WELDING Reference Text
JOURNEYMAN
PROGRAMME
WELDING
Reference Text
Foreword
MIC has produced this book for us in its Industrial Maintenance Journeyman Programme and it is specifically
designed to introduce the basics of maintenance.
This book is intended for use as a reference text to be supplemented by notes and explanations and does not
stand alone.
Compilation of this book was completed with standard published material, Tel-A-Train and resource personnel
at MIC. No claim is made to the ownership of any material contained herein.
THIS BOOK IS NOT FOR SALE
REFERENCE TEXT USED
TABLE OF CONTENTS
1WELDING SAFETY3
2FUNDAMENTALS OF ELECTRICITY7
3ARC WELDING EQUIPMENT11
4BASIC SCIENCE OF METALS20
5BASIC WELDING METALLURGY27
6ARC WELDING ELECTRODES32
7GAS WELDING EQUIPMENT38
8THE WELDING PROCESS46
9DISTORTION AND RESIDUAL STRESS61
10
WELDING SYMBOLS68
11
INSPECTION AND QUALITY CONTROL75
12
TEMPERATURE CONVERSION TABLE92
2
Welding Safety
3
4
5
1.2 CXYACETYLENE SAFETY RULES
1.
Blow out the cylinder valves before attaching the regulators to the cylinders.
2.
Release the adjusting screw on the regulator before opening up the cylinder valve.
3.
Stand to one side of the regulator before opening the cylinder valve.
4.
Open the cylinder valve slowly.
5.
Do not use or compress acetylene in a free state at pressures higher than 15 psi.
6.
Purge the oxygen plus acetylene gas passages individually before lighting the torch.,
7.
Light the acetylene before opening the oxygen valve on the torch.
8.
Never use oil on regulatorsk torches fittings or other equipment #1 contact with oxygen.
9.
Do not use oxygen as a substitute for air.
10.
Keep the work area clean of anything that will burn.
11.
Always wear eye protection.
12.
Never use a regulator designed for one type gas on a different type. gas.
13.
Never try to use gas fran a cylinder without a regulator..
14.
Cylinders should be secured on the cart or rack before the protective cap is removed.
15.
Replace the protective cap before moving tanks.
16.
Always close the valves on empty cylinders to prevent dust and moisture from entering the tank.
17.
Compressed gas cylinders should never be subjected to temperatures above 1250F.
6
Fundamentals
of Electricity
7
WELDING (CURRENT AND VOLTAGE)
In the behavior of a welding electrical current there are three distin¬guishable types of voltages.
OPEN CIRCUIT VOLTAGE
This is the voltage before beginning the arc (60 to 70 V approximately).
PRIMARY VOLTAGE
This is the voltage at the moment that the arc is struck (minimum)
WORKING VOLTAGE
This is the voltage during welding (30 V approximately).
In welding with alternating current, only the intensity of current (amperage) required can be
regulated. For welding with direct current, there are devices which enable regulating the voltage
also.
In direct current welding it is possible to change the direction of circula¬tion of the current (polarity) this change of polarity is indicated in the booklet on electrodes. To calculate the normal
working current of an electrode, 35 A for each millimeter of thickness of the nucleus is taken.
Example
For an electrode of 4 mm in diameter the normal intensity would be:
I = 4 mm x 35 A/mm
I = 140 A
The usual values are presented in the following table:
Observation:
These values may be increased or decreased from 5% to 15% depending on the electrode and
the machine to be used.
8
Electric machinery which transforms the alternating current, lowering the voltage of the supply
to an adequate voltage and intensity for welding. Such alternating current of low voltage (65 to
75 volts open circuit) and of constant amperage, supplies the heat source needed for welding.
THE TRANSFORMER IS MADE UP OF
A core, which is composed of steel sheets with silicon and
with two wire windings (coils); the high voltage coil is called
primary, and the low voltage primary is called secondary (fig.
1).
The current which comes from the supply lines, passes through
the primary coil.
Transformers are made adaptable for different voltages, with
the aim of facilitating their connection with supply nets. The action of the transformer is explained in this way. The electric current which moves through the primary coil, generates a field
of magnetic lines of force in the core. This field acts on the secondary coil, and produces in it a
low voltage current of high intensity which is used for welding.
CHARACTERISTICS:
The regulation of the intensity is commonly done in two ways:
1. Regulation by a displacement coil (fig. 2).
Consists in separating the primary and the secondary from each other
OBSERVATION:
This system is recommended because of its gradual regulation.
9
WELDING MACHINE (TRANSFORMER)
2 Regulation by plugs (fig. 3)
It works by increasing or decreasing the number of turns.
The transformers are also known as static machines, because they do not have moving parts.
Those made for high intensity carry a fan which is used to cool the system.
ADVANTAGES:
The use of the transformer has become generalized because of:
•
Low cost.
•
Longer duration and less maintenance expenses. - Greater output-and less open circuit consumption.
•
Less influence of the magnetic blow.
DISADVANTAGES:
Among its disadvantages that can be mentioned are:
•
Limitation of the use of some electrodes.
CARE:
It must be kept free from dust.
PRECAUTION:
ALL CLEANING JOBS MUST BE DONE WITH THE MACHINE DISCON-
NECTED. WHEN INSTALLING IT YOU OUGHT TO CHOOSE A DRY PLACE FIXING ON TO IT AN EARTH CONNECTION.
10
Arc Welding
Equipment
11
3.1 SAFETY WEAR FOR ARC WELDING
This is composed of elements made of leather and they are used by the welder to protect himself
from the heat and radiation produced be the electric arc. This equipment is composed of gloves,
apron, jackets, sleeves and leggings.
GLOVES:
Made of leather or asbestos, and their shape varies as can
be seen in figures 1 and 2. The asbestos gloves are used
only for jobs of a high temperature. Avoid handling very
hot work pieces with the gloves, because they lose their
shape and flexibility.
APRON:
It is of a common shape (fig. 3) OR with leg protectors (fig. 4). Its objective is to protect the
front part of the body and legs up to the knees.
JACKET:
Its shape can be seen in fig. 5. It is used to specially protect
the arms and part of the chest. It is used frequently when
welding is done in a vertical, horizontal or overhead position.
12
PROTECTIVE EQUIPMENT (LEATHER CLOTHING)
SLEEVES:
This garment is meant only to protect the arms of the welder (fig.6). it is mostly used for welding which is done at a work-bench and in a flat position
There are other types of sleeves in the shape of a vest which also covers part of the chest (fig.7)
LEGGINGS:
These are used to protect part of the legs and the welder’s feet (fig. 8). The leggings can be replaced by high, plain boots (fig. 9) with a steel tip.
CHARACTERISTICS:
They are cured leather, flexible, light weight, treated with lead salt to avoid radiation form the
electric arc.
CARE:
It is important to keep these elements in good working condition, free from tears and with their
fastenings in good condition. They must be kept clean and dry to ensure good insulation.
PROTECTIVE EQUIPMENT (MASK)
The protective mask is made of fiber glass or compressed fiber and it has an opening in which
neutralizing as well as protective glasses are placed. It is used to protect the eyes and to avoid
burns on the face.
13
TYPES
There are different types of welding masks (figs. 1,2
and 3). There are also combined masks with a safety
helmet to do construction work (fig.4) and with adapters for protection of the eyes when there is (……….) to
be cleaned (fig. 5). The hand shield (fig.6) is used when
assembling or tack welding is done. Its use is not convenient in jobs at a height where the welder needs to hold parts or
tools.
CONDITIONS OF USE
The masks must be used with the glasses in the quantity and sequence required (fig. 7).
The tinted glass must be selected according to the amperage used. It must maintain good visibility by changing the protective glass when this has too many splatters. Avoid light filtering into
the mask. It must not be exposed to heat or knocks.
They must be lightweight and must have a headband for adjusting to heat or knocks.
They must have a mechanism which allows them to be used comfortably. The changing of
glasses must be done by means of a mechanism that is east to handle.
14
BASIC MACHINE TYPES
Direct Current (D.C.) Equipment
The equipment used is classified into two groups i.e. generators and rectifiers.
Generator
A genereator may be driven by an electric motor cancelled to the main supply or by a petrol or
diesel motor.
Rectifier
A welding rectifier usually incorporates a transformer.
The welding lead from this power source carries power to the
electrode holder
Electrode Holder
This is used to hold the electrode during the
welding process. The handle is insulated to protect the welder from electric shock.
15
Power Sources
These are classified into two groups, Alternating Current (A.C.) or Direct Current (D.C.) according to the output current supplied by either a mains supply or mobiles generator.
Alternating Current (A.C.)
Where input is from tea minas the power source must:
•
Reduce the mains input voltage to give an output open-circuit voltage between 40 and 100 volts.
•
Increase the mains input current to give the output currents required for welding.
16
DESCRIPTION OF THE ARC (WELDING CIRCUIT)
Manual welding with an electric arc is a system that uses a heat source (electric arc) and a gaseous medium generated by the combustion of the coating of the electrode by means of which it is
possible to fuse the filler metal and the work piece.
This process is done by means of the electrical circuit (fig. 1)
The source of energy for welding is supplied by a direct current machine (D.C.) or one of alternating current (A.C.), which creates an electrical circuit, through the conducting cables from
the electrode to the work piece. This circuit is closed when the electrode makes contact with the
work piece. The arc produced is the part where the circuit encounters the greatest resistance and
is the point where the heat source is generated.
The high temperature generated in the arc enables the fusion of the base metal and the contributing rod.
This temperature also enables the igniting of the component elements of the coating which, on
gasifying, accomplish various functions, such as: deoxidizing, eliminating impurities, facilitating the flow of current and especially protecting the molten metal from atmospheric influences.
This system is characterized by its versatility and economy. This process may be applied in joining different metals, in small jobs or in those of great size.
THE FUNCTIONING OF THIS PROCESS SHOULD BE ADJUSTED TO THE TECHNICAL
INDICATIONS REQUIRED BY THE METAL TO BE WELDED AND THE ELECTRODES TO
BE USED.
17
These are accessories that make up part of the welding equipment. They are used to ensure a
good conduction of current through the work piece and the electrode. They are easy to handle,
they are balanced and provide a safe and fast functioning (fig. 1).
ELECTRODE HOLDER
Structure
The electrode holder consists of a hollow fiber shank which enables rapid cooling: the slots allow for easy handling as the hand fits perfectly on the handle. The trigger insulated with fiber is
to open the jaws and to change (by pressing the trigger downwards) the electrode held between
them.
The two jaws are made of steel and have copper bits on their ends which ensure a fluent flow of
the current, at the same time being protected at the back with an insulating Material to prevent
contact with the part. There are other types of electrode holders shown in figures 2 and 3.
Characteristics
Electrode holders should be light weight and balanced to prevent tiring and ensure a rapid manipulation. It should be thermally and elec¬trically insulated.
Conditions of use
The contact point in the electrode holder should be safe and allow the flow of current without
any electrical resistance. The jaws should be cleaned in such a way that the electrode may adjust perfectly to the slots of the bits. The electrode holder should not be subjected to amperages
(………………………)
18
GROUND CONNECTION
Structure
It consists of two arms (fig.4) connected to each other at the center by means of a metal bolt. It
is furnished with a spring situated around the bolt to keep the jaws tightly closed. These jaws
are fitted with copper contactors at their ends that ensure an efficient contact between the part
and the ground connection. The cable terminal is secured firmly to the ground connection with a
screw. The ends of the arms are covered with an insulating plastic tube.
Characteristics
The ground connection clamps are light weight for quick connection to the work. They are
made of steel and copper.
19
Basic Science
of Metals
20
21
METALLURGY
THE USES OF PLAIN CARBON STEEL
22
4.3 EFFECTS OF ALLOYING ELEMENTS
These are ferrous elements made by the fusion of steel with other elements such as:
•
Nickel (Ni)
•
Chromium (Cr)
•
Manganese (Mn)
•
Tungsten (W)
•
Molybdenum (Mo)
•
Vanadium (Va)
•
Silicon (Si)
•
Cobalt (Co)
•
Aluminum (A1)
Steel alloys are used to manufacture parts and tools which depending on their use, require in
their composition the presence of one or several of the above mentioned elements. The resulting
alloy receives the name of the element or elements, as the case may be, that compose it. Each
one of these elements gives the following properties to the steel.
NICKEL (Ni)
This is one of the first metals to be used successfully in rendering certain qualities to steel.
Nickel increases its resistance and toughness, raises its limit of elasticity, makes it a good conductor and increases its resistance to corrosion. Nickel steel contains 2 to 5% Ni and 0.1 to
0.5% carbon. The percentages 12 to 21% Ni and 0.1% carbon produce stainless steels which are
very hard and resistant.
CHROMIUM (Cr)
It also renders to the steel high resistance, hardness, high elasticity limit and good resistance to
corrosion.
Chromium steel contains 0.5 to 2% chromium and 0.1 to 1.5% C. The special chromium - steel
(stainless type) contains 11 to 17% Chromium.
MANGANESE (Mn)
Steels with 1.5 to 5% manganese are brittle. Manganese, nevertheless, when added in correct
quantities, increases the resistance of steel to wear and shock, maintaining its ductility. Manganese steel usually contains 11 to 14% Mn and 0.8 to 1.5% carbon.
TUNGSTEN (W)
It is generally added to steels with other elements. Tungsten increases hardness, the breaking
point, limit of elasticity and resistance to heat.
Steels with 3 to 18% W and 0.2 to 1.5% C are very resistant.
23
MOLYBDENUM (Mo)
Its effect on steel is similar to that of tungsten. It is used generally, added to chromium, to produce chrome-molybdenum steel of great stress, especially under repeated stress.
VANADIUM (Va)
It improves, in steels, the resistance to tension, without loss in ductility, and elevates the limits
of elasticity and fatigue. Chrome-vanadium steels generally contain, 0.5 to 1.5% Cr. 0.15 to 3%
Va and 0.13 to 1.1% C.
SILICON (Si)
Increases the elasticity and resistance of steels. Silicon steels contain 1 to 2% Si and 0.1 to 0.4%
C. Silicon has the property of insulating or suppressing magnetism.
COBALT (Co)
It favorably influences the magnetic properties of steels. Besides, cobalt associated with tungsten increase the resistance of steels to heat.
ALUMINIUM (Al)
It deoxidizes steel. In the thermo-chemical treatment process called nitriding, it is confined with
nitrogen to aid in forming a very hard superficial layer.
NOTES ON ENGINEERING MATERIALS
4.4 GENERAL: The Physical Properties of Materials
1.
DUCTILITY:
Is the property which enables a material to be drawn out to a considerable length without fracturing materials selected to make wire must be extremely ductile.
2.
ELASTICITY: Is the ability of a material which has been deformed in sane way to return
to its original shape and size after the deforming force has been removed.
3.
HARDNESS:
Is the property of resistance to surface wear of identation.
4.
MALLEABILITY: Is the property which allows a material to be deformed by hammering, rolling or pressing, without fracturing. Heating improves malleability.
5.
PLASTICITY: Is the ability of a material to flow into new shapes under pressure, and to
retain its new form.
6.
TENACITY:
Is the property of resistance under tensile force. This property is expressed as (lbf/in2 ) or (Tons.f/in2) (NM-2) or (N/m2) Tensile strength and is of great, impor24
tance to designers.
7.
TOUGHNESS: Is the property of resistance to fracture under sudden shock loads. It is
usually found in a material which carbines high tenacity with good ductility. Heating usually
lessens toughness.
8.
BRITTLENESS: Is the opposite of toughness: Brittle materials fracture cleanly under
shock loads but may with¬stand constant pressures. The property is also referred to as shortness. A material brittle at room temperatures is said to be cold short and when hot, as hot short.
Fig. 15.6 - stages of deformation exhibited by a Ductile Round Bar Tensile Specimen.
The original specimen is shown at A. Successive stages of elongated are shown in B,C and D. B
represents uniform strain; C and D illustrate necking
25
4.4.1 STRESS/STRAIN CURVES
The curve is plotted from information gathered during tensile (or compression) testing. The entire curve is shown at A, while an expanded view of the departure from as straight line relationship is drawn at B.
Some metals have a clearly defined yield point. This indicated that at some critical stress value,
a considerable extension occurs with no additional increase in stress.
26
Basic Welding
Met allurgy
27
28
5.3
HEAT TREATMENT
Welding inspectors normally are interested only in the left hand portion of the diagram up to
about 0.35% C. It must be recognized that the addition of alloying elements will cause a low
carbon steel to harden on rapid cooling in a manner similar to high carbon steels.
At this point, some of the terms relating to the iron-carbon alloy system will be reviewed.
Phases
Ferrite (α -Fe): The body-centered cubic (bcc) form of pure iron, stable below 912 C (1674 F).
Carbon content ranges from 0 to 0.022%.
Austenite (ϒ-Fe): A solid solution of one or more elements in face-centered cubic (fcc) iron.
Unless otherwise designated, the solute is generally assumed to be carbon, ranging in content
from 0 to 2.11%.
Ferrite (δ -Fe): A solid solution of one or more elements in body-centered cubic (bcc) iron. Unless otherwise designated, the solute is generally assumed to be carbon, ranging in content from
0 to about 0.10%.
Cementite: A compound of iron and carbon, known chemically as iron carbide and having the
approximate chemical formula Fe, C. It is characterized by an orthorhombic crystal structure.
Graphite: Carbon in the Free State occurring in several geometric forms.
Alloy Carbide: A chemical compound of carbon and alloying elements that form both simple
and complex chemical compounds with carbon, usually Fe, Mn, Cr, V, W, Mo, Cb and Ti.
The microstructures that can be produced within the system as a function of cooling rate and
chemical composition are listed below:
29
Microstructures
Ferrite: See definitions under Phases.
Austenite: See definition under Phases.
Cementite: See definition under Phases.
Pearlitic: coarse and fine. A mixture of ferrite arid cementite consisting of alternate platelets
with the thickness of the ferrite being about seven times the thickness of the cementite
Spheroidite: An aggregate of iron or alloy carbides of essentially spherical shape dispersed
throughout a matrix of ferrite.
Bainite: A decomposition product of austenite consisting of an aggregate of ferrite and carbide.
In general, it forms at temperatures lower than those where very fine pearlite forms and higher
than those where martensite begins to form on cooling. Its appearance is feathery if formed in
the upper part of the temperature range; acicular, resembling tempered martensite, if formed in
the lower part.
Tempered Martensite: A mixture of ferrite and cementite in which the carbides are very finely
dispersed in a submicroscopic spheroidal form
Martensite: A metastable phase of steel, formed by a transformation of austenite below the Ms
(or At”) temperature. It is an interstitial supersaturated solid solution of carbon in iron having a
body-centered tetragonal lattice. Its microstructure is characterized by an acicular, or needlelike,
pattern.
Graphite: See definition under Phases. Free carbon whose shape either as a flake, nodule or
spheroid describes the type of cast iron, which is gray, malleable or ductile; respectively.
Alloy Carbide: See definition under Phases.
Cooling rates are often described as follows:
Spheroidizing: Very slow, forming spheroidized carbides and ferrite.
Annealing: Furnace cool, forming lamellar, pearlite and ferrite.
Normalizing: Air cooling, forming fine lamellar pearlite and ferrite.
30
5.3.5 TTT CURVES
Fig. 135
idealized TTT curve for 0.6% carbon steel depicting time interval required for beginning, 50 and 100% transformation of austenite at a constant temperature.
A = Austenite
F = Ferrite
P = Pearlite
B = Bainite
31
Arc Welding
Electrodes
32
6.1 FLUX COATED ELECTRCOE
Metallic rod, specially prepared to be used as filler material in the arc welding processes.
It is made of ferrous or non-ferrous material.
TYPES:
There are two types: the coated electrode and the bare wire electrode.
Coated Electrode
It has a metallic core, a coat based with chemical substances and an uncoated end which is to be
fixed into the electrode holder (fig. 1).
The core is the metallic part of the electrode which is used as the filler metal. Its chemical components vary, and their selection is done according to the material to be welded.
The coating is a material composed of different chemical substances. Its functions are
a)
It directs the arc, leading to a balanced and uniform fusion.
b) It gives off gases which act as a protective shield keeping out oxygen and nitrogen.
c)
It forms a slag which covers the bead avoiding sudden cooling and contact with oxygen and nitrogen (fig. 2).
d) It contains certain elements which allow a good fusion with different types of metals.
e) Stabilizes the arc.
33
CARE:
1.
It must be free from humidity and its core must be concentric
(fig. 3).
2.
It must be kept in a dry place.
Naked electrode (without coating)
It is a drawn, laminated wire. Its use is limited by the high absorption of oxygen and nitrogen
from the air, and by the instability of its arc.
Technical Vocabulary
-Naked
-Bare
6.2 CLASSIFICATION OF STEEL ELECTRODES
Electrodes are classified by a combined system-of numbers and letters for their identification,
this permits one to select the type of electrode recommended for a particular job. The following
factors must be considered:
a)
b)
c)
Type of current available.
Position of the work piece to be welded.
Nature of the metal and resistance it must have.
This classification uses a system composed of a capital letter placed first, called a prefix, followed by four digits (fig. 1).
34
The prefix “E” means electric arc welding electrode. The first two digits from a total of four
indicate the resistance to tensile stress in thousands of pounds per square inch.
In figure 1 number 60 means 60.000 pounds per square inch, which is equivalent to 42.2 Kg per
square millimeter.
The third digit, from a total of four indicates the welding position. The number one means:
welding in any position.
The Last two digits jointly indicate the type of current to be used and the kind of coating. Number thirteen means coating with rutile, direct or alternate current, negative polarity.
To determine the meaning of the third digit, the following equivalence is used:
For third digit
1 - All positions.
2 - Joints on an inner angle in a horizontal position or flat position.
3 - Flat position only.
For the third and fourth digits jointly
10 - D C (+) cellulose coating.
11 - D C (+) cellulose coating.
12 - D C or A C (-) rutile coating.
13 - A C or D C (+) coating with rutile and iron powder
(30% approximately)
16 - D C (+) low hydrogen condition.
18 D C or A C (+) coating with low hydrogen content and iron powder.
20 - D C or A C (+) coating with low hydrogen content and iron powder (25% approximately).
24 - A C or 0 C (+) with rutile and iron powder (approximately 50% of this last element).
OBSERVATIONS:
D C direct current.
A C alternate current.
+ positive pole.
- negative pole.
Example
E. 9012 - is an electrode which has a resistance to a tensile strength of 90000 pounds per square
inch, which is equivalent to 63.2 Kg per square millimeter; it can be used with direct current,
negative pole or alternate current; its coating is rutile, and it is used in any position.
Because of the nature of the coating material, there are three basic types made industrially.
These are: basic which has a coating of calcium or calcite. Rutilic which has a high content of
rutile oxide (titanium) and the cellulose type, whose coating contains more than 12% of combustible organic matter.
35
ELECTRODE WITH BASIC COATING
thickness of coating
Generally is of a thick coating, on few occasions it has a medium size coating.
drop formation
Normally the drops are of medium size.
Current and polarity
These electrodes are used with D.C. current placing the electrode at the positive end. In some
cases it can be used with alternating current.
welding position
Welding in all positions.
depth of penetration
The depth of the penetration with this electrode is medium.
handling
The arc must be kept short.
type of slag
Dense with a brown appearance.
application
They are used for thick work pieces as in rigid constructions, for steels of low alloys and for
steels of high carbon content,
ELECTRODE WITH RUTILIC COATING
thickness of coating
Is generally of medium or thick coating, very seldom of thin coating.
drop formation
Thick when the coating is small, medium size when the coating is medium size and small when
the coating is thick.
current and polarity
The majority of these types of electrodes can be used with both currents. Generally the electrode is on the negative pole; only in some cases to the positive pole.
36
welding position
You can weld in any position.
depth of penetration
According to thickness of coating.
handling
Easy, giving a soft and smooth arc.
type of slag
Dense, uniform distribution.
application
Thin coating for small thickness, those of medium or thick coating for filling.
CELLULOSE COATED ELECTRODE
thickness of coating
In this case the coating is medium.
drop formation
Medium and large.
current and polarity
These electrodes can be used with both currents. Generally used with D.C. current and reverse
polarity that is to say the electrode into the positive pole and the work piece into the negative
pole
welding position
Any position.
depth of penetration
With these types of electrodes a good penetration is obtained.
handling
It is of easy handling with a short arc.
type of slag
Little slag formation; it makes a thin coat and it crystallizes rapidly.
application
These types of electrodes are especially suitable for difficult jobs and for jobs of high stress.
37
Gas Welding
Equipment
(Oxy-Acetylene)
38
7.1 PREPARATION OF OXY-ACETYLENE EQUIPMENT
This is a basic operation which the welder should correctly master as he will repeat it frequently
when doing different jobs of oxy-acetylene welding. It consists of mastering the knowledge
dealing with the functioning of the oxy-acetylene equipment so as to render it in working condition.
METHOD OF EXECUTION:
1st Step - Mount the regulators, thus:
SAFETY MEASURE:
THE CYLINDERS SHOULD BE IN A VERTICAL POSITION AND BE SECURED, SO AS
TO PREVENT TOPPLING.
a) Remove the cover of the cylinders.
b) Slightly open and close the valve to expel the impurities.
SAFETY MEASURES:
1) BEFORE OPENING THE ACETYLENE CYLINDER, MAKE SURE THAT THERE IS NO EXPOSED FLAME NEARBY.
2) WHEN HANDLING THE CYLINDERS YOUR HANDS SHOULD BE CLEAN OF GREASE AND OIL, AS THESE MAY CAUSE EXPLOSIVE COMBUSTIONS.
c) Connect the regulators to their respective cylinders.
OBSERVATIONS:
1) The connector nut should be tightened with the spanner for the equipment.
2) The dials should remain in such a way that the operator should be able to take the pressure readings with ease.
d) Turn the pressure regulating screw which regulates the flow of gas to the gauge which indicates the working pressure.
SAFETY MEASURE:
WHEN TURNING THE PRESSURE-REGULATING
SCREW DO SO IN AN ANTI-CLOCKWISE DIRECTION (fig. 1).
39
2nd Step – Position the hoses, thus –
a) Connect the hoses to the regulators (fig. 2 )
b)
Connect the hoses to the welding torch shank
OBSERVATIONS
1.
The hose that supplies acetylene is red in color and its connectors have left-handed threads.
2.
The hose that supplies oxygen is blue or green in color and its connectors have right-handed threads.
40
3rd Step – Install the nozzle, thus:
a)
b)
Adjust the nozzle manually
Place the nozzle in working condition (fig. 4)
4th Step – Regulate the pressures, thus:
a)
Open the cylinder valves
b) Turn the knobs that regulate the oxygen and acetylene
5th Step - Eliminate the pressures, thus:
a)
b)
Close the cylinder valves.
Loosen the pressure levers of the regulators.
SAFETY MEASURE:
OXYGEN IN CONTACT WITH OIL OR GREASE PRODUCES A RAPID COMBUSTION
THAT CAN ALSO AFFECT THE METALS OF THE VALVES AND THREADS.
c)
Open the valves in the torch to clear away the gases from the hoses; then close them.
NOTE:
During the welding, at any instant back-firing in the torch may occur.
THIS IS DANGEROUS; IT CAN PRODUCE AN EXPLOSION.
In this case, immediately, proceed in the following manner:
a)
Close the oxygen valve.
b) Close the acetylene valve.
c)
Cool the torch by placing it in a container with water.
d) Remove the torch and let the oxygen flow to clear away the water which entered it.
TECHNICAL VOCABULARY:
NOZZLE - tip
METHOD OF EXECUTION – PROCEDURE
41
6th Step - Ignite the torch, thus:
a) Open the acetylene valve in the torch for a 1/4 turn.
b) Operate the lighter (fig. 5).
SAFETY MEASURE:
WHEN THE TORCH IS IGNITED, AIM THE NOZZLE OVER A FREE SECTION AND
MANIPULATE THE LIGHTER, WITHOUT PUTTING OUT THE FLAME, SO AS TO PREVENT ACCIDENTS.
c) Slowly open the oxygen valve of the torch until obtaining a well regulated flame, “neutral”.
OBSERVATION:
It is important that the welder be able to distinguish between the neutral, oxidizing and carburizing flames (figs. 6, 7, and 8).
42
7th Step - Turn off the torch, thus:
a) Shut off the acetylene valve in the torch.
b) Shut off the oxygen valve in the torch.
SAFETY MEASURE:
EACH TIME YOU TURN OFF THE TORCH, FIRST SHUT OFF THE ACETYLENE
VALVE.
7.2 SAFETY WEAR FOR GAS WELDING
Safety goggles are devices used to protect the eyes of the operator when carrying out cleaning,
grinding, turning, machining, welding or another operation which requires protection of your
eyes.
There are various types of goggles (figs. 1, 2 and 3).
The body is generally made of plastic or metal, making it possible to exchange the glass or
transparent plastic when the latter deteriorates. Safety goggles must be easy to put on, resistant
and adaptable to the configuration of the face. There are also protective
elements in the shape of a mask (fig. 4), which besides protecting the
eyes, also protect the face; this mask must be well adjusted to the head
to avoid it dropping.
CONDITIONS OF USE:
Clean the goggles before using them to obtain better vision.
Change the elastic when necessary.
CARE:
Keep the goggles in the case when not in use, thus protecting them in
case of dropping or knocking.
Avoid putting the goggles in direct contact with hot work pieces.
43
OBSERVATION:
l) For the oxyacetylene welding, goggles with a greenish color are used, graduated in numbers
of which the most frequently used is No. 6.
2) For annealing, a bluish color is used.
7.3 CLEANING ACCESSORIES
These are suitable tools for cleaning work pieces before and after welding are done.
They are studied jointly even though they have different characteristics.
WIRE BRUSH:
This is made of steel wires and a wooden handle by which it is held (fig. 1).
CHIPPING HAMMER:
It is made with a handle which can be of wood as shown in figure 2 or made of steel as in figures 3, 4 and 5.
44
Its body is long; one of its ends has a point and the other has the shape of a chisel. The Chipping
hammer has its end hardened and pointed.
There are other types of chipping hammers combined with the wire brush as shown in figure 6.
45
The Welding
Processes
46
8.1 SMAW
Learning To Weld
The serviceability of a product or structure utilizing this type of information is and must be the
sole responsibility of the builder/user. Many variables beyond the control of The Lincoln Electric Company affect the results obtained in applying this type of information. These variables
include, but are not limited to, welding procedure, plate chemistry and temperature, weldment
design, fabrication methods and service requirements.
No one can learn to weld simply by reading about it. Skill comes only with practice. The following pages will help the inexperienced welder to understand welding and develop his skill.
For more detailed information order a copy of “New Lessons in Arc Welding” listed on the back
cover.
The Arc-Welding Circuit
The operator’s knowledge of arc welding must go beyond the arc itself. He must know how
to control the arc, and this requires a knowledge of the welding circuit and the equipment that
provides the electric current used in the arc. Figure 1 is a diagram of the welding circuit. The
circuit begins where the electrode cable is attached to the welding machine and ends where the
work cable is attached to the welding machine. Current flows through the electrode cable to the
electrode holder, through the holder to the electrode and across the arc. On the work side of the
arc, the current flows through base metal to the work cable and back to the welding machine.
The circuit must be complete for the current to flow. To weld, the work clamp must be tightly
connected to clean base metal. Remove paint, rust, etc. as necessary to get a good connection.
Connect the work clamp as close as possible to the area you wish to weld. Avoid allowing the
welding circuit to pass through hinges, bearings, electronic components or similar devices that
can be damaged.
This arc-welding circuit has a voltage output of up to 79 volts which can shock and kill. This
“open circuit” voltage (79 VAC) is present when not welding. The chance of receiving an electric shock is greatest when no welding is being done.
Do not weld if you or your work area is wet or if you
cannot insulate yourself from work using insulation
such as a rubber mat or dry wood.
The electric arc is made between the work and the
tip end of a small metal wire, the electrode, which
is clamped in a holder and the holder is held by the
operator. A gap is made in the welding circuit (see
Figure 1) by holding the tip of the electrode 1/161/8” (1.6-3.2mm) away from the work or base metal
being welded. The electric arc is established in this
47
gap and is held and moved along the joint to be welded, melting the metal as it is moved.
Arc welding is a manual skill requiring a steady hand, good physical condition, and good eyesight. The operator controls the welding arc and, therefore, the quality of the weld made.
What Happens in the Arc?
Figure 2 illustrates the action that takes place in the electric arc. It closely resembles what is
actually seen during welding.
The “arc stream” is seen in the middle of the picture. This is the electric arc created by the electric current flowing through the space between the end of the electrode and the work. The temperature of this arc is about 6000°F. (3315°C) which is more than enough to melt metal. The arc
is very bright, as well as hot, and cannot be looked at with the naked eye without risking painful
injury. The very dark lens, specifically designed for arc welding, must be used with the hand or
face shield whenever viewing the arc.
The arc melts the base metal and actually digs into it, much as the water’ through a nozzle on
a garden hose digs into the earth. The molten metal forms a molten pool or crater and tends to
flow away from the arc. As it moves away from the arc, it cools and solidifies. A slag forms on
top of the weld to protect it during cooling.
The function of the covered electrode is much more than simply to carry current to the arc. The
electrode is composed of a core of metal wire around which has been extruded and baked a
chemical covering. The core wire melts in the arc and tiny droplets of molten metal shoot across
the arc into the molten pool. The electrode provides additional filler metal for the joint to fill
the groove or gap between the two pieces of the base metal. The covering also melts or burns in
the arc. It has several functions. It makes the arc steadier, provides a shield of smoke-like gas
around the arc to keep oxygen and nitrogen in the air away from the molten metal, and provides
a flux for the molten pool. The flux picks up impurities and forms the protective slag. The principal differences between the various types of electrodes arc in their coatings. By varying the
coating, it is possible to greatly alter the operating characteristics of electrodes. By understanding the differences in the various coatings, you will gain a better understanding of selecting the
best electrode for the job you have at hand. In selecting an electrode you should consider:
1.
48
The type of deposit you want, e.g. mild steel, stainless, low alloy, hardfacing.
2.
3.
4.
5.
The thickness of the plate you want to weld.
The position it must be welded in (downhand, out-of-position).
The surface condition of the metal to be welded.
Your ability to handle and obtain the desired electrode.
Four simple manipulations are of prime importance. Without complete mastery of these four,
further welding is more or less futile. With complete mastery of the four, welding will be easy.
1. The Correct Welding Position
Illustrated is the correct welding position for right-handed people. (For left-handed it is opposite):
a.
Hold the electrode holder in your right hand.
b.
Touch left hand to the underside of right hand.
c.
Put the left elbow into your left side.
Weld with two hands whenever possible. This gives complete control over the movements of the electrode.
Whenever possible, weld from left to right (if right-handed). This
enables you to see clearly what you are doing.
Hold the electrode at a slight angle as shown.
(missing photo of correct right-handed welding position)
2.
The Correct Way to Strike an Arc
Be sure the work clamp makes good electrical contact to the work.
Lower your headshield and scratch the electrode slowly over the metal, and you will see sparks
flying. While scratching, lift the electrode 1/8”- (3.2mm) and the arc is established.
NOTE: If you stop moving the electrode while scratching, the electrode will stick.
NOTE: Most beginners try to strike the arc by a fast jabbing motion down on the plate. Result:
They either stick or their motion is so fast that they break the arc immediately.
3.
The Correct Arc Length
The arc length is the distance from the tip of the electrode core wire to the base metal.
Once the arc has been established, maintaining the correct arc length becomes extremely important. The arc should be short, approximately 1/16 to 1/8” (1.6 to 3.2mm) long. As the electrode
burns off the electrode must be fed to the work to maintain correct arc length.
The easiest way to tell whether the arc has the correct length is by listening to its sound. A nice,
short arc has a distinctive, “crackling” sound, very much like eggs frying in a pan. The incorrect, long arc has a hollow, blowing or hissing sound.
49
4.
The Correct Welding Speed
The important thing to watch while welding is the puddle of molten metal right behind the arc.
Do NOT watch the arc itself. It is the appearance of the puddle and the ridge where the molten
puddle solidifies that indicate correct welding speed. The ridge should be approximately 3/8”
(9.5mm) behind the electrode.
Most beginners tend to weld too fast, resulting in a thin, uneven, “wormy” looking bead. They
are not watching the molten metal.
IMPORTANT: For general welding it is not necessary to weave the arc; neither forwards and
backwards nor sideways. Weld along at a steady pace. You will find it easier.
NOTE: When welding on thin plate, you will find that you will have to increase the welding
speed, whereas when welding on heavy plate, it is necessary to go more slowly in order to get
good penetration.
PRACTICE
The best way of getting practice in the four skills that enable you to maintain:
1.
Correct Welding Position
2.
Correct Way to Strike An Arc
3.
Correct Arc Length
4.
Correct Welding Speed
is to spend a little more time on the following exercise.
Use the following:
Mild Steel Plate:
Electrode:
Current Setting:
3/16” (4.8mm) or heavier
1 /8” (3.2mm) Fleetweld® 180
105 Amps AC or 95 Amps DC(+)
Do the following:
1.
Learn to strike the arc by scratching the electrode over the plate. Be sure the angle of the
electrode is right and be sure to use both hands.
2.
When you can strike an arc without sticking, practice the correct arc length. Learn to distinguish it by its sound.
3.
When you are sure that you can hold a short, crackling arc, start moving. Look at the
50
molten puddle constantly, and look for the ridge where the metal solidifies.
4.
Run beads on a flat plate. Run them parallel to the top edge (the edge farthest away from
you). This gives you practice in running straight welds, and also, it gives you an easy way to
check your progress. The 10th weld will look considerably better than the first weld. By constantly checking on your mistakes and your progress, welding will soon be a matter of routine.
The Advantages of DC
DC welding permits a uniform, continuous flow of current to help maintain a smooth welding
arc. DC(+), electrode positive, causes the current to flow in one direction and DC(-), electrode
negative, in the opposite direction. Usually DC(-) is preferred for very thin material or when
minimum penetration is desired. Electrodes designed to operate on DC(+) usually give maximum penetration. This makes them very well suited for such applications as welding pipe and
burning through rust, mill scale or other materials which cannot be easily removed.
Stainless, low hydrogen, non-ferrous, hardfacing and other specialty electrodes usually run
smoother on DC than AC. However,- LH-73 operates well on either AC or DC. The smoother
DC arc also makes it easier to weld with small diameter electrodes, so important for critical outof-position work.
When welding with AC the current changes from DC(+) to DC(-) 60 times each second (50
times in most overseas areas) which means the welding current goes through zero 120 times
each second. AC electrodes are designed to maintain the arc through this rapid cycling. AC
welding is characterized by the hissing sound of the arc due to this alternating current. AC is
usually preferred for larger electrodes and for applications where arc blow is encountered.
Common Metals
Most metals found around the farm, small shop or home are low carbon steel, sometimes referred to as mild steel. Typical items made with this type of steel include most sheet metal,
plate, pipe and rolled shapes such as channels, angle irons and “I” beams. This type of steel
can usually be easily welded without special precautions. Some steel, however, contains higher
carbon. Typical applications include wear plates, axles, connecting rods, shafts, plowshares and
scraper blades. These higher carbon steels can be welded successfully in most cases, however,
care must be taken to follow proper procedures, including preheating the metal to be welded
and, in some cases, carefully controlling the temperature during and after the welding process.
For further information on identifying various types of steels and other metals, and for proper
procedures for welding them, we again suggest you purchase a copy of “New Lessons in Arc
Welding” (see page 19).
Regardless of the type of metal being welded, it is important in order to get a quality weld that it
be free of oil, paint, rust or other contaminants.
51
Types of Welds
Five types of welding joints are: Butt Welds, Fillet Welds, Lap Welds, Edge Welds and Corner
Welds.
Of these, the Butt Weld and Fillet Weld are the two most common welds.
Welding Procedures
Butt Welds
Place two plates side by side, leaving 1/16” (1.6mm) (for thin metal) to 1/8” (3.2mm) (for heavy
metal) space between them in order to get deep penetration.
Tack the plates at both ends; otherwise the heat will cause the plates to move apart. (See drawing):
Now weld the two plates together. Weld from left to right (if right handed). Point the electrode
down in the crack between the two plates, keeping the electrode slightly tilted in the direction of
travel.
Watch the molten metal to be sure it distributes itself evenly on both edges and in between the
plates.
Penetration
Unless a weld penetrates close to 100%, a butt weld will be weaker than the material welded
together.
52
In this example, the total weld is only 1/2
the thickness of the material; thus the weld
is only approximately half as strong as the
metal.
In this example, the joint has been flame beveled or ground prior to welding so that 100% penetration could be achieved. The weld, if properly made, is as strong or stronger, than the original
metal.
Successive passes must be used to build up butt welds on heavier metals.
Fillet Welds
When welding fillet welds, it is very important to hold
the electrode at a 45° angle between the two sides, or the
metal will not distribute itself evenly.
To make it easy to get the 45° angle, it is best to put the
electrode in the holder at a 45° angle, as shown:
Multiple Pass Welds
Make multiple pass horizontal fillets as shown in the
sketch. Put the first bead in the corner with fairly high
current. Hold the electrode angle needed to deposit
the filler beads as shown putting the final bead against
the vertical plate.
Welding In the Vertical Position
Welding in the vertical position can be done either vertical-up or vertical-down. Vertical-up is
used whenever a large, strong weld is desired. Vertical-down is used primarily on sheet metal
for fast, low penetration welds.
53
Vertical-Up Welding
The problem, when welding vertical-up, is to put the molten metal
where it is wanted and make it stay there. If too much molten
metal is deposited, gravity will pull it downwards and make it
“drip’ Therefore a certain technique has to be followed:
1.
Use 1 /8” (3.2mm) at 90-105 amps or 3/32” (2.4mm) at 60 amps Fleetweld® 180 elec-
trode.
2.
When welding, the electrode should be kept horizontal or pointing slightly upwards. (See drawing)
3.
The arc is struck and metal deposited at the bottom of the two pieces to be welded togeth-
er.
4.
Before too much molten metal is deposited, the arc is SLOWLY moved 1/2-3/4” (12.5-
19mm) upwards. This takes the heat away from the molten puddle, which solidifies. (If the arc is not taken away soon enough, too much metal will be deposited, and it will “drip’)
5.
The upward motion of the arc is caused by a very slight wrist motion. Most definitely, the arm must not move in and out, as this makes the entire process very complicated and dif-
ficult to learn.
6.
If the upward motion of the arc is done correctly with a wrist motion, the arc will auto-
matically become a long arc that deposits little or no metal. (See drawing)
7.
During this entire process, the ONLY thing to watch is the molten metal. As soon as it has solidified, the arc is SLOWLY brought back, and another few drops of metal are depos- ited. DO NOT FOLLOW THE UP AND DOWN MOVEMENT OF THE ARC WITH THE EYES. KEEP THEM ON THE MOLTEN METAL.
8.
When the arc is brought back to the now solidified puddle. IT MUST BE SHORT, other- wise no metal will be deposited, the puddle will melt again, and it will “drip”
9.
It is important to realize that the entire process consists of SLOW, DELIBERATE move-
ments. There are no fast motions.
Vertical-Down Welding
Vertical-down welds are applied at a fast pace These welds are therefore shallow and narrow,
and as such are excellent for sheet metal. Do not use the vertical-down technique on heavy
metal. The welds will not be strong enough.
1.
Use 1/8” (3.2mm) or 3/32” (2.4mm) Fleetweld® 180 electrode.
2.
On thin metal, use 60-75 amps. 14 ga 75 amps - 16 ga 60 amps (1.9mm 75 amps - 1.5mm 60 amps).
3.
Hold the electrode at a 30-45° angle with the tip of the electrode point
ing upwards.
4.
Hold a VERY SHORT arc, but do not let the electrode touch the metal.
54
5.
6.
An up and down whipping motion will help prevent burn-through on very thin plate.
Watch the molten metal carefully.
The important thing is to continue lowering the entire arm as the weld is made so the angle of
the electrode does not change. Move the electrode so fast that the slag does not catch up with
the arc. Vertical-down welding gives thin, shallow welds. It should not be used on heavy material where large welds are required.
Overhead Welding
Various techniques are used for overhead welding. However, in the interest of simplicity for the
inexperienced welders the following technique will probably take care of most of his needs for
overhead welding:
8.2 OXY-ACETYLENE WELDING
This is the operation by which two thin plates are joined with the same material, fusing their
edges by means of a flame produced by the combustion of oxygen and acetylene mixed in a
welding torch. It is a basic operation to initiate the oxy-acetylene welder into uniform movement with the torch. It is used frequently in sheet-metal work.
METHOD OF EXECUTION:
1st Step - Prepare the equipment.
OBSERVATIONS:
1) The tip is selected according to the base metal.
2) Before mounting the tip, ensure that its hole is clean.
3) Use the needle that is suited to the hole of the tip.
2nd Step – Prepare the material, thus:
a)
Clean the plates
b) Even-up the plates
c)
Position the plates for tacking (fig.1)
3rd Step – Tack, thus:
a)
b)
Ignite and regulate the torch flame
Put on the welding goggles
OBSERVATION
The pressures and tip are selected according to the tables provided by the manufacturers.
55
SAFETY MEASURES
FOR WELDING, USE GOGGLES WITH THE APPROPRIATE GLASS
NUMBER
c)
d)
e)
Bring the cone to a distance of 3mm from the base material (fig.2)
Preheat the area to be tacked
Fuse the edges with a circular movement.
OBSERVATION:
The tacking should be kept at a distance equal to 25 times the thickness of the base material.
SAFETY MEASURE:
TAKE CARE! THE FLAME BACK-FIRE CAN PRODUCE AN EXPLOSION.
4th Step - Weld, thus:
a)
Incline the tip through 45° and direct the cone to the center
of the joint (fig. 3)
b) Preheat the base material in the area to be welded.
c)
Begin the bead, maintaining the cone at a constant height
d) Advance the tip while oscillating (fig. 4).
e)
Finish the bead.
f)
Turn off the torch.
56
This type of joint is done in a flat position, with the filler material. It allows the welder to make
joints in the sheets. It is extensively used in car bodies, ventilation ducts and metal furniture.
METHOD OF EXECUTION:
1st Step - Prepare equipment.
OBSERVATION:
Look at table for selection of tips with relation to thickness.
2nd Step - Prepare material, thus:a)
Clean sheets eliminating rust and impurities.
b) Even up the work-piece.
c)
Assemble and tack according to fig. 1.
PRECAUTION:
WEAR THE SAFETY EQUIPMENT.
d)
e)
Even up by correcting distortions, after tacking.
Start a small bead in the opposite direction to the advance (fig. 2).
OBSERVATION:
This small bead is known by the name of a heel.
3rd Step - Weld the joint, thus:
a)
Incline tip with regard to the work piece (fig. 3) and preheat the base metal on starting the bead.
b) Incline the rod as seen in figure 4.
c)
Advance making a semi-circular movement with the tip (fig. 4).
d) Oscillate the filler rod as you advance in a zig-zag manner.
OBSERVATIONS:
1) When oscillating the rod, avoid coming out of the fusing zone.
2) The movements of the tip and the filler rod must be uniform and coordinated between them.
57
8.3
THERMAL CUTTING
Fissures are small or moderately sized separations along grain boundaries. They are readily
formed here because of the large grains. High restraint stresses cause the separation.
Thermal Cutting
Processes allied to welding that interest the weld-ing inspector are oxyfuel gas cutting (OFC)
and air carbon-arc cutting or gouging (AAC). A welding inspector may also encounter plasma
arc cutting (PAC). These processes are used for edge preparation, repairing welds and backgouging.
Quality, of cut surfaces varies with the nature of the weldment. The welding inspector should
know what difficulties the welder may have with irregularities in the cut surface. Unacceptable sizes and locations of irregularities must be marked for repair, just as defective welds are
marked.
Gouging grooves for welding and back-gouging the root passes from the reverse side are usually done manually, but they also can be done by machine and automatic methods.
The air carbon-arc process may allow carbon to dissolve in the surface of stainless steel grooves
or cuts. To minimize carbide precipitation, the inspector should recommend grinding the cut
stainless steel surfaces to remove the outer 0.4 mm (1/64 in.).
Oxyfuel Gas Cutting
Oxyfuel gas cutting (OFC) was at one time exclusively oxyacetylene cutting (OFC-A), but in
present day practice, the inspector will encounter natural gas (OFC-N), propane (OFC-p) and
a proprietary mixture of stabilized methylacetylene and propadiene (Mapp gas). Metal powder
cutting (Poc) of stainless steel, aluminum and copper alloys may also be encountered. Oxygen
cutting severs ferrous metals by burning the iron in oxygen to form iron oxide. Above a kindling
temperature of about 940 C (1700 F) the familiar oxidation of iron (rusting) becomes a combustion, which may be confined to a narrow well-defined zone of controlled width called the kerf. Fig. 5-19. Oxygen culling attachment on a welding lorch.
(Linde Division, Union Carbide)
58
The parts to be cut are heated to the kindling temperature by preheat flames disposed around the
oxygen cutting jet. Torch modifications to suit each fuel gas are required.
An oxygen cutting attachment for a welding torch is shown in Fig. 5-19. The lever opens the
oxygen jet.
Oxyfuel gas cutting is often applied as a manual, machine or fully automatic method of plate
preparation. The quality of cut surface varies over wide limits. The skill of the operator affects
all operations, because the cutting flame must be manually adjusted even for automatic cutting.
Plasma arc cutting is less susceptible to this variable.
Air Carbon-Arc Cutting and Gouging
The air carbon arc (ARC-AIR) process removes metal by melting the metal with a carbon arc,
then blowing out the molten metal by compressed air. A high velocity air jet, traveling parallel
to the electrode and striking the puddle just behind the arc, blows the molten metal away. The
principle of air carbon-arc gouging is shown in Fig. 5-20.
The basic equipment requires a power source with suitable capacity (up to 1300 A for 16 mm-or
5/8 zinc electrodes), compressed air at 0.55 to 0.7 MPa (80 to 100 psi), welding cables and work
clamp, an air carbon arc torch and carbon electrodes.
Air carbon arc gouging is performed both manually and automatically.
Plasma Arc Cutting
The greater heat of the plasma arc (15,000 C or 27,000 F) will melt a kerf through any metal,
ferrous or nonferrous, removing the molten material with its high velocity jet of hot ionized gas.
The process operates on direct current electrode negative (dcen) with a constricted arc, struck
between the water-cooled electrode in the torch and the work piece.
Fig. 5-20. Schematic diagram for air carbon-arc gouging.
(Hobart Brothers Co.)
The orifice, which constricts the arc, also is water-cooled. The schematic view in Fig. 5-21
shows the power supply and _controls, the pilot arc circuit through a current-limiting resistor,
the dcen circuit through the work piece and the water-tooled torch with gas inlet for either ni59
trogen or argon, plus 0 to 30% hydrogen. Water may be injected into the torch nozzle to further
constrict the arc and square up the kerf.
Plasma arc cutting is a machine or automatic process.
Mechanical Cutting
Joints are also prepared for welding either in part or entirely by mechanical means, such as milling, grinding, shaping, sawing, shearing and chipping.
Fig. 5-21. Schematic diagram for plasma-arc cutting. (Aluminum Company of America)
The inspector is frequently concerned about residues of sulfurized cutting oils used to lubricate
the cutting tools, not only because sulfur may cause cracking in welds, but also because all oils
are a source of hydrogen. The unfused root in passes chipped or ground back from the other side
to expose sound metal may be hidden by metal smeared over the unfused root of the joint by a
blunt tool or a loaded wheel. On critical jobs, the inspector should have the supposedly exposed
bead etched to remove such smeared metal and verify that the weld metal is revealed.
60
Distortion and
Residual Stress
61
RESIDUAL STRESSES:
The strength of a welded joint depends a great
deal on the way expansion and contraction of the
metal is controlled during the welding operation.
Whenever heat is applied to a piece of metal,
expansion forces are created which tend to distort’ the dimension of the piece. Upon cooling,
the metal undergoes a change again as it attempts
to resume its original shape. When free movement is restricted there is likely to occur warping
or distortion if the metal is malleable or ductile, and fracture if the metal is brittle, as with cast
iron. If expansion is restricted in its movement the building up force will find another direction
for uneven expansion in the heated area and some of the original displaced metal will contract
unevenly on cooling and the work piece will become permanently distorted.
CONTROLLING RESIDUAL STRESSES:
Few simple procedures will help control undue
forces caused by expansion and contraction;
Proper edge preparation and fit up; Reduced bevel
angle with sufficient roan in the joint to permit
proper manipulation of the electrode. Weld joint
nearest to the neutral axis first, followed by welding the unit farthest from the neutral axis. On
long seams especially thin section allow 3mm per
300mm length for weld expansion during set up
procedures. Preset pieces of the joint slightly out of alignment to counteract direction of distortion.
FORMS OF DISTORTION:
Angular distortion in a fillet weld is caused by unrestrained plates being drawn together as the plates
and weld cool. Distortion will increase as further
unbalanced runs are deposited. A flat, thin plate
will bend upwards as the plate and weld cool. Two
butt welded plates which are free to move will be
drawn together as welding takes place. Distortion
may occur if the welds are made on one side of the
joint. Double sided butt joints will distort less. In a T-joint the weld along the seam will bend
both the upright and flat piece.
Distortion is counteracted by welding to a specified pattern given in weld procedures. This is
known as balanced welding.
(= 15.1 x 10-6/0c)
62
LIMITING DISTORTION BY WELD SEQUENCE:
Track welds are also used to hold plates in position and
control undue expansion on long seams. A long longitudinal (end ways) seam is welded before a short transverse
(side-ways) seam. Jigs and fixtures are used to preset and
hold plates and prevent excessive movement, heavy fixture plates not only control distortion but they also serve
as chill blocks to avoid excessive heat building up in the
work.
Intermittent or skip weld is employed to minimize heat input by making a short weld at the beginning (1) center of seam (2) end of joint (3) and repeating the cycle for completing the seam.
In back - step or step - back technique instead of laying a continuous bead from left to right,
deposit off short sections of the beads from right to left to complete the pass.
PREHEATING:
On many work pieces, particularly alloy steel and cast
iron, expansion and contraction forces can be better
controlled if the entire structure or large parts of the
weld area can be preheated before the welding is started.
Ensure uniform temperature during welding operation
and care for slow cooling.
POST HEATING (STRESS RELIEVING):
Stresses set up during welding may be relieved by heating the work piece for a stated temperature and period
of time and cooling in controlled rate.
63
9.4 PREVENTION, CONTROL AND CORRECTION OF DISTORTION
TRAINING NOTES
Fixtures
Fixtures may temporarily or specially made for quick and correct positioning of materials to be
welded.
Tack Weld
A short weld used to help assembly by holding work
pieces in position during welding.
Backing Bar
A piece of metal tacked behind a butt or corner joint
to aid the welding operation but not intended to become part of the finished joint.
Backing Strip
A piece of metal tacked at the root of the joint which
becomes part of the joint when welding is complete.
64
Bridge
Pieces of metal of the same shape which are tack welded in several places round pipes to hold them in position. A bridge may be
used to hold the clamp of an earth return lead close to the weld
area.
A short continuous weld in the root gap may also be called a
bridge.
Strongback
Similar to a bridge and used to hold work pieces in position for butt welding in the flat, horizontal, vertical and overhead positions.
Temporary Fixtures
a) Temporary fixtures may be tack welded to work pieces to hold materials which are to be
welded.
Care must be taken to ensure that the fixture does not move during welding. Dimensions should
always be checked for accuracy after tacking and before starting to weld.
Tack Welded Cleats as an aid to alignment
65
b) Work pieces may be lap jointed in position for welding,
using clamps and angle bars.
c) Cleats temporarily attached to work pieces to hold a T
joint in position for welding.
d) A strongback, bolt and clamp may be attached to work
pieces to hold them in position whilst a butt joint is welded.
e) A bolt, clamp and hard wood blocks may be used to hold an
angle section in position during welding.
66
Positioners
The universal balance type of positioner allows for positioning of the work piece in any direction.
Manipulators
The most widely used types of manipulators are motor driven
and designed to rotate and move work at the required welding
speed
Manipulation and Positioning of Work pieces
Ensure that components placed in or on a fixture, positioner
or welding manipulator are properly balanced and secured.
67
Welding
Symbols
And
Blueprint
Reading
68
69
70
Welding Terminology
The following are the meaning of some of the terms used in welding
Angle of Bevel
The angle of an edge or end which is cut or chamfered.
Arc Length
The distance between the end of an electrode and
the surface of the weld pool.
Fusion Face
The surfaces or edges of the parent metal to be
fused by welding.
Fusion Zone
The place where the deposited metal fuses with the
work pieces.
71
Gap
The distance between the parts to be joined
Included Angle
The total angle between fusion faces of the parts in
position ready for welding.
Parent Metal (or work piece)
The materials and or the parts to be welded
Penetration
The depth the molten metal from the electrode penetrates the parent metal
Reinforcement (Excess weld metal)
Weld metal lying outside the plane joining the toes.
Root
The position in a prepared butt joint where the parts
are to be joined is nearest together
Or
The corner of the angle formed by the two fusion
faces of a fillet joint
72
Root Face
The square faces at the root of prepared work pieces
Toe of Weld
The top and bottom position where the weld face joins the
parent metal
Leg Length
The distance from the root to the toe of the weld
Throat Thickness
The shortest distance from the root of the weld to the weld
face in a fillet weld
Tack Weld
A short weld used to help assembly by holding work pieces
in position during welding
Run or Pass
The molten metal deposited during the passage of the electrode.
Root Run
The first run deposited in the root of a joint where there is to
be more than one run
73
Filler Runs or Passes
The build up run(s) between root and capping run(s)
Capping or Final Runs
The weld runs which make up the top layer of the joint
Sealing Run
A weld deposited on the roof side of a butt or corner joint after
completion of the main weld.
Sealing Weld
A weld used to make a fluid tight joint
JOINT & WELD SYMBOLS – MANUAL METAL ARC WELDING
TRAINING NOTES
Types of Joint
Butt Welded Joint
A joint where a weld is made between the ends or edges of
metals.
Fillet Welded Joint
A weld approximately triangular in shape and external to the
surfaces to be joined.
Square Butt Joint
A joint with permanent backing strip held in place by fillet
welds.
Double Lap Joint
A joint made by two fillet welds
74
Inspection And
Quality Control
75
BASIC WELDING
TECHNOLOGY
WELDING INSPECTION
& QUALITY CONTROL
LECTURER:
C. DAVIS
M.I.C. TRAINING DEPARTMENT
11. 1. COMMON WELD DEFECTS - HANDOUT
2.
CODES GOVERNING WELDING INSPECTION
Every * country has some code for Welding Inspection or adapts to some acceptable international code e.g. DIN, BS, AWS, ASTM,
Lloyds of London. The American codes are commonly used in our region and are also widely accepted internationally.
Some organisations which have formulated codes and rules for welding are as follows:(a) AWS American Welding Society
(b) ASME
American Society for Mechanical Engineers.
(c) ANSI
American National Standards Institute
(d) API
American Petroleum Institute
(e) MIL
United States Department of Defence
(f) ABS
American Bureau of Shipping
(g) AWWS
American Water Works Association
(h) ASNT
American Society for Non-Destructive Testing.
(i) ASTM
American Society for Testing Materials
(j) SAE
Society of Automotive Engineers.
All these societies require that records are kept and be made available for the Inspector. All
drawings must be kept together with inspection records.
* Japan, Germany, France, British, Italy, Norwegian
It should be noted that most of these codes were first developed by private organisations and
after some time were adopted by various government agencies, and eventually some because of
laws governing weldments.
WELDING INSPECTION METHODS
1.
Macro-Inspection:Visual, low magnification x 5
2.
Micro-Inspection: High Magnification x 101 - 106
3.
Non-Destructive - Inspection: Using aids to assist in the identification of flaws.
Job must not be physically damaged during testing process.
4. Destructive Testing - To verify physical and chemical properties of the weld or for failure
analysis.
1.1 MACRO INSPECTION
This method of inspection requires that the inspector be an experienced welder who is very
familiar with welding processes and various defects associated with the processes, preferably a
coded inspector or one who has completed a welding inspection course of studies from a recognized institution, e.g. , AWS, ASM, API, etc.
Defects in this category of inspection require direct observation with the eyes or very low powered magnification after etching. Etching requires the use of small quantities of acids in solution
76
with alcohol, water or other solvents. Penetration, porosity, fusion zones and other phenomena
may be observed.
1.2MICRO-INSPECTION
This is a destructive method of testing since a sample has to be cut out from the welded member to be tested. Samples are prepared and place under a high power microscope to observe
grain structures, Fin, and other micro defects that cannot be seen by macro-inspection. Chemical analysis also falls under the category and requires that small pieces of materials be removed
from the job and mounted, polished and view under the optical microscope or Scanning Election Microscope (S.C.M.)
1.3 NON-DESTRUCTIVE TESTING
There is a range of tests done on the actual job to detect surface or sub-surface flaws, the component remains completely functional after the tests.
The following are NDT test commonly done on weldments:
(a) Liquid Penetrant Inspection. (LP)
(b) Magnetic Particle Inspection (MP)
(c)
Ultrasonic Inspection(UT)
(d) Radiographic Inspection
(RI)
(e) Eddy-current Inspection
(EI)
(f) Acoustic Emission Inspection (AE)
(g)
Proof Tests(PT)
(h)
Leak Tests(LT)
1.4 DESTRUCTIVE TESTING
There are test done to verify the physical properties and also the chemical properties of weld
and base metals. This method of testing requires that the component be destroyed in order to
obtain a sample for testing. The tests are done to verify:
(a) Tensile strength of weld and base metal
(b) Impact strength “” “” “” “” “” “””
(c) Compression strength “” “” “” “” “” “””
(d) Shear strength “” “” “” “” “” “””
(e) Bending strength of welded joint
f) Metallographic Inspection
2.1.0 INSPECTION PERIODS
2.1.1 BEFORE WELDING
(a) Material to be used for fabrication - scabs, seams, scales, laminations plate dimension, tensile testing.
(b) After assemblies - root opening, edge preparation and other features of joint geometry.
(c) Checks on backing strips, run on and run off plates etc.
77
(d) Cleanliness of material and joint before welding begins.
2.1.2 INSPECTION DURING WELDING
Among details to be checked are:
(a) Welding Process
(b) Filler metal
(a) Flux or shielding gas
(d) Pre-heat and interpass temperature/Post heating and time
(e) Cleaning (interPass)
(f) Chipping, grinding or gonging
(g) Joint preparation for other side
(h) Distortion control
2.1.3 INSPECTION AFTER WELDING
(a) Conformity to drawings
(b) Appearance of weldment
(c) Presence of structural discontinuties
(d) Any defects that may be visible.
78
WELDING: QUALITIES, CHARACTERISTICS AND RECCMMENDATICNS
Among other things, a good weld must offer safety and quality. To attain these objectives, it is
necessary that the welding beads be made with a maximum of skill, good regulation of the current and proper selection of electrodes.
CHARACTERISTICS OF A GOOD WELD
A good weld must have the following characteristics:
a)
Good penetration.
b) No undercutting
c)
Complete fusion.
d) No porosity.
e)
Good appearance.
f)
No cracks.
Good penetration
This is obtained when the filler metal fuses the root and is extended under the surface of the
welded parts.
No undercutting
A weld without undercut is obtained when near to its root (toe) there is not produced on the base
metal any digging ‘which damages the work piece.
Complete fusion
A good fusion is obtained when the base metal and the filler metal form a homogeneous mass.
No porosity
A good weld is free of porosity, when in its inner structure there are no gas pockets nor slag
inclusion.
Good appearance
A weld has good appearance, when there is seen in the whole extension of the joint an even
welding bead, without cracks or overlapping.
No cracks
A weld is considered without cracks when the finished bead has no fissures throughout its
length.
Following are some recommendations for producing a good weld.
79
80
81
82
APPENDIX
SAE Steel Numbering System
83
PIPE AND PIPE FITTINGS
Wrought Steel Pipe — ANSI B36.10-1979 covers dimensions of welded and seamless wrought
steel pipe, for high or low temperatures or pressures.
The word pipe as distinguished from tube is used to apply to tubular products of dimensions
commonly used for pipelines and piping systems. Pipe dimensions of sizes 12 inches and smaller have outside diameters numerically larger than the corresponding nominal sizes whereas
outside diameters of tubes are identical to nominal sizes.
Size: The size of all pipes is identified by the nominal pipe size. The manufacture of pipe in the
nominal sizes of V8 inch to 12 inches, inclusive, is based on a standardized outside diameter
(OD). This OD was originally selected so that pipe with a standard OD and having a wall thickness which was typical of the period would have an inside diameter (ID) approximately equal
to the nominal size. Although there is now no such relation between the existing standard thicknesses, ODs and nominal sizes, these nominal sizes and standard ODs continue in use as “standard.”
The manufacture of pipe in nominal sizes of 14-inch OD and larger, proceeds on the basis of an
OD corresponding to the nominal size
Weight: The nominal weights of steel pipe are calculated values and are tabulated in Table 1.
They are based on the following formula:
Wpe = 10.68(D –t)t
Where Wpe = nominal plain end weight to the nearest 0.01 lb/ft.
D = outside diameter to the nearest 0.001 in.
t = specified wall thickness rounded to the nearest o.00t in.
Wall thickness: The nominal wall thicknesses are given in Tablet which also indicates the wall
thicknesses in API Standards 5L and 5LX. Thicknesses listed in API Standard 5LS are not indicated but may be found in that Standard or in ANSI B36.10-1979.
The wall thickness designations “Standard,” “Extra-Strong,” and “Double Extra-Strong” have
been commercially used designations for many years. The Schedule Numbers were subsequently added as a convenient designation for use in ordering pipe. “Standard” and Schedule 40 are
identical for nominal pipe sizes up to 10 inches, inclusive. All larger sizes of “Standard” have
1/2-inch wall thickness. “Extra-Strong” and Schedule 8o are identical for nominal pipe sizes up
to 8 inch, inclusive. All larger sizes of “Extra-Strong” have 1/2-inch wall thickness.
Wall Thickness Selection: When the selection of wall thickness depends primarily on capacity
to resist internal pressure under given conditions, the designer shall compute the exact value of
wall thickness suitable for conditions for which the pipe is required as prescribed in the “ASME
Boiler and Pressure Vessel Code,” “ANSI B31 Code for Pressure Piping,” or other similar
codes, whichever governs the construction. A thickness can then be selected from Tablet to suit
the value computed to fulfil the conditions for which the pipe is desired.
Metric Weights and Mass: Standard SI metric dimensions in millimetres for outside diameters
and wall thicknesses may be found by multiplying the inch dimensions by 25.4. Outside diameters converted from those shown in Table I should be rounded to the nearest o.t mm and wail
thicknesses to the nearest 0.01 mm.
The following formula may be used to calculate the SI metric plain end mass in kg/m using the
84
converted metric diameters and thicknesses:
Wpe = 0.02466(D t)t
where Wpe = nominal plain end mass rounded to the nearest 0.01 kg/m.
D = outside diameter to the nearest o.t mm for sizes shown in Table I.
t= specified wall thickness rounded to the nearest 0.01 mm.
85
86
87
88
89
90
91
Temperature
Con version
Table
92
93
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement