion
is produced by an electric arc maintained between a continuously fed wire electrode
and the part to be welded. The heated weld zone, the molten weld metal, and the
consumable electrode are shielded from the atmosphere by a shroud of gas, fed
through the welding torch.
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Metals& Fabrication
Arc Welding 1
Direction
of travel
Wire guide and contact tip
Gas nozzle
Solidified
weld metal
Shielding gas
Work
Arc
Molten weld metal
Fig 7.1 – The gas metal arc welding process
Advantages
The major advantage of the GMAW process is its high deposition rate compared with
the manual metal arc, and gas tungsten arc welding processes. This is brought about
by the automatic wire feed, the high ratio of current to wire diameter, and the removal
of the need to change electrodes, chip slag etc. The advantages of this include:
•
high deposition rates when compared to manual metal arc welding
•
high operating factor
•
no wastage from electrode stubs
•
elimination of slag removal
•
has a wide range of applications
•
low hydrogen deposit
•
reduced distortion on thin materials.
Limitations
Whilst GMAW is a popular and versatile welding process offering the advantages listed
above, it is also limited by the following.
•
High initial equipment cost.
•
High maintenance requirements and low mechanical reliability.
•
Cannot be used in windy conditions Australian Standard® AS/NZS 1554.1
Structural steel welding – Welding of steel structures, limits the use of gas
shielded processes where the wind velocity exceeds 10 km/hr. This makes the
process generally unsuitable for site work.
•
Lack of fusion defects can be a major problem under some circumstances.
•
More variables to set.
123
Chapter 7 – Gas metal arc welding (GMAW)
Safety in gas metal arc welding
Darker welding filters
The primary concern in regard to safety is the open arc and intensity, which is much
greater than that associated with MMAW electrodes. Thus a darker welding filter than
is normally used is required for GMAW – a filter one shade darker than that used for
welding at the same amperage with the MMAW process is required.
For example:
•
up to 200 amps – a shade 11 is recommended
•
200−300 amps – a shade 12 is recommended.
Clear safety glasses must be worn at all times, due to the high emission of UV
radiation, resulting in more frequent and severe arc flashes.
Body protection
This same arc intensity will also require the operator to ensure their body is completely
covered with protective clothing. Even extraneous light from the arc (ie UV radiation
bouncing from a reflecting wall) can result in a rather uncomfortable ‘ray burn’.
Experience has shown that cotton materials have less resistance to ultraviolet rays
than woollen materials. Cotton, and particularly synthetics, quickly break down and
eventually disintegrate. Consequently, it is preferable to wear leather or woollen
materials.
Term
UV radiation
Definition
ultraviolet radiation sources are invisible rays
of energy from the sun (natural), and welding
processes (artificial). Ultraviolet radiation can
damage the skin causing varying types of cancer
Ventilation
When arc welding, a toxic gas called ozone (O3) is given off from the arc. Processes
which employ higher current densities produce more ozone. Although ozone is not
dangerous under most conditions, it is advisable when working in confined spaces
(where ventilation is restricted) to use exhaust extraction. Natural ventilation and
exhaust fans can also be advantageous. Any ventilation system used must not interfere
with the gas shielding of the weld zone.
Term
ozone
124
Definition
an unstable, poisonous allotrope of oxygen (O3) that is
formed naturally in the ozone layer from atmospheric
oxygen and in the vicinity of an electric arc
Metals& Fabrication
Arc Welding 1
Equipment
The major equipment items which make up a GMAW plant are:
•
the power source
•
the wire feeder
•
the welding gun cable assembly
•
the gas supply system
•
the inter-connecting cables.
Power source
Transformer
rectifier
Wire feeder
Wire
reel
Feed
rolls
Flow meter
Heater
(if required)
Inductor
Regulator
Power cable
Welding wire
Work lead
Cooling water
(if required)
Contact tip
Nozzle
Shielding
gas
supply
Shielding
gas
Work
Fig 7.2 – Gas metal arc welding equipment
Power source
A heavy duty constant voltage (constant potential) power source is required for GMAW.
This is commonly a transformer/rectifier or an inverter. The output requirement is for
direct current with a constant voltage type characteristic but this may be varied to suit
different applications. All solid wires for GMAW run on DC electrode positive (DC+).
The GMAW process is intolerant to variations in arc voltage, and the constant voltage
type output provided by the power source ensures that the arc length is self-adjusting,
and the burn-off remains constant despite uneven gun movement and variations in arc
length.
Term
DC
Definition
direct current. An electric current flowing in one
direction only
125
Chapter 7 – Gas metal arc welding (GMAW)
Wire feed unit
The primary function of the wire feed unit is to feed wire to the arc. The unit houses
a reel of wire and a DC motor, to which feed rollers are attached. The feed rollers
feed wire to the arc down through a hollow conduit. The speed of the drive motor
is governed by a potentiometer (the wire feed control). Increasing the wire speed
rate usually increases amperage because the increased wire feed rate effectively
decreases the arc length slightly. This loads up the arc voltage and the machine will
then increase welding current to compensate. Incorporated into the wire feed unit are
the shielding gas connections, gas solenoid, and water connections (in the case of a
water-cooled torch).
Most wire feed units have a gas purge control so that the gas flow can be set without
any current or wire flow, and a wire inch control so that wire may be fed through without
the welding current being turned on. Some wire feeders may also have pre-gas flow
and post-gas flow (useful for aluminium and stainless steel).
Gun cable assembly
The gun cable assembly is the vehicle by which wire, current and shielding gas are
conveyed to the welding arc. It connects to the wire feeder and terminates at the gun or
handpiece.
The electrode wire travels through the wire conduit or ‘liner’ which runs through the
centre of the gun cable. Welding current is carried through the cable by a heavy copper
lead within the cable.
Shielding gas is also carried through the cable, and is distributed at the weld via the
gas diffuser and gas nozzle.
Welding is commenced by depressing the torch trigger. This initiates three separate
functions, as follows.
1.
The welding current contactor is ‘pulled in’ (closed) and welding current becomes
available. Welding current is transferred to the wire as it passes through the
contact tip.
2.
The gas solenoid valve opens and allows shielding gas to flow.
3.
The wire feed motor starts up and feeds wire at the preset, constant speed
through the torch conduit.
Due to the heat generated in the weld pool and through electrical resistance at the
contact tip, torches have to be efficiently cooled. The majority of torches are
gas-cooled, however, water-cooled torches may be required when high amperages are
used on a continuous basis.
Welding guns are usually provided with a bent neck to improve operator comfort but
some guns may have a straight neck, which allows better wire feed.
126
Metals& Fabrication
Term
Arc Welding 1
Definition
gas solenoid valve
an electromechanical valve controlled by running
(or stopping) an electrical current through a
solenoid (a coil of wire) thus changing the state of
the valve
electrical resistance
a measurement of properties which limit the ability
of a substance to conduct electricity
Gas supply system
Shielding gases for GMAW are usually supplied from a single cylinder, however, large
consumers may use manifolded systems. The components of the gas supply system are:
•
a cylinder of gas − CO2 or argon/CO2 mixtures for carbon steels
•
a regulator − to reduce cylinder pressure
•
a flowmeter − to control shielding gas flow rate
•
a heater − when CO2 is used as a shielding gas, a heater is fitted between the
cylinder and the regulator to prevent freezing at the regulator.
Interconnecting cables
These consist of:
•
the work return lead
•
the electrode lead – from the power source to the gun cable adaptor of the wire
feeder
•
the control cable from the power source to the wire feeder.
Wire feed systems
There are three basic types of GMAW wire feed systems, each requiring different
torches.
1.
The push system
The push system is by far the most popular wire feed system. The wire feed unit
pushes the electrode wire from the drive rolls along the gun conduit, through the
gun and contact tip and to the weld pool. Push systems are generally robust,
lightweight and very functional (also the least expensive). The system works very
well with hard wires such as steel and stainless steel, in cables up to 4.5 metres
in length. Wires in spools of 15 kg or larger are usually used with this system.
This keeps costs down and increases convenience.
The major disadvantage of the push system is the unreliability of wire feeding
caused by friction which causes dust to accumulate in the conduit. Wires may
also become kinked and this is a particular problem when feeding soft wires
such as aluminium. Because the conduit in most wire feed systems is live
(connected to the wire feeder and/or the contact tip) the conduit may experience
internal arcing caused by dust, or a faulty or dirty contact tip and this results in
127
Chapter 7 – Gas metal arc welding (GMAW)
wire feed problems. Any wire feed rate problem caused by a dirty contact tip
and/or faulty wire feed will reflect itself in altered or changing weld parameters
(voltage/amperage).
Drive
rolls
Welding
torch
Fig 7.3 – Push system
2.
The pull system
The pull system is sometimes known as spool on gun and is ideally suited to
feeding soft wires such as aluminium, or where welding is to be carried out at a
location remote from the power source. The drive motor and drive rollers are built
into the handle of the gun. This offers short, direct wire travel, with little friction
through the conduit.
The drawbacks of this system are the high initial cost of equipment, the
susceptibility of damage to the torch, the cost of consumable wire on small spools,
and the weight of wire carried on the gun. Though this system is mainly used for
aluminium work, mild steel and stainless steel wires can also be used.
Drive
rolls
Wire
reel
Welding
torch
Fig 7.4 – Pull system
3.
The push/pull system
As the name implies, both the push motor at the wire feeder and a pull motor at
the torch are employed. In the best brands the two motors are synchronised to
feed the wire at the same speed, although there are some cheaper brands on the
market that allow the torch motor to only apply a set tension to the wire feed whilst
all the speed control is maintained at the main wire feeder. The push/pull wire feed
system enables the feeding of both hard and soft wires up to 10 metres from the
welding machine and still offers the economy of 15 kg (or larger) spools of wire.
The push/pull system is versatile, particularly suited to aluminium but may also be
used for hard wires as well.
128
Metals& Fabrication
Drive
rolls
Arc Welding 1
Drive
rolls
Wire
reel
Welding
torch
Fig 7.5 – Push/pull system
Drive rollers
Friction, caused by pressure applied to the wire as it passes through the rotating drive
rolls, is the mechanism by which the wire is fed. Resistance in the gun cable may
cause the wire to slip as it passes through the drive rolls. Increasing the pressure of the
top roller increases friction and prevents this slippage. However, excessive pressure
can deform the wire making it more difficult to feed (Fig 7.6).
Top roll
Vee drive roll
Fig 7.6 – Deformation caused by excessive roll pressure
Wire feeders use either a two or four-roller drive system. Two-roll systems are cheaper
to buy and are best suited to feeding hard wires such as carbon and stainless steels
through short gun cables.
Four-roll feeders allow greater friction between the rollers and the wire with less roller
pressure, giving a smoother feed with less slippage and less distortion of the wire.
129
Chapter 7 – Gas metal arc welding (GMAW)
The four-roll system offers advantages for:
•
feeding soft wires such as aluminium
•
feeding wires through long gun cables
•
use with cored wires.
Wire direction
Drive
rolls
Fig 7.7 – Two roller feeder
Wire direction
Drive
rolls
Fig 7.8 – Four roller feeder
The cross sectional shape of the rollers used with any particular wire feeder, for any
particular application, varies according to the manufacturer.
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Metals& Fabrication
Arc Welding 1
Common configurations/sections of drive rolls and their uses include the following.
Drive roll
Type
Application
flat top roll/‘V’
bottom roll (three
point universal
system)
used for general
purpose feeding
of hard wires
such as steel and
stainless steel
flat top roll/‘U’
bottom roll
(contour system
– the rolls must
suit wire size)
used mainly
for aluminium
wires. The ‘U’
profile reduces
deformation of the
soft wire
top and bottom
rolls have
serrated ‘V’
grooves (knurled)
used for cored
wires and large
diameter solid
wires
Top
Bottom
Drive
rolls
Top
Bottom
Top
Bottom
Drive
rolls
Separate
Drive
rolls
Fig 7.9 – Drive rolls
This list is not exhaustive, but these are in most common use.
131
Chapter 7 – Gas metal arc welding (GMAW)
Wire conduit (liner)
The liner is used to guide the wire through the gun cable to the handpiece, and through
to the contact tip. The liner is made of spiral steel wound-wire for feeding hard wires
such as carbon and stainless steels, and of Teflon®, for feeding aluminium wire. To
ensure reliable wire feeding, it is imperative that the liner is cut to the correct length,
and properly fitted in the gun cable and kept free of dust. Additionally, the gun cable
should be kept as straight as possible when in use.
Contact tip
The contact tip serves two functions:
•
to guide the wire to the arc
•
to transfer welding current to the wire.
The contact tip is a most important component of the welding torch. It is here that the
filler wire is energised or ‘picks-up’ the welding current. It is usually made from copper
and is, via the gas diffuser and torch body, directly attached to the power lead. Contact
tips are matched to each wire size. It is important that the contact tip is maintained in
a clean condition, free from spatter on the end, and with a smooth internal bore. Worn
contact tips reduce the efficiency with which current is transferred to the wire, and
contribute to uneven wire feeding. They should be replaced when worn.
Metal transfer
With most of the commonly used welding processes, the operator has little control over
the way metal is transferred across the arc. With GMAW the operator can select and
control the type of metal transfer. This is done essentially by selecting the arc voltage,
although wire diameter and shielding gas also influence metal transfer.
The metal transfer mode determines the characteristics of the GMAW process. The
operator must select the most appropriate mode of transfer and set the machine
accordingly, prior to commencing welding.
Apart from the pulsed transfer mode, which requires sophisticated power sources, the
welding operator can select from three transfer modes, which are:
•
dip (or short arc) transfer
•
globular transfer
•
spray transfer.
Dip transfer
Dip transfer is also known as ‘short arc’ transfer (short for short circuiting arc). In the dip
transfer mode, low current and voltage settings are used. The low voltage employed is
easily overcome by electrical resistance across the arc, preventing continuous current
flow as arc length increases.
When welding commences, the tip of the electrode wire contacts the plate and a short
circuit occurs. This results in a rapid temperature rise in the wire (caused by the short
circuit current flowing through to the work piece) and the end of the electrode wire is
melted off. An arc is immediately formed between the tip of the wire and the weld pool.
This arc maintains the electrical circuit for a short time until the electrical resistance
across the increasing arc gap causes the arc to be extinguished.
132
Metals& Fabrication
Arc Welding 1
The electrode wire continues to feed, and the tip once again dips into the pool and
the cycle is repeated. This sequence of events is repeated at a frequency of up to 200
times per second. It produces sufficient heat for fusion, and to keep the weld pool fluid.
This method of transfer is suitable for positional welding due to rapid freezing of the
weld pool, and has the advantage that the heat input to the work piece is kept to a
minimum. This limits distortion and enables thin sheet material to be welded. However,
on thicker material, the low heat input tends to result in a lack of fusion defects if care is
not taken with machine adjustment and welding technique.
Direction of
wire travel
1
2
3
4
5
1
2
Work
Fig 7.10 – Schematic diagram of short arc transfer
1.
Trigger depressed − wire starts to feed.
2.
Wire contacts the work piece − heats up due to electrical resistance and starts to
melt.
3.
Wire melts off and an arc is established.
4.
Arc length increases as the end of the wire melts slightly.
5.
Arcing ceases due to the low arc voltage being unable to overcome the electrical
resistance across the arc gap.
6.
Wire is fed into the weld pool which has been created and the cycle begins again.
The following are features of dip transfer:
•
low currents are used
•
low heat input
•
low penetration
•
moderate spatter
•
low deposition rate
•
relatively cold weld pool
•
ideal for thin materials
•
used for positional welding
•
tends to result in lack of fusion defects − particularly when plate thickness
exceeds 5 mm.
133
Chapter 7 – Gas metal arc welding (GMAW)
Globular transfer
Globular transfer occurs at current levels between those used for dip and spray
transfer. Voltages are high enough to ensure a constant arc, but amperage is set below
the threshold current that produces spray transfer. The result is that the wire melts in
the arc, and a molten globule forms on the end of the wire. As melting continues, the
size of the globule grows until its own weight causes detachment of the droplet due
to gravitational forces. This droplet detachment is erratic and, along with arc forces
repelling the droplet away from the wire, high spatter levels result. Droplet size is
considerably larger than the wire diameter.
Globular
metal drops
Fig 7.11 – Globular transfer
The following are features of globular transfer:
•
moderate amperages are used
•
low/moderate penetration
•
moderate/high spatter levels
•
coarse appearance
•
metal droplets are detached by gravitational forces
•
largely unsuitable for positional welding
•
occurs even at high amperages when the shielding gas contains in excess of
23% CO2.
Spray transfer
Unlike dip transfer, where the low arc voltage used precludes the use of a continuous
arc, spray transfer employs an arc which burns continuously. To achieve this, the arc
voltage, when welding steel, must be above approximately 23V (depending on wire
size and shielding gas composition).
Additionally, the amperage used must be above the ‘threshold current’. The threshold
current is the current above which tiny droplets are pinched off and projected axially
across the arc gap. Below the threshold current, droplet detachment is brought about
by the molten droplet of wire growing in size until it is heavy enough to be detached by
gravitational forces.
134
Metals& Fabrication
Arc Welding 1
Fig 7.12 – Spray transfer
Spray transfer offers greatly increased deposition rates compared to dip transfer,
produces minimal spatter, and doesn’t result in the lack of fusion defects sometimes
associated with dip transfer. Due to the hot, fluid weld pool associated with spray
transfer, it is only suitable for use on plates above approximately 5 mm thick, and in the
downhand (flat) position.
The following are features of spray transfer:
•
high currents are used
•
high heat input
•
moderate/deep penetration
•
high deposition rates
•
low spatter
•
good appearance
•
fluid weld pool
•
unsuitable for positional welding
•
requires a shielding gas with high argon content.
The following chart (Fig 7.13) shows the amperage and voltage ranges for the
above-mentioned transfer modes.
40
v
o
l
t
a
g
e
30
dip
20
globular
10
spray
0
100
200
300
amperage
400
500
Fig 7.13 – Volt AMP ranges for gas metal arc welding
135
Chapter 7 – Gas metal arc welding (GMAW)
Pulsed current
Pulsed current may be available on some GMAW equipment. The welding current
is set to fluctuate between a high current level for fusion and a low solidification or
background current level; both of which are adjustable, as is the time for which each
current level is effective. The number of pulses can be varied from ten per second
down to about one per second.
Pulsed current is particularly useful for welding very thin materials, providing good
penetration during the high cycle with cooling of the molten pool during the low cycle.
In effect, pulsed current produces a series of spot welds, penetration is good, distortion
is minimised, and control is improved for difficult welding situations involving thin
materials and positional welds.
A number of machine manufacturers are providing a droplet transfer option. In this
mode the variable related to background current, pulse frequency and pulse current are
controlled to provide a cool/heat cycle that produces sufficient current and voltage to
melt off a droplet of wire at each pulse cycle. Refer to Fig 7.14 and 7.15.
Pulse peak
current
Streaming
Threshold
current
c
u
r
r
e
n
t
Globular
Pulse
average current
Pulse
background
current
time
Fig 7.14 – Pulsed current terms
136
Metals& Fabrication
Arc Welding 1
a
m
p
s
Aver.
amps
a
m
p
s
Aver.
amps
Aver.
amps
a
m
p
s
TIME
Fig 7.15 – Increasing pulse rate increases average amperage
137
Chapter 7 – Gas metal arc welding (GMAW)
Classification of consumables
There are many different types of solid and flux-cored electrode wires commercially
available. They are classified to a particular standard, which makes it possible to
identify and select the most suitable type of wire for a job. It is important to understand
classification systems and the information they represent.
Consumable classification systems list a number of essential features about the
consumable. For example, consumables are classified in construction, filler metal
composition, shielding method, mechanical strength of the weld deposit and so on.
The two systems in this text specify features of solid and/or flux-cored wire electrodes.
Solid wire electrodes classification system
Australian Standard® AS/NZS 2717.1 Welding – Electrodes – Gas metal arc – Part 1:
Ferritic steel electrodes.
This Standard classifies solid wire electrodes under three groups of elements
separated by hyphens. Each group consists of a letter(s) and number(s).
Example:
ES2-GMp-W502H
Group 1 (ES2)
The first group of letters denotes a solid electrode, and indicates the chemical
composition of the wire. ES stands for ‘electrode solid’. After ES, a number indicates
the chemical composition of the wire. From the chart below you can see that a wire
ES2 contains 0.07% carbon, and 0.9 to 1.4% manganese.
Chemical composition chart
Classification
Carbon (%)
Manganese (%)
Silicon (%)
ES2
0.07
0.9–1.40
0.40–0.70
ES3
0.06–0.15
0.9–1.40
0.45–0.70
ES4
0.07–0.15
1.00–1.50
0.60–0.85
ES5
0.07–0.19
0.90–1.40
0.30–0.60
ES6
0.07–0.15
1.40–1.85
0.80–1.15
ES7
0.07–0.15
1.50–2.00
0.50–0.80
Based on AS/NZS 2717.1: 1996 – Table 2.2 (www.saiglobal.com)
Electrodes can also contain very small additions of copper, titanium, zirconium and
aluminium.
138
Metals& Fabrication
Arc Welding 1
Group 2 (GMp)
The second group consists of G for gas shielded and then two letters that indicate the
type of shielding gas used during qualification tests and the welding current required.
G – gas shielding which is then followed by one of the following:
C = shielded with carbon dioxide (CO2)
M = shielding with a mixture of gases
I
= shielded with an inert gas
p = positive electrode.
For example, GMp indicates that the wire is to be shielded by use of mixed gas with
positive electrode.
Group 3 (W502H)
The third group involves a letter W followed by a three-digit number. W stands for
weld metal. The first two digits refer to the minimum strength of the deposited weld,
which is measured in megapascals. The third digit refers to the minimum impact value.
The letter H generally completes the classification which indicates that the process is
hydrogen controlled.
W – weld metal properties:
50 = 500 MPa strength
2
= degree of impact test
H = hydrogen controlled.
For example, W502H indicates the weld strength is 500 MPa and low in hydrogen.
An example of the full classification system is shown below.
E S 2-G Mp-W 50 2 H
Denotes
solid
electrode
Indicates chemical
composition of the
wire
Denotes gas
shielded
Type of
shielding gas
Denotes
weld metal
Approximately 0.1
x tensile strength
in MPa
Hydrogen
controlled
Impact
energy
grade no.
139
Chapter 7 – Gas metal arc welding (GMAW)
Here are some examples of the system.
ES-GMp-W502H
A plain carbon steel wire electrode.
The chemical composition can be found in the chart on the previous page. When
deposited with an Ar/CO2 gas shield, the weld metal will have a minimum tensile
strength of 500 MPa and an impact value 60 J at 0 °C. The weld is hydrogen-controlled.
Term
Definition
measures the maximum force required to pull
material to the point where it breaks
tensile strength
ES4-GCp-W503H
A plain carbon steel wire electrode.
The chemical composition can be found in the chart on the previous page. When
deposited with CO2 shielding gas, the weld metal will have a minimum tensile strength
of 500 MPa and an impact value of 60 J at 0 °C. The weld is hydrogen-controlled.
Filler wires for the welding of steels are de-oxidised with manganese and silicon, and
are generally copper coated (nickel is sometimes used). The copper coating of the wire
serves three purposes:
•
prevents corrosion of the wire
•
improves current pickup
•
improves feeding characteristics.
Common wire sizes for GMAW of steels is as follows.
0.6 mm
0.8 mm
Generally used for sheet metal
and other light applications
0.9 mm
1.0 mm
General purpose GMAW
1.2 mm
1.6 mm
140
Welding of heavy plates
Metals& Fabrication
Arc Welding 1
Gas metal arc welding variables
The variables affecting the GMAW process are:
•
wire speed/amperage
•
arc voltage
•
travel speed
•
electrical stick-out
•
torch angle
•
shielding gases and flow rate.
Wire speed/amperage
Wire speed and amperage are controlled by the same potentiometer on a GMAW plant.
Consequently, these variables cannot be adjusted independently of each other.
As amperage is increased, the current density in the wire increases and the melt-off
rate of the wire increases. Amperage is the most important factor when determining
heat input into the metal being welded. Increasing wire speed/amperage control will:
•
increase the wire feed speed
•
increase amperage
•
increase deposition rate
•
increase penetration
•
increase heat input
•
for a given travel speed, increase the size of the weld bead.
Decreasing wire speed will have the opposite effect.
Weld profile and
penetration
Increased amperage
Fig 7.16 – Effect of amperage
141
Chapter 7 – Gas metal arc welding (GMAW)
Arc voltage
Arc voltage determines the mode of metal transfer when GMA welding. At low arc
voltages, resistance across the arc causes extinguishment of the arc, which results
in dip transfer. Higher arc voltages are enough to maintain the arc by overcoming the
electrical resistance. As the arc voltage is increased, arc length is increased. This
enables more wire to be melted off without ‘stubbing’ as sometimes occurs when high
wire feed speeds and low arc voltages are used. Increased arc length also increases
the width of the weld bead.
Increased voltage
Fig 7.17 – Effect of arc voltage
It can be seen therefore, that if arc voltage is increased without changing the wire
speed or travel speed, the arc gap is increased, resulting in a wider, flatter bead.
Travel speed
As travel speed is reduced, the weld bead becomes more convex due to a greater
deposition of filler wire. Heat input is increased due to the arc remaining above any
particular point for a greater period of time.
The opposite is achieved when travel speed is increased.
Increased speed
Fig 7.18 – Effect of travel rate
Electrical stick-out
When discussing GMAW, two types of stick-out are referred to:
142
•
visible stick-out – the distance that the electrode protrudes beyond the gas nozzle
•
electrical stick-out – the distance that the electrode protrudes from the contact tip.
Metals& Fabrication
Arc Welding 1
Contact
tip
Nozzle
Visible stick-out
Electrical stick-out
Fig 7.19 – Stick-out length
Visible stick-out has little effect upon welding conditions except that, if excessive,
shielding efficiency will be reduced. However, electrical stick-out is an important
consideration. Welding current is transferred to the wire via the contact tip. The wire
between the end of the contact tip and the arc offers electrical resistance. As the
electrical stick-out is increased so is the electrical resistance (Fig 7.20).
Work
Wire
Work
Fig 7.20 – Increased resistance due to increased electrical stick-out
The effect of this increased resistance is:
•
reduced amperage
•
reduced penetration
•
reduced heat input
•
higher deposition rate.
The increased deposition rate is brought about by:
•
preheating of the wire
•
the wire feed rate.
143
Chapter 7 – Gas metal arc welding (GMAW)
As the increased electrical resistance due to the increase in electrical stick-out
preheats the wire, it tends to melt off sooner. This has the effect of increasing the arc
length, which in turn tends to increase arc voltage – because of the power source
characteristics (constant voltage) the current reduces and thus compensates. If the
drive motor speed is now increased there will be an increase in wire deposition rates.
Torch angle
As with any welding process, the angle of approach must be adjusted to distribute the
weld metal evenly in the joint (Fig 7.21).
90°
45°
Fillet weld
Butt weld
Fig 7.21 – Angle of approach
Angle of travel
The angle of the gun is maintained such that it is ‘pushed’ in the direction of travel
(Figure 7.22).
10° to 30°
Direction
of travel
Nozzle
Weld
Work
Fig 7.22 – Angle of travel
The exception to this is when making heavy welds in spray transfer where the gun is
‘dragged’. This is done to direct shielding gas over the solidifying/cooling weld metal,
which remains hot for an extended period of time.
The operator determines the actual angle of travel used, by seeking the best
compromise between good visibility and efficient shielding.
As the torch angle is lowered shielding efficiency is reduced due to the Venturi effect,
which draws air into the gas shield.
144
Metals& Fabrication
Arc Welding 1
(b)
(a)
(c)
Air
Note: Angle varies with
direction of travel drag
or push
Fig 7.23 – Nozzle angle
(a) correct (b) incorrect (c) incorrect
Shielding gases
In Australia GMAW was also commonly known as ‘MIG welding’ (metal inert gas).
This is in fact misleading, as it suggests that the shielding gas is inert. All GMAW of
carbon and low-alloy steels employ the use of an active shielding gas, ie there is a
reaction between the shielding gas and the metal droplets as they travel across the arc.
Inert shielding gasses are used for welding stainless steels and non-ferrous metals.
To achieve the desired arc stability when welding carbon and low-alloy steels, some
oxidising action is required in the arc. This can be achieved in one of two ways:
•
using CO2 (carbon dioxide) as a shielding gas
•
using Ar (argon) as the base with the addition of CO2 and/or O2 (oxygen).
Term
Definition
inert
refers to unmoving or unchanging. Having only
a limited ability to react chemically; chemically
inactive
non-ferrous
the term given to metal that does not contain iron
145
Chapter 7 – Gas metal arc welding (GMAW)
the metal that has been deposited in the weld joint,
after it has cooled off
(weld) bead
Carbon dioxide
CO2 when used as a shielding gas, produces a highly reactive arc. CO2 has the
following welding arc characteristics:
•
deep penetration
•
high spatter levels
•
high deposition rates
•
high heat input
•
true spray transfer cannot be achieved.
CO2 is best suited to the making of welds using dip transfer. The additional heat of CO2
helps to overcome the tendency towards lack of fusion, and increases deposition rates.
CO2 tends to produce convex bead shapes and high spatter levels.
Argon
Argon is a true inert gas, which by itself cannot be used to weld carbon and low-alloy
steels. When used by itself to weld non-ferrous metals, it produces an arc which, when
compared to CO2, has the following characteristics:
•
smooth arc
•
lower penetration
•
lower heat input
•
lower spatter
•
improved bead shape
•
promotes spray transfer.
Gas mixtures
Gas mixtures for welding steel employ the use of argon as a base, with the addition of
differing levels of CO2 and/or O2 to achieve desirable arc characteristics.
The greater the O2/CO2 addition, the more the arc characteristics align to the
characteristics of CO2. The lower the addition of CO2/O2 the more the arc aligns toward
the characteristics produced by argon shielding gas.
Shielding gas
146
Chemical
behaviour
Effect/uses
Argon
inert
For welding all metals except carbon and low alloy
steels
CO2
oxidising
Produces high spatter and deep penetration. Used
with de-oxidised wire on carbon steels
Metals& Fabrication
Arc Welding 1
Argon/CO2
oxidising
For welding carbon and low-alloy steels. Produces
low spatter and moderate penetration
Argon/CO2/O2
oxidising
Additional oxygen increases penetration. Used with
de-oxidised wire to weld carbon and low-alloy steels
Each gas company will supply mixtures of their own formulation. However, as a rough
guide for welding carbon and low-alloy steels, uses for mixtures approximating the
following compositions are:
•
CO2
•
Ar + 25% CO2 − general use in dip transfer
•
Ar + 15% CO2 − multi-purpose for dip and spray transfer
•
Ar + 5% CO2 − for spray transfer.
− dip transfer, particularly on thicker plates
The ionising effect of the shielding gas influences bead shape as well as the amount of
penetration obtained. The effect of shielding gas upon bead shape can be seen in the
following graphic (Fig 7.24).
(a)
Argon
Helium
Work
Weld
penetration
(b)
Weld
profile
Fig 7.24 – (a) Effect of a change from argon to helium and (b) Effect of various shielding
gases on bead shape
Term
ionising
Definition
a process in which an atom or molecule loses or
gains electrons, acquiring an electric charge or
changing an existing charge
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Chapter 7 – Gas metal arc welding (GMAW)
Flow rates
Flow rates for CO2 should be set at 16–18 L/min. Flow rates for Ar/CO2 mixtures should
be set at 12–14 L/min.
Gas flow rates should be set so as to provide adequate shielding.
Recommended rate of flow for Argon / CO2 mixtures = 12–14 L/min.
Recommended rate of flow for CO2 mixtures = 16−18 L/min.
It should be kept in mind that excessively high flow rates cause turbulence and
increase the Venturi effect when torch angles are too low.
Other machine controls
Spot timer
Allows the weld time to be preset as a means of making consistent weld sizes for spot
welding. The timer is activated when the gun trigger is depressed.
Burnback control
Enables wire to feed for a small amount of time after current flow is terminated when
the gun trigger is released. This can be adjusted to prevent the wire fusing to the
contact tip, or stop it sticking to the weld pool when welding is terminated.
Spool brake
The wire spool carrier employs a braking device to prevent over-run of the wire due
to the inertia of the spool of wire. It should be adjusted to provide enough braking to
prevent over-run, but with no unnecessary drag which would cause slippage of the wire
at the drive rollers.
Photographed with permission of Lincoln Electric Co. (Aust) Pty Ltd
Fig 7.25 – Typical wire feeder
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Arc Welding 1
Joint design for gas metal arc welding
Pre-qualified joint preparation for GMAW of steel structures can be found in Australian
Standard® AS/NZS 1554.1 Structural steel welding. It can be seen that joint design is
similar to that used for MMAW butt welds in steels but with the following variations.
•
Included angles of butt welds are reduced by ten degrees. This is because the
thinner electrode and lack of flux provides easier access to the root of the joint.
•
The root face for butt welds is decreased when dip transfer is used due to the fact
that penetration is limited, and increased when spray transfer is used as a means
of preventing burn through.
Term
burn through
Definition
a localised collapse of the molten pool due to melt
through
Gas metal arc welding defects
Apart from slag inclusions, all the common weld defects that occur with other
processes may occur with GMAW. Defects such as porosity and lack of fusion can be a
particular problem with GMAW.
The defects commonly encountered in GMAW are:
•
porosity
•
cold lap/lack of fusion
•
lack of root penetration
•
excessive penetration
•
contour defects
•
undercut
•
weld cracking
•
excessive spatter
•
stray arcing.
Porosity
Defined as a pore or group of gas pores in the weld metal. Porosity may be
conveniently differentiated according to size and distribution. A number of different
terms are used related to size. These are:
•
gas pore − a cavity (usually spherical) formed by entrapped gas during the
solidification of molten metal
•
wormhole − an elongated or tubular cavity in the weld metal caused by entrapped
gas being forced away from the solidifying weld metal
•
cluster − a group of pores in close proximity to each other.
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Chapter 7 – Gas metal arc welding (GMAW)
As is the case with other welding processes, porosity may be caused by moisture, or
surface contaminants on the plate. With GMAW, by far the greatest cause of porosity is
due to inadequate gas shielding. This may be due to:
•
flow rate set too low
•
flow rate set too high
•
no gas flow at all
•
excessive wind or air movement at the gun
•
contaminated shielding gas
•
stick-out length too long
•
gun angle too low.
Lack of fusion
Defined as portions of the weld deposit which do not fuse to the surface of the metal
or the edge of the joint. With GMAW, lack of fusion is commonly referred to as ‘cold
lapping’ as it usually takes the form of lack of sidewall fusion over an extensive part of
the joint.
Cold lapping is common when welding in the dip transfer mode − particularly when the
plate thickness exceeds 5 mm. Welding downhill, or with high wire speed and low arc
voltage settings, further increases the risk of occurrence. Plates that are dirty or heavily
scaled further exacerbate the problem.
Cold lapping does not generally occur when welding in the spray transfer mode.
To minimise the likelihood of cold lapping, one or more of the following should be
employed:
•
weld in the spray transfer mode
•
clean plates
•
if in doubt, set the arc voltage slightly higher
•
set enough amperage to ensure sufficient heat for fusion
•
keep the electrical stickout short
•
use CO2 shielding gas or a mixed gas high in CO2.
Lack of root penetration
Defined as the failure of the weld metal to completely fill the root of the joint.
Root runs in butt welds are normally made in the dip transfer mode except for those
in heavy plate, in which case spray transfer would be used. The dip transfer mode
is inherently ‘cold’, employing low amperages and voltages. This means that root
penetration is limited in this mode.
The solution to overcoming lack of root fusion is to use thinner root faces on butt welds
than would be the case with other processes − typically in the range of ½ mm to 1 mm.
In fillet welds, the solution is to use comparatively high amperage settings when in the
dip transfer mode. Additionally, CO2 or a gas mixture high in CO2 will help.
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Excessive penetration
Defined as excess weld metal protruding through the root of a butt weld. This defect
normally only occurs on thin (sheet) materials or when the spray mode of transfer is
used. Adjustment of wire speed and arc voltage will usually overcome this problem with
relative ease.
Another form of this defect is electrode wire protruding through the root of the butt in
the form of ‘spikes’ or ‘icicles’. This is caused when arcing to the root face of the butt
weld momentarily ceases, a small amount of wire penetrates the butt, and the arc is
re-established when the wire contacts the parent metal.
The solution to this problem is to limit the width of the root gap and/or increase the
arc voltage, which results in a wider spread of the arc so that arcing to one or both
sides of the weld is always present.
Contour defects
Contour defects may be in the form of overroll or overlap, excessive convexity or
excessive concavity of the bead, or simply a rough, uneven appearance.
Travel speed and torch angle adjustments may fix many of these problems. The
GMAW operator can also control the weld profile by adjusting the arc voltage.
Excessive convexity may be remedied by increasing the arc voltage, and beads which
are too wide or too concave may be remedied by decreasing the arc voltage.
Undercut
Defined as a groove or channel in the parent metal, occurring continuously or
intermittently along the toes or edge of a weld.
Undercut is not a common problem in GMAW, however, it is likely to be encountered in
two situations.
1.
When fillet welding in spray transfer – This is normally caused by setting the arc
voltage too high, causing a long arc length which results in undercutting of the toe
of the weld of the vertical plate. To remedy this, set a smooth spray transfer mode
using the lowest arc voltage that will facilitate this. This solution is quite simple
and it is good practice for all welds in spray transfer.
2.
Vertical-up welds – Solid wires are largely unsuitable for making stringer beads
in the vertical-up position. Convex beads with some undercut generally result.
When a weave technique is used, a bead that is convex in the middle, with
undercut toes may result. The solution is to:
ο
ο
reduce the arc voltage
reduce the overall heat of the welding
ο
pause longer at the toes.
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Chapter 7 – Gas metal arc welding (GMAW)
Cracking
Defined as discontinuities produced either by the tearing of metal in the plastic
condition (hot cracks) or by fracturing when cold (cold cracks). Hot cracks are
common in materials with high co-efficients of expansion, and/or which suffer from hot
shortness. Hot cracking occurs at elevated temperatures soon after solidification. This
mode of cracking is common in aluminium and stainless steel. Cold cracking is most
common in hardenable materials, particularly when cooling rates are rapid. Cracking is
considered to be a serious defect and rarely is any amount of cracking tolerated.
Cracks may be also be described depending on how, when and where they occur,
for example longitudinal, transverse, crater, centre line, hot, cold, toe and underbead.
Cracks may occur in either the parent metal, usually as fusion or heat affected zone
cracks, or in the weld metal.
•
Hot cracking − Usually occurs in metals that are hot short and/or have high rates
of thermal expansion. Hot cracking most commonly occurs in the weld metal, with
longitudinal cracks and crater cracks being the most common examples.
•
Cold cracking − Most commonly occurs in the base metal adjacent to the fusion
zone. The most common example of this is underbead cracking in hardenable
steels.
•
Crater cracks − These come from hot shrinkage. The crater solidifies from all
sides toward the centre, leading to a high concentration of stress at the centre of
the crater. If the metal lacks ductility, or the hollow crater cannot accommodate
the shrinkage, cracking may result. Crater cracks may, under stress, propagate
from the crater and lead to failure of the weldment.
Cracking in GMA welds is not generally a major problem due to the following factors.
•
GMAW is a ‘low-hydrogen’ process.
•
Hollow craters are not usually a characteristic of GMA welds.
•
The inherent low heat input is ideal for stainless steels and other metals which
are prone to hot cracking.
Stray arcing
Defined as damage on the parent metal resulting from the accidental striking of an arc
away from the weld, or the accidental striking of an arc away from the weld.
Stray arcing is not a major problem associated with GMAW as the electrode is usually
only live when the gun trigger is depressed. Care should be taken that the gun is not
put down with the weight resting on the trigger, and also that arcing does not occur
between the job and the work return lead connection.
Excessive spatter
Defined as the metal particles expelled onto the surface of the parent metal or weld,
during welding, and not forming part of the weld.
This usually occurs due to one of the following factors:
152
•
shielding gas or plate contaminated with moisture
•
high levels of CO2 or O2 in the shielding gas
•
excessive arc voltage in the dip transfer mode
•
welding in the globular transfer mode.
Metals& Fabrication
Arc Welding 1
Spatter is not usually present in the spray transfer mode.
Trouble shooting/equipment malfunction
Compared to the manual welding processes, GMAW requires higher levels of care and
maintenance. Major sources of frustration are the problems associated with the feeding
of the electrode wire.
This is a particular problem when welding with aluminium wire, feeding wire through
long gun cables, or when using a gun cable that has been poorly maintained.
Equipment malfunctions with GMAW fall into two main categories; which are:
•
electrical
•
mechanical.
The main problems with regard to electrical malfunctions and their likely causes areas
follows.
Problem
No power at machine
Likely cause
Solution
Mains switch off
Check switches and fuses
Machine switched off
If intact, call an electrician
Blown fuse
Mains power on but no
welding power
Trigger switch not working
Wire feeds, but no arc
Work return not connected
Check work return
Blown fuse
Check fuses
Wire feeder not connected
Check if trigger is working, and
whether wire feeder will operate
and wire will feed
153
Chapter 7 – Gas metal arc welding (GMAW)
Mechanical problems manifest themselves in the form of wire feeding problems.
Common wire feeding problems and their likely causes are as follows.
Problem
No wire feed at all
Likely cause
Solution
Spool brake excessively tight
Check tension on spool brake
No friction at drive rolls
Check drive rolls and adjust as
necessary
Wire jammed at drive rolls or in
gun cable
Check guide tubes
Check wire conduit
Uneven wire feed
Dirty or damaged liner
Clean or replace
Slippage at drive rolls
Increase pressure
Liner cut too short
Replace
Kinks in gun cable
Keep as straight as possible
Insufficient roll pressure
Tighten drive rolls
Wire distorted due to excessive roll
pressure
Misalignment of drive rolls
Contact tip worn or dirty
Spool brake excessively tight
Damaged liner
Inspect and replace
Check tension on spool brake
Spool overrun
Spool brake too loose
Tighten
Wire fused to
contact tip
Excessive arc voltage
Reduce arc voltage
Excessive burnback time
Reduce burnback time
Intermittent wire feed
See above
GMAW equipment requires a regular inspection and maintenance schedule.
•
Contact tips should be inspected at least daily.
•
Liners, and drive rolls, and spool brake should be inspected weekly.
•
Gas and electrical connections should be inspected monthly.
Feeding aluminium wire presents additional problems. It is essential that all sources of
friction upon the wire be minimised. Recommendations are:
154
•
reduce spool braking
•
use a Teflon® liner
•
ensure the correct liner is used
•
keep the gun cable as straight as possible
•
avoid small diameter wire if possible
•
fit a straighter gooseneck to the gun
•
pay particular attention to drive roll pressure
•
use good quality wire.
Metals& Fabrication
Arc Welding 1
Additionally, a welding machine with the following features is highly recommended:
•
a push/pull gun
•
a four-roll wire feeder (‘U’ shaped bottom groove) and late-top roll
•
a soft-start feature.
Term
Teflon®
Definition
Teflon® is the brand name of a polymer compound.
A micro porous laminate on various nylon fabrics
forming a protective layer
Activity
Carry out the following in your workshop.
•
Identify safe working practices and protective equipment in the workshop.
•
Locate a suitable GMAW plant.
•
With the mains power switch OFF, set up the GMAW plant (start by checking
the electrical cable connections to the wire feed unit, work lead and hand piece).
•
Check the appropriate general purpose wire is fitted correctly into the wire feed
unit.
•
Remove the gas nozzle and check to see if the correct contact tip is fitted and the
nozzle and tip are clean.
•
Refit gas nozzle to give correct electrode stick-out.
•
Ask your lecturer to help you to set up the shielding gas flow at 12–14 L/min and
in setting the voltage and wire feed rate for dip transfer mode.
•
Strike an arc between the electrode and a practice piece of material.
•
With your lecturer’s assistance, produce welds using dip transfer, globular
transfer and spray arc on the practice material.
Refer to Australian Standard® AS/NZS 2717.1 Welding – Electrodes – Gas metal arc
– Ferritic steel electrodes, for a full understanding of the classification system for solid
wire.
155
156
Metals& Fabrication
Arc Welding 1
Chapter 8 –
Flux-cored arc welding (FCAW)
Introduction
The introduction of flux-cored wires (creating the newer and closely related process
of flux-cored arc welding) extended the range of work carried out by hand-held
semi-automatic welding. Flux-cored welding brought with it the advantages of
greater penetration, higher welding speeds, site welding capability, and the ability
for it to be applied to a variety of plate thicknesses.
In this chapter you will look at the following.
•
Principles
ο
advantages
ο
limitations
•
Equipment
•
Techniques for gas-shielded flux-cored arc welding
ο
electrode stick-out
ο
direction of travel
ο
position and angle of torch
ο
self-shielding flux-cored arc welding
■
advantages of self-shielding FCAW
■
limitations of self-shielding FCAW
■
techniques of self-shielding FCAW
■
electrode stick-out
■
electrode angles
■
vertical welding
ο
Welding procedures for FCAW
ο
Effects of the operating variables with FCAW
■
polarity
■
arc voltage
■
current (wire feed speed)
■
travel speed
■
electrode stick-out
ο
electrodes for flux-cored arc welding
ο
classification of flux-cored wire electrodes
ο
safety recommendations with FCAW
157
Chapter 8 – Flux-cored arc welding (FCAW)
•
Flux-cored arc welding faults
ο
cracking
ο
porosity
ο
slag inclusions
ο
lack of fusion/lack of penetration
ο
excessive penetration
ο
contour defects
ο
undercut
ο
excessive spatter
ο
stray arcing.
At the end of the chapter, you will complete an activity.
Principles
As the name implies, the flux-cored arc welding process employs an electrode which
is essentially a formed steel sheath in which is contained a core of flux. The electrode
has been described as a ‘stick’ electrode turned inside out and made into a continuous
wire.
There are two distinct types of FCAW welding.
158
•
Self-shielding FCAW, in which all of the shielding is provided by the
decomposition of the flux core. Self-shielding wire has the advantage that it is
suitable for use in windy conditions and is therefore ideally suited to site work.
Further to this, no shielding gas system is required.
•
Gas-shielded FCAW, which requires additional shielding gas. Gas-shielded wires
have the disadvantage of requiring a shielding gas system, but they produce
lower fume levels.
Metals& Fabrication
Direction
of travel
Arc Welding 1
Thread
protector
Current carrying
contact tip
Molten slag
Powered metal, vapour
or gas forming materials,
deoxidisers and scavengers
Solidified
slag
Arc shield composed of
vaporised and slag forming
compounds protects metal
transfer through arc
Molten
weld metal
Solidified
weld metal
Metal droplets covered
with thin slag coating,
forming molten puddle
Fig 8.1 – Flux-cored arc welding
A major advantage of FCAW compared to GMAW is that the higher current densities
used means that the mode of metal transfer across the arc is always spray transfer.
The advantages of this are:
•
higher deposition rates
•
deeper penetration
•
excellent fusion to the base metal.
The down side of this is:
•
higher emission of UV radiation
•
higher fume levels
•
more heat is generated.
The flux core serves as a medium to introduce de-oxidants and alloy elements into
the weld. The flux is low in hydrogen and the process is therefore suitable for welding
hardenable steels and other carbon and low-alloy steels.
Due to the limitations of the manufacturing technology available at the time, early
flux-cored wires were produced by applying the flux to a strip of metal, and then
forming it into a tube. Wires smaller than about 2 mm to 2.4 mm diameter could not be
produced by this method. This meant that, when low welding currents were required,
the current density in the wire was relatively low and the metal transfer across the arc
was relatively coarse and rough.
159
Chapter 8 – Flux-cored arc welding (FCAW)
Currently, flux-cored wires are produced in a number of configurations designed to
improve burn-off, as shown in Fig 8.2. They are also being manufactured by filling
a tube with flux, and drawing the wire to produce a seamless electrode in sizes as
low as 0.9 mm in diameter. This is a major advantage in that even though welding
current used may be low, the current density is high enough to ensure ideal transfer
characteristics across the range.
Steel sheath
Flux core
Fig 8.2 – Flux-cored wire
Advantages
Penetration – Compared with iron powder electrodes for MMAW, the depth of
penetration is much greater. This makes it possible to reduce the fillet leg length
without decreasing the strength of the weld (Fig 8.3).
Tubular
electrode
IP
electrode
Fig 8.3 – Comparison of penetration
Deposition rate – Compared with manual metal arc electrodes, the deposition rate is
very high.
Slag detachability – Providing that the operating conditions are correct, the slag is
virtually self-detaching. In a deep groove, the slag is easily removed when the weld has
cooled.
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Metals& Fabrication
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Appearance – Providing the operating conditions are correct the weld appearance
is bright and neatly rippled with a good ‘wash’ into the parent metal at the toes. Fillet
welds tend to be mitred or slightly concave rather than convex.
Weld quality – The weld deposit is low in hydrogen content and has good mechanical
properties. Sound radiographic quality welds can be achieved.
Low spatter – Assuming the correct operating conditions have been selected, spatter
should be minimal.
Visibility – Because of its high deposition and high penetration characteristics,
gas-shielded flux-cored arc welding is often compared with submerged arc welding,
which can offer similar advantages. With the FCAW process, however, the operator can
see the arc and be in a position to allow for variations in the joint fit-up.
Limitations
•
Limited applications – The range of FCAW consumable currently available are
limited to ferrous-based alloys such as steel. Constant development means there
is potential for a much greater range of materials that may be welded with FCAW
in the future.
•
Loss of gas shielding – The gas-shielded FCAW is only suitable for sheltered
conditions, away from any wind that may interfere with the gas shielding. For
this reason the process is not usually suitable for outdoor work, unless adequate
steps are taken to screen the arc from the wind. Loss of gas shielding can cause
severe porosity in the weld. Self-shielding wires do not suffer from this problem.
•
Operator fatigue – With the smaller diameter of positional wires, operator fatigue
is no greater than that experienced with GMAW. However, when used as a
high deposition process, the welding gun and cables must be robust enough to
withstand the heat generated and are usually rather heavy. This, together with
the hot conditions, makes operator fatigue a significant factor. This problem can
be overcome by mechanising the process.
•
Fumes – Many FCAW wires (particularly self-shielding wires) emit a substantial
volume of fumes which add to the discomfort of the operator. Special precautions
may be required to eliminate these fumes, such as the use of fume extractor
nozzles fitted to the gun. In confined spaces, fume extraction units will be needed
to remove fumes from the work area into filter banks or outside the workshop.
Equipment
The equipment required is essentially the same as that used for GMAW however the
component parts may be heavier duty. Electrode positive is generally required for gas
assisted wires whilst most of the self-shielding wires use electrode negative.
A constant voltage DC power source is generally used, however there are some newer
wire feeders that incorporate a wire feed-rate compensating circuit and these will
operate successfully on constant current power supplies.
The wire feed unit used for GMA welding can usually be adapted for FCAW. The wire
reel holder may need to be changed to carry the spool of flux core wire which is usually
supplied in 30 kg reels. The wire drive rolls may be serrated or have a 15° ‘V’ groove.
Care must be taken to minimise the pressure on the feed rolls so that the wire is not
squeezed out of round.
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Chapter 8 – Flux-cored arc welding (FCAW)
The welding gun preferred is the pistol type where the wire is kept straight as it passes
through. Goose necked guns with a small radius bend tend to create a ‘drag’ on the
wire thus giving rise to wire feed problems.
Due to the high amperages employed, heat radiation is intense and therefore the
welding torch is sometimes fitted with a heat shield at the handle.
Power
cable
Contactor
leads
Electrode
Goose neck
Gas line
Water lines
Electrode
Contactor
leads
Gas line
Pistol
Fig 8.4 – FCAW equipment and pistol and goose necked guns
The welding cable and return lead must be of sufficient size to carry the high currents
without overheating.
Techniques for gas-shielded flux-cored arc welding
A welding operator with a reasonable degree of skill in MMA or GMA welding can
readily adapt to gas-shielded FCAW, however, a few factors need attention.
Electrode stick-out
Recommended stick-out lengths must be adhered to – they tend to be greater than
the stick-out lengths used with GMAW. Stick-out is the length of the wire from the end
of the contact tip to the surface of the work piece. A shorter stick-out could result in a
poorly shaped weld due to an increase in amps and a decrease in voltage. A longer
stick-out could give rise to excessive spatter and porosity in the weld due to poor gas
shielding when using gas-shielded wires.
Direction of travel
The direction of travel (whether pushing the torch or dragging it) is usually a matter of
personal preference on the part of the operator. However, where the work is to be of a
particularly high quality, the backhand or drag method is regarded as superior.
162
Metals& Fabrication
Arc Welding 1
Position and angle of torch
2° to 15°
Direction
of travel
Fig 8.5 – Angle of torch to direction of travel
As already mentioned, it is preferable to push the arc with gas assisted wires and most
self-shielded wires should be dragged (although not always essential). Fig 8.5 shows
the recommended angle of the torch in relation to direction of travel.
In the flat position, the torch is angled at 90° to the plate (Fig 8.6).
90°
Fig 8.6 – Position of torch for flat butt weld
Self-shielding flux-cored arc welding
This is probably best regarded as a semi-automatic version of the manual metal arc
process. Like MMAW, the flux-cored wire generates sufficient vaporised gases around
the arc to completely protect it from the atmosphere.
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Chapter 8 – Flux-cored arc welding (FCAW)
Advantages of self-shielding flux-cored arc welding
•
No external shielding gas or flux is required, therefore the process can easily be
used outdoors even in draughty conditions.
•
All positional wires, hard facing and stainless steel wires are available.
•
Deposition rates are high when compared with MMA welding.
•
Slag is easily detached except where tacks have been made with cellulose or
rutile electrodes.
•
The weld deposit is low in hydrogen and resists cracking in many crack-sensitive
applications.
•
Poor fit-ups (gaps) can be handled easily by increasing wire stick-out.
Limitations of self-shielding flux-cored arc welding
•
Penetration is not as great as the gas-shielded FCAW process. It is more akin to
that achieved with MMA low hydrogen electrodes.
•
Slag removal is difficult when welding over tacks or a previous weld made
with cellulose or general purpose electrodes. The use of low-silicon cellulose
electrodes or certain low-hydrogen electrodes can overcome this problem.
•
Fumes can also be a problem and the precautions outlined previously for
gas-shielded FCAW may be necessary.
Techniques of self-shielding flux-cored arc welding
Welding techniques are similar to those employed with hydrogen-controlled MMAW
electrodes, however, a few additional factors should be considered.
Electrode stick-out
For all positional self-shielding electrode wires, the recommended electrode stick-out
is usually 18−20 mm. If no gas nozzle is used with these wires, the electrode stick-out
is visible from the contact tip to the work (Fig 8.7). Even though no gas shielding is
employed, a nozzle is commonly used to give the operator the feel of ‘normal’ electrical
stick-out.
164
Metals& Fabrication
Arc Welding 1
Thread
protector
Contact tip
Electrode
stick-out
Work
Fig 8.7 – Electrode stick-out is visible stick-out
Some self-shielding electrode wires are designed to give high deposition rates in the
downhand positions by employing long electrical stick-out (Fig 8.8).
A long electrical stick-out is used to increase the deposition rate by pre-heating the wire
before it is melted at the arc. The recommended electrical stick-out varies, depending
on the type and size of wire and the wire manufacturer’s recommendations. To assist
the operator in maintaining the correct electrical stick-out for these wires, the welding
gun can be fitted with a nozzle incorporating an insulated extension guide.
165
Chapter 8 – Flux-cored arc welding (FCAW)
Electrode
guide tube
Insulated
extension guide
Contact tip
Electrical
stick-out
Electrode
Visible
stick-out
Fig 8.8 – Insulated extension guide nozzle for electrical stick-out
Electrode angles
When welding with self-shielding flux-cored wires, the electrode angles are much the
same as for MMAW electrodes, as shown in Fig 8.9.
Drag angle
Travel
Fig 8.9 – Self-shielding wires are dragged similarly to MMAW electrodes
166
Metals& Fabrication
Arc Welding 1
For horizontal/vertical welds the wire is pointed directly into the root of the joint at an
approach of 40° (Fig 8.10).
40°
Fig 8.10 – Recommended electrode angle for horizontal/vertical fillet weld
Vertical welding
Most all-positional self-shielding wires can be used vertical-down or vertical-up.
Vertical-down is usually preferred for welds in thinner sections or for the first pass in a
butt weld. The gun is tilted to a drag angle of 10−15° from the horizontal so that the arc
force helps hold the molten metal in the joint (Fig 8.11).
Techniques for welding vertical-up are the same as for low-hydrogen MMAW
electrodes. Vertical-up welding is recommended for welds in thick sections. The first
pass in a vertical-up fillet or butt is best made using a triangular weave technique and
subsequent passes are made with a side-to-side weave.
Direction of travel
10° - 15°
Fig 8.11 – Electrode angle for welding vertical-down
Welding procedures for flux-cored arc welding
There is a wide variety of flux-cored electrode wires for both the gas-shielded and
self-shielding processes. Each wire has its own set of optimum operating conditions
and procedures and therefore it is best to consult the wire manufacturer’s table
to obtain the recommended welding procedure for a particular wire. However, the
following effects of the operating variables associated with the FCAW process may
help to refine the set procedures.
167
Chapter 8 – Flux-cored arc welding (FCAW)
Effects of the operating variables with flux-cored arc welding
With FCAW there are five major operating variables, which are:
•
polarity
•
arc voltage
•
current (wire feed speed)
•
travel speed
•
electrode stick-out.
Polarity
Whereas all solid wires for GMAW run on DC + ve, some flux-cored wires are designed
to run on negative polarity.
Arc voltage
If the other variables are held constant, arc voltage variations have the following
effects.
•
Higher arc voltage gives a wider and flatter bead shape.
•
Excessive arc voltage can cause porosity.
•
Low voltage causes a convex, ropey bead shape.
•
Extremely low voltage will cause the electrode wire to stub on the parent metal.
The arc voltage should be set according to the wire manufacturer’s recommendations
and, if necessary, be fine-tuned to give the desired bead shape.
Term
ammeter
Definition
an instrument that measures electric current in
amperes
Current (wire feed speed)
In setting critical procedures, wire feed speed is a better measure than the welding
current. The wire feed speed is constant whereas the current reading at the ammeter
tends to fluctuate.
If the other variables are held constant, current variations have the following major
effects:
•
increasing the current increases penetration and deposition rate
•
excessive current produces convex, ropey bead shapes
•
current that is too low gives a large droplet transfer and may give porosity.
As the current is increased or decreased, the arc voltage must be increased or
decreased to maintain the proper bead shape. The correct current range should be
obtained from the wire manufacturer’s tables.
168
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Arc Welding 1
Travel speed
If the other variables are held constant, travel speed variations have the following
effects.
•
A too high travel speed increases the convexity of the bead and causes uneven
edges.
•
A too slow travel speed results in slag interference, possible slag inclusions and a
rough uneven bead shape.
Electrode stick-out
If the other variables are held constant, variations in stick-out have the following effects.
•
Increasing stick-out decreases the welding current.
•
Decreasing stick-out increases the welding current.
•
With self-shielding wires the stick-out can be increased to reduce the penetration
thereby allowing poor fit-ups to be bridged.
•
Decreasing stick-out can lead to spatter build-up on the contact tip or overheating
of the contact tip.
Electrodes for flux-cored arc welding
Early electrode wires developed for gas-shielded flux-cored arc welding used a basic
type flux with low-hydrogen content and required electrode positive polarity. This type
of flux-cored wire electrode is still popular.
More recent developments have led to the availability of electrode wires with a rutile
flux suitable for more general purpose work. Many of these rutile flux-cored wires
perform better with electrode negative polarity.
Virtually all flux-cored wires, whether gas shielded or gasless, require a constant
voltage power source. However, there are a few types which can operate satisfactorily
with constant current power sources.
Due to this range of variables stated, it became necessary to provide a system of
classification for flux-cored electrode wires, an outline of which follows.
Classification of flux-cored wire electrodes
There are many different types of solid and flux-cored electrode wires commercially
available. They are classified to a particular standard, which makes it possible to
identify and select the most suitable type of wire for a job. It is important to understand
classification systems and the information they represent.
Consumable classification systems list a number of essential features about the
consumable. For example, consumables are classified by construction, filler metal
composition, shielding method, mechanical strength of the weld deposit and so on.
AS/NZS 18276 and 17632 classifies flux-cored electrodes under three groups of
symbols separated by hyphens. Each group consists of a number of letters or letters
and numbers.
eg ETD - GCp – W503H
169
Chapter 8 – Flux-cored arc welding (FCAW)
First group – construction and recommended welding position (ETD)
The first group of letters denotes a tubular electrode, and indicates the following.
E
=
electrode
T
=
tubular construction
D
=
horizontal fillet or flat position
P
=
any position
S
=
single run (if applicable)
Second group – shielding requirements and current type (GCp)
The second group consists of G for gas shielded and then two letters that indicate the
type of shielding gas used during qualification tests and the welding current required.
G =
gas shielding which is then followed by one of the following.
C
=
shielded with carbon dioxide (CO2)
M
=
shielding with a mixture of gases
N
=
no shielding required
p
=
d.c. constant potential, electrode positive
n
=
d.c. constant potential, electrode negative
a
=
a.c.
For example, Nn indicates that the wire is self shielded with negative electrode.
Third group – properties of the weld metal
The third group involves a letter W followed by a three-digit number. W stands for
weld metal. The first two digits refer to the minimum strength of the deposited weld,
which is measured in megapascals. The third digit refers to the minimum impact value.
The letter H generally completes the classification which indicates that the process is
hydrogen-controlled.
170
W
=
weld metal properties:
50
=
500 MPa strength
3
=
degree of impact test
H
=
hydrogen-controlled
Metals& Fabrication
Arc Welding 1
From the reference chart in the code, this electrode will meet an expected impact value
of 47 joules at -20 °C.
a
b
c
Symbol
Temperature for minimum
average impact energy of
47 J a,b or J c
za
no requirements
Ab of Yc
+ 20
0
0
2
-20
3
-30
4
-40
5
-50
Only the symbol Z is used for electrodes for the single-run technique.
Classification by yield strength and 47 J impact energy.
Classification by tensile strength and impact energy.
Based on AS/NZS 17632: 2006 – Table 3 (www.saiglobal.com)
Due to earlier problems related to flux-cored wires achieving satisfactory impact values
in the completed welds, further information is sometimes added to the final group of
numbers. For example, an electrode is classified in the following way.
AS/18276 and 17632 ETDS – GM/Cp – W503A CM1 H10
A
= classified in the as-weld condition or P
heat treatment
=
classified after post-weld
CM = carbon manganese or K = killed (double deoxidised)
H5
diffusible hydrogen content of deposited weld metal < 5 ml/100 g of weld
H10
diffusible hydrogen content of deposited weld metal < 10 mL/100 g of weld
H15
diffusible hydrogen content of deposited weld metal < 15 ml/100 g of weld
For example, this weld deposit would be carbon manganese alloy of 500 MPa,
minimum strength in the as-welded state and contain 10 mL of hydrogen per 100 g of
weld.
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Chapter 8 – Flux-cored arc welding (FCAW)
An example of the full classification system is shown below.
Electrode
Weld metal
Shielding
Tubular
Type of
shielding gas
Approximately
0.1 x tensile
strength in MPa
Impact energy
grade number
Position
Hydrogencontrolled
E
T
X
s
-
G
X
-
W
XX
X
H
The classification system consists of three groups, separated by a hyphen. Each group
consists of a letter or letters and figures. To assist with the above explanation the
symbols X1, X2, X etc are used to represent the variables.
Example1
Fluxed-cored electrode
Metal-Cor 2
E
= Electrode
T
= Tubular
D
= Downhand
M
= Mixed gas
p
= DC constant voltage electrode positive
AS/NZS 18276 and 17632
ETD − Mp − W502 (H)
W = Weld metal
50 = 500 MPa minimum tensile strength
2
= Charpy v-notch impact test 47 J at 0 °C
H
= Hydrogen-controlled
Example 2
Fluxed-cored electrode
Lincoln innershield NR211
172
E
=
Electrode
T
=
Tubular
P
=
Any position
N
=
No external shielding (self-shielding)
n
=
DC constant voltage electrode negative
W =
Weld metal
50 =
500 MPa minimum tensile strength
0
Charpy v-notch impact test not required
=
AS/NZS 18276 and 17632
ETP − Nn − W500
Metals& Fabrication
Arc Welding 1
Safety recommendations with flux-cored arc welding
The following recommendations apply to FCAW.
•
Due to of the greater arc intensity, particularly with the gas-shielded FCAW
process, a welding lens one to two shades darker than for MMAW should be
used.
•
A heat shield fitted to the torch handle is desirable to protect the operator’s hand
from radiated heat. Reflective backed leather gloves are also recommended.
•
Additional care should be taken regarding clothing and protective leathers. Dark
woollen clothing is most desirable, and leather gloves, apron, jacket and spats
should be worn.
•
Attention should be given to ensuring adequate ventilation. If natural ventilation is
inadequate, exhaust fans and respirators should be used.
Flux-cored arc welding faults
The defects commonly encountered in FCAW are:
•
weld cracking
•
porosity
•
slag inclusions
•
lack of fusion
•
insufficient or excessive penetration
•
contour faults
•
undercut
•
excessive spatter
•
stray arcing.
Cracking
Cracking is considered to be a serious weld fault, and rarely is any amount of cracking
tolerated.
Cracks may be described depending on how, when and where they occur, eg
longitudinal, transverse, crater, centre line, hot, cold, toe and underbead. Cracks may
occur in either the parent metal, usually as fusion or heat affected zone cracks, or in
the weld metal.
Crater cracks occur when the weld solidifies from all sides toward the centre leading
to a high concentration of stress at the centre of the crater. If the metal lacks ductility,
or the hollow crater cannot accommodate the shrinkage, cracking may result. Crater
cracks may, under stress, propagate from the crater and lead to failure of the weld.
Cracking in FCAW welds on mild steel is not generally a major problem.
173
Chapter 8 – Flux-cored arc welding (FCAW)
Porosity
Porosity in FCA welds may be the result of welding on a parent metal that is
susceptible to this condition (such as steel that contains high amounts of dissolved
gases or sulphur). Porosity may also be caused by welding on dirty material or
material contaminated with moisture, oil, paint or grease. The electrode may have
been contaminated, or too much voltage or current has been used. The shielding gas
may not be the correct type to suit the wire. The gas flow may be set incorrectly or be
affected by wind. Too long an arc length may have been used.
Slag inclusions
Slag inclusions are not generally a problem in FCA due to the high heat input. If they
do occur in FCAW they can occur at the weld root, between weld runs, or on the weld
surface. They may occur as a result of low voltage or amperage, or poor electrode
manipulation. Slag inclusions can occur when incorrect joint preparations are used, or
when material is dirty or contaminated.
Lack of fusion/lack of root penetration
With FCAW, lack of fusion or lack of root penetration is not normally a problem but may
be caused by working with incorrect joint configuration, low amperage, working on dirty
or contaminated material or using the wrong electrode angles or travel rate.
Excessive penetration
Excess weld metal protruding through the root of a butt weld may occur in FCAW
because of incorrect joint preparation, wrong electrode choice, excessive amperage or
incorrect variables.
Contour defects
Contour defects may be in the form of insufficient or excessive leg size, overroll or
overlap, excessive convexity or concavity of the bead, or simply a rough, uneven
appearance.
These are mainly caused by the operator but using the correct electrode, voltage,
amperage, travel speed and electrode angle adjustments may solve many of the
problems.
Undercut
Undercut in FCAW is defined as a groove or channel in the parent metal, occurring
continuously or intermittently along the toes or edge of a weld.
Undercut is a common problem in FCAW and may be caused by excessive voltage or
amperage, too long an arc length, wrong electrode angles, or wrong travel rate.
Excessive spatter
Spatter is a normal part of welding and FCAW does not normally produce excessive
spatter.
Stray arcing
Defined as damage to the parent metal resulting from the accidental striking of an arc
away from the weld.
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Arc Welding 1
Even though stray arcing is not usually a major problem associated with the FCAW
of mild steel, it is good practice to take precautions against accidental arcing of the
electrode anywhere other than in the weld zone.
Stray arcing can lead to serious weld failure in a material that is crack sensitive, or is
going to be put into a stressed situation.
Activity
•
Identify safe working practices and protective equipment in the workshop.
•
Locate a suitable gas shielded FCAW plant.
•
With the mains power switch OFF, set up the FCAW welding plant (start by
checking the electrical cable connections to the wire feed unit, work lead and
hand piece).
•
Check the appropriate flux-cored wire is fitted into the wire feed unit.
•
Remove the gas nozzle and check to see if the correct contact tip is fitted and
that the nozzle and tip are clean.
•
Refit the gas nozzle to give correct electrode stick-out.
•
Ask your lecturer to help you set up the shielding gas flow at 12–14 L/min and in
setting the correct voltage and wire feed rate.
•
Strike an arc between the electrode and a practice piece of material.
•
With your lecturer’s assistance, produce welds on a practice piece of material.
Your lecturer should also be able to demonstrate the equipment and set up required for
self-shielding wires.
Refer to Australian Standard®:
•
AS/NZS ISO 17632:2006 (in part) Welding consumables – Tubular cored
electrodes for gas shielded and non-gas shielded metal arc welding of non-alloy
and fine grain steels – Classification (ISO 17632:2004, MOD).
•
AS/NZS ISO 17634:2006 (in part) Welding consumables – Tubular cored
electrodes for gas shielded metal arc welding of creep-resisting steels –
Classification (ISO 17634:2004, MOD).
•
AS/NZS ISO 18276:2006 (in part) Welding consumables – Tubular cored
electrodes for gas shielded and non-gas shielded metal arc welding of
high-strength steels – Classification (ISO 18276:2005, MOD).
175
176
Metals& Fabrication
Arc Welding 1
Chapter 9 –
Submerged arc and electro-slag
welding processes (SAW)
Introduction
The principles of submerged arc and electro-slag welding processes are similar, to
the extent that they are ideally suited to joints in very heavy materials. Both processes
make use of a continuously fed filler wire and a granular flux. Deposition rates are high
and weld quality is excellent, providing very economical welded joints.
The basic difference between the two processes is that submerged arc is applied to
joints in the flat position, and electro-slag to joints in the vertical position.
In this chapter you will look at the following.
•
Submerged arc welding
ο
ο
principles
■
the process
■
limitations of the process
Metals weldable
■
process advantages
■
process limitations
ο
power source
ο
current selection: alternating current or direct current
ο
control box/head unit
■
•
submerged arc welding consumables classification
Variables
ο
the effects of welding variables
ο
arc voltage
ο
wire feed control (amperage)
ο
travel rate
ο
flux
ο
process requirements
ο
weld backing
ο
causes of cracking
177
Chapter 9 – Submerged arc and electro-slag welding processes (SAW)
•
Electro-slag welding
ο
preparation
ο
copper mould
ο
flux
ο
wires
ο
power source
ο
metallurgical aspects
ο
summary
■
advantages
■
limitations.
At the end of this chapter, you will complete an activity.
Submerged arc welding
Principles
Submerged arc welding is defined as the process where the heat required for welding
by an electric arc (or arcs) is created between a bare metal electrode (or electrodes)
and the work piece. The weld area is completely shielded by a blanket of finely crushed
mineral composition (flux) making the arc invisible; hence the term `submerged arc’
(see Fig 9.1). The flux, when cold, is a non-conductor of electricity but in the molten
state is highly conductive.
Term
Definition
conductor
a material that permits the easy flow of electricity
electrode
an electrically conductive structure which transfers
electrons to or from reactant atoms or molecules
During the welding operation, the flux in the vicinity of the arc fuses and forms an
airtight slag to protect the molten metal from oxygen and nitrogen in the atmosphere
and to slow down the cooling rate. As welding progresses there is no visible arc and
a complete absence of spatter. The fused flux is easily removed when cool and the
unfused flux is recovered for re-use.
The process was developed primarily for the production of high quality butt welds
at increased welding rates. The operation is carried out by a unit which moves at a
controlled speed along the joint to be welded. For circumferential joints, the work piece
is rotated beneath a stationary welding head.
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Arc Welding 1
Molten flux
Slag
Direction of travel
From flux
hopper
Electrode
Granular
flux blanket
Solidified weld
metal
Molten welded
metal
Arc path
Base metal
or work
Fig 9.1 – Submerged arc welding
The process
Although the process is particularly suited to welding heavy plate, (plate 75 mm thick
has been welded in one pass) with smaller filler rod and low amperages, it can be used
successfully on material as light as 2.6 mm. Usually high welding currents can be used
on heavy sections – in some cases as high as 4000 amps. This allows faster weld
deposits to be made with very deep penetration. Plates up to 12.5 mm thick can be
welded without edge preparation. On thicker plate a relatively small preparation is used
permitting the use of smaller amounts of deposited metal.
Submerged arc welding has been carried out using two or three wires simultaneously,
with welding speeds as high as 2.5 metres per minute. It can also be used manually,
which makes the process flexible for repetition work where complicated shapes make
fixturing too difficult. Submerged arc welding is a fast and economical method of
welding when large diameter rod and high amperage are used. Absence of spatter
and easy slag removal facilitates post cleaning. Completed welds are of a high quality
with uniform appearance. Travel speed is predetermined and arc length is adjusted
automatically eliminating human errors which often occur with manual arc welding
processes.
Term
alloy
Definition
a mixture of two or more metals
179
Chapter 9 – Submerged arc and electro-slag welding processes (SAW)
Limitations of the process
Major limitations of submerged arc welding are the inability to cope with positional
welding and the almost exclusive use on mild and low-alloy, high strength steels.
The high heat input, slow cooling cycle can be a problem when welding quenched
and tempered steels. The heat input limitations of the steel being used must be strictly
adhered to when using submerged arc welding. This may require the making of
multipass welds where a single pass weld would be acceptable in mild steel. In some
cases, the economics may be reduced to the point where flux-cored arc welding or
some other process should be considered.
In semi-automatic submerged arc welding, the inability to see the arc and puddle can
be a disadvantage in reaching the root of a groove weld and properly filling or sizing.
Metals weldable
•
Low and medium carbon steels.
•
Low alloy, high strength steels.
•
Quenched and tempered steels.
•
Many types of stainless steels.
•
Copper and nickel alloys have been welded experimentally.
Process advantages
•
High quality of the weld metal (ie good mechanical properties).
•
Extremely high deposition rate and speed due to high current densities (ie amps
to wire ratio).
•
Uniform bead appearance with no spatter.
•
Little or no fumes.
•
No visible arc flash.
•
Easily automated for high operator factor.
•
Multiple electrodes can be used (ie tandem or twin arcs).
•
Deep penetration can be attained.
•
Penetration can be closely controlled.
•
Thick joints can be welded in one pass.
•
Less distortion compared to other processes.
•
Flux blanket prevents any rapid escape of heat.
•
Small preparations only are required, hence there is a minimum of filler metal
required per metre of joint.
Process limitations
180
•
High initial cost of equipment.
•
Not usually suited for jobs requiring only short runs.
•
The process is intolerant to such factors as poor fit-up, dirt.
Metals& Fabrication
Flux hopper
Arc Welding 1
Wire reel
Handheld gun
Electrode
wire
Wire feeder
Power source
Control system
OR
Electrode lead
Direction of travel
Flux
Work lead
Base metal
Auto-torch
Fig 9.2 –Submerged arc welding equipment
Power source
The power source for submerged arc welding can be either constant voltage or
constant current type. Constant voltage is often used with small wire sizes where the
self-adjusting arc properties are most useful. Constant current type power sources are
generally used on larger wires and these require feedback circuits that change the wire
feed rate to compensate for any arc length or wire burn off variations.
Machines can be either generator or transformer – rectifier power sources, with the
latter being the most common. These machines range in capacity from 300 amperes to
1500 amperes and must be rated for a 100% duty cycle. They may also be connected
in parallel to provide extra power for high-current applications. Both AC or DC current
can be used. Multiple electrode systems require specialised types of circuits especially
when AC is employed.
Current selection: alternating current or direct current
The differences in arcing and operating characteristics obtained from AC and DC
welding supply with the submerged arc welding process has a slight bearing upon the
suitability for a particular application. Generally, the majority of applications can be
carried out equally effectively using AC or DC, but there are definite advantages at the
extremities of the application range.
When selecting the most suitable welding supply, direct current gives the operator
greater control over bead shape and penetration. Direct current electrode negative has
a slight advantage in regard to deposition rate (this is the reverse of GMAW), whilst
electrode positive has a slight advantage where deep penetration is required. Light
gauge applications up to 3 mm are best carried out with DC current. Weld starting is
more positive with DC and the arc is more stable at the lower currents with increased
travel speeds possible.
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Chapter 9 – Submerged arc and electro-slag welding processes (SAW)
Square-edge and prepared-edge butt welds, and standing and positioned fillet welds
from 5 mm and upwards, may be carried out with either type of power supply with equal
results. However, arc blow or magnetic disturbance may cause deposit deformation
when welding inside circular vessels with a DC supply.
Arc blow is more likely to occur at higher currents with DC, especially on applications
such as heavy wall thickness circular vessels of comparatively small dimensions, and
also geometrically complex weldments.
Photographs reproduced with permission of Lincoln Electric (Aust) Pty Ltd
Fig 9.3 – Twin/tandem arc
Multiple electrode techniques such as twin arc (side-by-side) or tandem arc (where one
automatic welding head follows another in a common molten pool) are used for higher
productivity rates. Where more than one power supply is used, one of the electrodes
is connected to an AC welding supply to cancel any magnetic effects and help arc
stability. Both electrodes can be operated from an AC welding supply, but higher
welding speeds are possible with an AC/DC combination.
Control box/head unit
The head unit consists of a control box that provides adjustment for the arc voltage,
amperage and travel rate. It may have additional functions such as wire inch, start, stop
and crater fill. The welding head also houses a large reel of wire that is lightly covered
with copper to improve its transport and electrical properties, a wire straightener and
a wire feed unit. A flux feed hopper and delivery system is also provided to deposit the
dry, finely divided, free-flowing flux automatically along the weld joint. The unit may
also have a travel motor that can allow the unit to travel at a predetermined speed
(forward or reverse) along a weld joint. It may also have arc length and seam tracking
adjustments.
182
Metals& Fabrication
Arc Welding 1
Photographs reproduced with permission of Lincoln Electric (Aust) Pty Ltd
Fig 9.4 – Head unit
Submerged arc welding consumables classification
There are many different types of electrode wires commercially available. They are
classified to a particular standard, which makes it possible to identify and select the
most suitable type of wire for a job. It is important to understand classification systems
and the information they represent.
Consumable classification systems list a number of essential features about the
consumable, for example, consumables are classified according to filler metal
composition, flux method, weld deposit and so on.
Australian Standard® AS 1858.1 Electrodes and fluxes for submerged-arc welding
– Carbon steels and carbon-manganese steels classifies solid wire electrodes under
three groups of elements separated by hyphens. Each group consists of a letter(s) and
number(s).
Example
EL12K - FMM – W504
Group 1 (EL12K)
The first group of letters denotes a solid electrode, and indicates the following.
E
=
electrode
L
=
low manganese content:
L
(low)
M
(medium)
H
(high)
183
Chapter 9 – Submerged arc and electro-slag welding processes (SAW)
12
K
=
a number indicating percentage of carbon:
=
12
(0.12%)
8
(0.8%)
13
(0.13%)
14
(0.14%)
killed (double de-oxidised)
Group 2 (FMM)
The second group consists of F for flux shield and then two letters that indicate the type
of flux and contribution to weld metal
F – flux shield which is then followed by one of the following.
M
=
multi run
S
=
single run
B
=
basic flux
G
=
general flux
M
=
increase in manganese
M
(little or no increase)
L
(moderate)
H
(high)
Group 3 (W504)
The third group involves a letter W followed by a three-digit number. W stands for weld
metal. The first two digits refer to the minimum strength of the deposited weld, which is
measured in megapascals. The third digit refers to the minimum impact value.
W – weld metal properties.
50
=
500 MPa strength
4
=
degree of impact test
For example, W502H indicates the weld strength is 500 Mpa and low in hydrogen.
An example of the full classification system is shown below.
E L 12 K – F M M – W 50 4
Denotes
solid
electrode
Low
manganese
content
184
Killed
(de-oxidised)
Percentage
of carbon
Multi run
Flux shield
Weld metal
properties
Moderate
increase in
manganese
Degree of
impact test
Approximately
0.1 x tensile
strength in MPa
Metals& Fabrication
Arc Welding 1
Variables
The effect of welding variables
The major variables that affect the weld involve heat input (including the arc voltage),
the welding current, and the travel speed. The quality of the finished weld depends
almost entirely upon their proper selection and control.
The variables, in the approximate order of importance, which must be set and
maintained during welding are voltage, the current, weld speed, and width/depth of flux.
Arc voltage
As the arc voltage is reduced, so the tip of the welding wires will operate at a lower
level, giving a narrower weld with deeper penetration than a higher arc voltage would
give under the same current and speed condition. With high arc voltage, the wire tip
operates at a high level, allowing the metal to spread out, giving a wider weld with less
penetration. It also allows the fusing of slightly more flux than in the former case.
An extremely low arc voltage for a given current setting, with the tip of the welding
wire operating at a lower level (which could be well below the surface of plate), will
cause the molten deposited metal to be forced up around the sides and the rear of the
crater. The resultant bead will be rough, irregular, and comparatively high and narrow,
with visible gas holes sometimes occurring in the crater. With an excessively high arc
voltage under the same current conditions, the tip of the welding wire being rather
high above the plate surface will mean that the covering flux will tend to extinguish
the arc. The resultant bead will again tend to be rough and irregular, but in this case
comparatively flat and wide.
Mechanical adjustments may be necessary to keep the process in the line of weld and
to maintain a uniform stick-out length of wire.
Wire feed control (amperage)
This may be either constant feed or voltage control.
Constant feed control maintains the wire speed by means of some form of governor.
This control is often used with constant potential power sources. The desired arc length
(or arc voltage) is selected by setting the constant voltage power source output voltage
at a suitable value.
Voltage controlled, wire feed motors are used for constant current. The control used
may be a ‘series control’, which is essentially an electric motor that is highly responsive
to arc voltage variations, or it could be an electronic device which senses arc voltage
variations and promotes motor response to these changes as they occur.
If for any short period of time the current melts-off filler wire at a faster rate than it is
being fed, the distance between the wire and the work will increase as will the arc
voltage. This increase in arc voltage speeds up the wire speed motor and restores the
wire tip-work relationship as previously established.
With either control system, the most critical variables; arc voltage and arc current, are
maintained at constant levels.
Typically, any increase in wire feed rate will increase amperage, penetration, and
deposition rate.
185
Chapter 9 – Submerged arc and electro-slag welding processes (SAW)
Travel rate
Weld size and shape are affected by travel speed. Any increase in travel speed will
reduce weld size and produce a narrower weld bead. Penetration is also affected by
travel speed, an increase over normal settings will give a proportionate decrease in
depth of penetration.
Flux
Submerged arc welding flux shields the arc, and the molten weld metal, from the
harmful effect of atmospheric oxygen and nitrogen. The flux contains de-oxidisers and
scavengers which help to remove impurities from the molten weld metal. Flux also
provides a means of introducing alloys into the weld metal. As the molten flux cools to a
glassy slag, it forms a covering that protects the surface of the weld.
The non-melted portion of the flux does not change its form, its properties are not
affected, and it can be recovered and re-used. The flux that does melt forming the slag
covering must be removed from the weld bead. This is easily done after the weld has
cooled and in many cases will actually peel without requiring special effort for removal.
In groove welds, the solidified slag may have to be removed by a chipping hammer.
Fluxes are available in various types similar to MMAW, namely rutile, acid, or basic
types and these are formulated for specific applications and for specific types of weld
deposits. Due to a large part of the flux interacting with the molten weld pool, another
method is often used to differentiate between various types of submerged arc fluxes.
•
A neutral flux has no effect on the finished weld, in spite of any variable change.
•
Active fluxes contain elements such as manganese and/or silicon and these can
be picked up in the arc and thus contribute to the weld metal properties. The flux/
wire combination must be carefully selected and is often critical in predicting weld
metal properties.
•
Submerged arc fluxes are also available in different particle sizes and methods of
manufacturing.
Process requirements
The successful application of the process of submerged arc welding depends on the
following.
•
Welding conditions and preparation to suit the work. Correct voltage, welding
current and travel rate create the necessary bead width, weld contour and
penetration. The joint often contains more of the base metal, 50–70% than
applied filler metal and hence the composition of the base metal plays an
important part in this process. Base metal composition and thickness go hand in
hand in determining the mechanical properties of the joint.
•
Correct selection of welding wire and flux combination to suit the base metal to
be welded. The manganese wire and flux contribution should be matched and
the depth of flux covering applied should be no greater than is required to obtain
a quiet action and an absence of porosity in the finished weld. If too deep a layer
is used, the rough and uneven surface which results is due to the entrapment of
gases generated during the welding process, and which cannot escape through
the thick layer of flux.
Too shallow a flux results in porosity and ‘open-arcing’ occurring.
186
Metals& Fabrication
Term
porosity
Arc Welding 1
Definition
the state or property of being porous (having pores,
easily penetrated)
•
Plate surface preparation other than joint preparation. It is important that no
foreign material is picked up during flux reclamation and to prevent this, a
suitable width of plate on either side of the joint is cleared prior to welding. It is
essential that the plate and joint surfaces are clean and dry. Oil, grease, paint
and other gas producing materials remaining in the joint area cause porosity.
Even a crayon mark on the joint surface can ruin an otherwise good weld.
•
Heat treatment prior to, during and after welding has been completed.
Calculation of pre-heat temperatures and the requirements of post-weld heat
treatment is extremely important. For most plain carbon and alloy steels, only
pre-heat is needed, if any treatment is required at all.
Weld backing
Due to the large volume of molten metal which remains fluid for a length of time, it is
essential to provide support to contain the weld until it solidifies.
Methods used are:
•
non-fusible backing – for example, copper backing strip
•
weld backing – the most widely used method of applying support.
Some further points to consider on weld backing are as follows.
•
In a ‘root backed’ joint, the root face is thick enough to support the incompletely
penetrated first pass of weld. It is most important that the joint edges are tightly
butted.
•
Manual welds are sometimes used as backing when it is not convenient to use
other backing methods because of inaccessibility, poor joint preparation of fitting,
or difficulty in positioning the job.
•
The manual weld may become part of the complete joint or it may be removed
and replaced.
•
E4112 and E4113 electrodes are not recommended as backing welds, as they
tend to cause porosity in the finished weld.
•
Fusible metallic backing which the weld penetrates into and fuses with the
backing material either temporarily or permanently becomes part of the weld.
•
Preparation is provided to aid weld penetration and control the amount of weld
reinforcement. Preparation is usually provided in accordance with the quality of
the weld metal required in the finished weld.
•
The root face should be thick enough for the weld to fuse down into, but not
through the nose of the joint. Sufficient thickness of nose must be provided to
absorb the heat of the molten metal in the joint area.
187
Chapter 9 – Submerged arc and electro-slag welding processes (SAW)
Causes of cracking
The principal causes of cracking when submerged arc welding are as follows.
Result
Cause
Rigidity
Cause by thick plates or reinforced/braced structures.
Low ambient
temperatures and fast
cooling rates
Causes an increase in quench rates that might be detrimental on
susceptible material.
Steels of low
weldability
High alloy content, high carbon equivalent or large mass can all
contribute to poor weldability.
Polarity
If cracking is due to plate composition, electrode positive polarity is
recommended. The 20–30% better burn-off will help to build up an
adequate cross section of weld with a proper convex bead which
resists cracking.
Manual first pass weld
backing
Use only recommended hydrogen controlled electrode types for this
function.
Bead shape and
dimensions
Particularly the ratio between the width and depth of deposit.
Internal shrinkage
To prevent internal shrinkage cracking, the bead surface must be
flat to slightly convex and the width of the weld must not be greater
than penetration depth.
Electrode stick-out
This determines the burn-off rate. A high burn-off rate gives less
penetration and weld dilution and reduces cracking but bead shape
is hard to control.
Bead shape
Particularly below the plate surface. What is termed `hat-shaped
beads’ provide stress rises due to their below plate cross-section
and could lead to cracking where fatigue loading is possible.
All operating factors are important, and with adequate supervision, submerged arc
welding will provide the most consistent and trouble-free welding of all production
processes.
188
Metals& Fabrication
Arc Welding 1
Electro-slag welding
The electro-slag process was first developed for vertical welding of tank seams on site.
The process potential for single pass welds on heavy materials in the vertical position
was soon realised. Unique features of the process are the absence of an arc, because
once the molten pool is established, welding heat is developed by resistance heating
as the current passes through the molten slag. Preparation of plate edges is minimised,
since square oxy-cut edges can safely be welded, but the advantages of this feature
are limited to some extent by the need to provide substantial clamping arrangements
to secure the parts during welding. The slag has no metallurgical function because
very limited amounts of flux are used compared with other processes; its function is to
develop and distribute welding heat and to protect the molten pool of weld metal.
In the electro-slag process, the guide and contact tube for the continuously fed
electrode wire is mounted just above the weld pool and mechanically raised as the
weld progresses. An adaptation of this process employs a continuous guide from top
to bottom of the weld, passing down the centre of the joint, and this process is referred
to as ‘consumable guide welding’, because the guide is melted into the weld pool as
it rises. The advantage of a consumable guide is that it can be set up before welding
commences, including the wire feed mechanism which remains static, and joints which
change direction and slope away from the vertical can be better catered for.
Guide and contact tube
Unfused
plate
edge
Wire feed
Wire
on reel
Vertical drive
mechanism
Flux
added
Joint
alignment
plates
Water
flow
Solidified weld
Copper shoes or slippers
Fig 9.5 – Electro-slag arrangement
189
Chapter 9 – Submerged arc and electro-slag welding processes (SAW)
Preparation
A gap between square plate edges of between 25 mm and 50 mm depends on the
thickness of the material. The gap is often set to increase from bottom to top in order to
cope with the considerable contraction forces as the weld progresses. ‘Strong-backs’ or
shaped plates are usually welded on to hold joint plates in position and maintain good
alignment across the face of the joint.
These strong backs must be cut out to provide free passage for the copper strips and
the vertical drive gear (see Fig 9.6).
Fig 9.6 – Strong backs to secure joint for welding
Preparation also requires the provision of ‘run-on’ and ‘run-off’ plates at the beginning
and end of joints. These short extensions of joint plates, approximately 75 mm, allow
full size and strength welds to be maintained throughout. They can be seen on the
drawing of the consumable guide arrangement in Fig 9.7.
Copper moulds
Copper moulds, or shoes, retain the molten metal and slag and are moved forward
as the weld proceeds. Copper is a good conductor of heat and the moulds tend to
cool the weld quickly. The moulds are water cooled internally and form a solidified flux
coating against the weld face which also serves to protect the copper. Reinforcement
of the weld is created by the shape of the moulds. They are held in close contact with
the joint faces to prevent leakage from the molten pool, and are moved upwards as
welding progresses. The molten metal solidifies and becomes self-supporting before
the bottom of the mould moves forward. An alternative to continuously moving shoes is
the stepping of moulds as indicated in Fig 9.7.
Flux
Flux is similar to submerged arc fluxes but has additional amounts of:
•
calcium fluoride – to prevent arcing
•
manganese and aluminium silicates – to raise boiling point
•
fluorspar and magnesia – to improve conductivity and ionisation.
Flux is added as necessary to compensate for losses, and the level of molten slag is
maintained between 38 mm and 50 mm. Molten slag which is too deep may trap gas or
slag, and if too shallow may allow metal to run out or cause unstable current flow.
190
Metals& Fabrication
Arc Welding 1
Continuous
wire
Consumable
guide
Consumable
guide
Flux
ferrule
Run-off
block
Water cooled
copper shoes
Cooling
water
Consumable
guide and wire
Liquid
weld metal
Molten slag
Cooling water
Run-off
block
Fig 9.7 – Electro-slag process
Wires
Standard wires are used, as in submerged arc welding, but since flux cannot be used
to add elements to the weld, these must added via the wires, and/or the consumable
guide when it is used, and wire oscillation can be arranged to ensure even heat and
metal distribution.
The molten slag maintains a temperature between 1700 °C and 2400 °C, which melts
the filter wire and the plate edges. The actual melting point of flux is much lower than
the melting point of steel, which is designed to prevent slag being trapped. The molten
slag must not be allowed to boil.
191
Chapter 9 – Submerged arc and electro-slag welding processes (SAW)
Power source
Ordinary arc welding transformers or generators can be used. Constant potential
machines give better control of weld conditions than constant current machines.
Alternating or direct current can be used, but alternating is preferred. Welding current
is relatively high, depending on the size of joint and the number and size of electrodes;
for example, three wire feeds may require 3000 amps at 40 to 55 volts.
Metallurgical aspects
Owing to prolonged heating and slow solidification, the welds produced have a very
coarse grain structure and a wide HAZ, often returning low impact values. However,
normalising can give the necessary grain refinement and a resulting improvement in
mechanical properties.
Term
Definition
HAZ (heat affected zone)
the part of a metal that is not melted during cutting,
brazing, or welding, but its microstructure and
physical properties are altered by these processes
Summary
Advantages
•
Single pass only required.
•
Square edge preparation.
•
No arc and little heat given off.
•
Excellent on heavy material.
•
No angular distortion.
•
More metal is deposited per unit of electrical power than other processes.
•
Flux consumption is very low compared to submerged arc welding.
Limitations
192
•
High cost of equipment.
•
Can only weld vertical.
•
Setting up may be difficult – strongbacks must be used.
•
Heat treatment is often required.
Metals& Fabrication
Arc Welding 1
Activity
Carry out the following in your workshop.
•
Identify safe working practices and protective equipment in the workshop.
•
Locate a suitable submerged arc welding (SAW) plant.
•
With the mains power switch OFF, set up the SAW plant (start by checking the
electrical cable connections to the wire feed unit, work lead and handpiece.
•
Check the appropriate wire is fitted into the wire feed unit.
•
Remove the flux distribution nozzle and check to see that the correct contact tip
is fitted.
•
Set up the correct electrical stick-out and flux height as the nozzle is refitted.
•
Ask your lecturer to help you set up a typical voltage, welding current and travel
rate. Your lecturer should be able to also demonstrate the equipment.
•
With your lecturer’s assistance, produce practice welds on practice material.
Conduct three test welds.
1.
Try variations in arc voltage while keeping all other parameters constant.
2.
Alter amperage while keeping all other parameters constant
3.
Alter travel rate, while keeping all other parameters constant.
Refer to Australian Standard® AS 1858 – Electrodes and fluxes for submerged-arc
welding – Carbon steels and carbon-manganese steels.
193
194
Arc welding Vol 1
Book title
2. Electricity and welding machines
1. Safety
Chapter title
Perform routine gas metal arc welding
Weld using gas metal arc welding
Perform advanced welding gas metal arc welding
Weld using submerged arc welding
Select welding processes
Apply safe welding practices
MEM 5.50B
MEM 5.17C
MEM 5.18C
MEM 5.23C
MEM 5.51A
MEM 5.52A
Perform routine gas tungsten arc welding
Perform advanced welding manual metal arc welding
MEM 5.16C
MEM 5.49B
Weld using manual metal arc welding
MEM 5.15C
Perform manual production welding
Weld using gas tungsten arc welding
MEM 5.19C
MEM 5.13C
Perform routine gas tungsten arc welding
MEM 5.49B
Perform routine manual metal arc welding
Perform manual production welding
MEM 5.13C
MEM 5.12C
Perform routine manual metal arc welding
Competency title
MEM 5.12C
Comp
code
Arc welding 1
Metals and fabrication competency mapping
Appendix
‘
‘
‘
‘
‘
‘
‘
‘
‘
‘
‘
‘
‘
‘
‘
Full = 9
Partial =‘
Metals& Fabrication
Arc Welding 1
Book title
4. Air-arc gouging
3. Welding preparation and
workmanship
Chapter title
‘
Weld using gas metal arc welding
Perform advanced welding gas metal arc welding
Select welding processes
Perform advanced manual thermal cutting, gouging and
shaping
MEM 5.17C
MEM 5.18C
MEM 5.51A
MEM 5.8C
‘
Perform routine gas metal arc welding
MEM 5.50B
‘
‘
‘
‘
‘
‘
‘
Perform advanced welding manual metal arc welding
‘
‘
MEM 5.16C
Apply safe welding practices
MEM 5.52A
‘
Weld using manual metal arc welding
Select welding processes
MEM 5.51A
‘
MEM 5.15C
Weld using submerged arc welding
MEM 5.23C
‘
Perform manual production welding
Perform advanced welding gas metal arc welding
MEM 5.18C
‘
MEM 5.13C
Weld using gas metal arc welding
MEM 5.17C
‘
Perform routine manual metal arc welding
Perform routine gas metal arc welding
MEM 5.50B
‘
MEM 5.12C
Perform advanced welding manual metal arc welding
MEM 5.16C
‘
Perform routine oxy acetylene welding
Weld using manual metal arc welding
MEM 5.15C
‘
Full = 9
Partial =‘
MEM 5.4C
Weld using gas tungsten arc welding
Competency title
MEM 5.19C
Comp
code
Appendix – Competency mapping
Book title
MEM 5.23C
9. Sub-arc and electro-slag welding
processes
Perform advanced welding gas metal arc welding
MEM 5.18C
Weld using submerged arc welding
Weld using flux-cored arc welding
Weld using gas metal arc welding
MEM 5.17C
MEM 5.47B
Perform routine gas metal arc welding
MEM 5.50B
Weld using gas tungsten arc welding
MEM 5.19C
Perform advanced welding manual metal arc welding
MEM 5.16
Perform routine gas tungsten arc welding
Weld using manual metal arc welding
MEM 5.15C
MEM 5.49B
Perform routine manual metal arc welding
Competency title
MEM 5.12C
Comp
code
8. Flux-cored arc welding
7. Gas metal arc welding
6. Gas tungsten arc welding
5. Manual metal arc welding
Chapter title
‘
‘
‘
‘
‘
‘
‘
‘
‘
‘
Full = 9
Partial =‘
Metals& Fabrication
Arc Welding 1
Assessors must be satisfied that the candidate can competently and consistently perform all elements of the units as specified by the criteria,
including required knowledge, and be capable of applying the competency in new and different situations and contexts.
Consistency of performance
Assessors should gather a range of evidence that is valid, sufficient, current and authentic. Evidence can be gathered through a variety of
ways including direct observation, supervisor’s reports, project work, samples and questioning. Questioning should not require language,
literacy and numeracy skills beyond those required in this unit. The candidate must have access to all tools, equipment, materials and
documentation required. The candidate must be permitted to refer to any relevant workplace procedures, product and manufacturing
specifications, codes, standards, manuals and reference materials.
Method of assessment
These units could be assessed in conjunction with mandatory units addressing the safety, quality, communication, mathematics etc. Units
may also be assessed with other units requiring the exercise of the skills and knowledge.
Project work, integration
Assessors are reminded the individual units may be assessed on the job, off the job or a combination of both on and off the job. Where
assessment occurs off the job, that is the candidate is not in productive work, then an appropriate simulation must be used where the range
of conditions reflects realistic workplace situations.
Context of assessment
This resource is specifically designed to provide basic underpinning knowledge related to a number of competency units used in the
Engineering Tradesperson Fabrication (Heavy) pathway across TAFEWA from January 2007. This pathway was specifically designed to meet
the needs of the heavy metal fabrication industry after industry consultation and TAFE WA moderation sessions held in 2006. This pathway
is also designed to be common across all colleges of TAFEWA (customisation to suit local conditions is however encouraged). The pathway
meets the requirements and guidelines of the MEM05 Training Package.
Arc welding 1
Appendix – Competency mapping
ARC WELDING 1
Basic Arc Welding Information Book
DESCRIPTION
This resource supports learners to develop the basic underpinning skills and
knowledge relating to a number of competency units used in the Engineering
Tradesperson learning pathway, with a particular focus on introductory level arc
welding.
Topics covered include the following.
•
•
•
•
•
•
•
•
•
Arc welding safety
Electricity and welding machines
Weld preparation and workmanship
Air-arc gouging
Manual metal arc welding (MMAW)
Gas tungsten arc welding (GTAW)
Gas metal arc welding (GMAW)
Flux-cored arc welding (FCAW)
Submerged arc and electro-slag welding processes (SAW)
The book is divided into separate chapters, each containing workshop-based
activities that will provide opportunities for practice before assessment.
A comprehensive mapping guide is included, to show where the content in this
resource aligns with the relevant competencies.
EDITION
2007
CATEGORY
Metals & Engineering
TRAINING PACKAGE
• MEM05
ENG093
BASIC ARC WELDING,
INFORMATION
ISBN 978-0-7307-9807-1
ORDERING INFORMATION:
Tel: (08) 6212 9700 Fax: (08) 9227 8393 Email: [email protected]
Orders can also be placed through the website: www.vetinfonet.dtwd.wa.gov.au
9 780730 798071
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