Chapter Chapter 3 The Physics of Welding The Physics of Welding

Chapter Chapter 3 The Physics of Welding The Physics of Welding
This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved.
Welding Theory
Chapter
C
hapt er 3
The Physics
of Welding
Learning
bjectives
After studying this chapter, you will be able to:
•
•
•
•
•
•
•
•
Identify the three general methods by which a
weld is achieved.
Describe the difference between chemical and
mechanical properties and give examples of
each.
Explain the effects of welding on metal.
Identify processes used to heat-treat metal.
Describe the relationship between voltage and
current.
Give examples of US conventional and SI metric
units of measurement.
Convert US conventional units of measurement
to SI metric units.
Convert SI metric units of measurement to US
conventional units.
Welding with Heat
Electrodes
Heat is used to create welds in many welding
processes. Filler material is commonly used when
welding with heat only. Shielded metal arc welding
(SMAW) is a process that uses heat and filler material.
In the SMAW process, heat is created by an arc that is
struck between an electrode and base metal (metal to
be welded). See Figure 3-1. The heat causes the end of
the electrode and an area of the base metal to melt.
When two pieces of base metal are placed together,
both are heated by the arc. A portion of each piece
melts and the liquid areas flow together. The molten
filler material combines with the molten base metal.
The molten material cools and becomes solid, creating
a weld.
Welding with Heat and Pressure
Technical
Terms
amperes
annealing
base metal
brittleness
chemical
composition
chemical properties
compressive strength
contraction
corrosion resistance
current
density
ductility
electrode
expansion
grain size
hardness
interpass heating
mechanical
properties
normalizing
Some welding processes use both heat and pressure. Filler material is generally not used in these
processes. In resistance spot welding, pressure is
applied through opposing electrodes. Electrical current
flows from one electrode through the base metal to the
other electrode. Resistance to the electrical current provides the heat required to join the pieces. A resistance
spot welding machine is shown in Figure 3-2.
ohms
open circuit voltage
(OCV)
oxidation resistance
physical properties
preheating
quenching
resistance
root opening
shielded metal arc
welding (SMAW)
SI metric system
strength
stress-relieving
tempering
tensile strength
toughness
US conventional
system
volts
voltage
Figure 3-2. This is a rocker-arm-type spot welding machine.
Electrodes apply pressure to pieces to be welded. Resistance to
electric current between the electrodes results in heat. (LORS
Machinery Company)
Welding with Pressure
Metal or other material can be welded together
using pressure alone. Heat and filler material are not
required. Cold welding is an example of a welding
process that does not require the use of heat or filler
material. In the cold welding process, very clean pieces
of metal are forced together under considerable pressure. The pressure forces the atoms of the materials
together to create a weld.
Properties of Metals
Figure 3-1. Shielded metal arc welding (SMAW) is being used
by this welder to assemble parts of a metal building. (Lincoln
Electric Company)
Chapter 3 The Physics of Welding
Rocker arm
Welding is a group of processes used to join
metallic or nonmetallic materials. Welding is often
done using heat. It can also be done using pressure or
a combination of heat and pressure. A filler material
may or may not be added to the weld joint.
29
30
The properties of a metal determine how it is
used. Properties can be grouped into three categories:
physical, chemical, and mechanical. These properties
are largely determined by the chemical composition
of a metal.
The chemical composition of a metal alloy consists of the different metals or elements that are combined to produce that alloy. For example, the chemical
composition of low carbon steel is a combination of
iron and carbon. The chemical composition of stainless steel includes iron, chromium, nickel, manganese,
and carbon.
Welding Technology Fundamentals
This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved.
Properties of metals are affected by the method of
processing. Processing includes bending, rolling,
forming, heat-treating, and welding.
Physical Properties
Physical properties are used to identify or describe
a metal. These include color, melting temperature, and
density. Density is the weight of a particular material
per unit volume.
Chemical Properties
Chemical properties determine the way a material
reacts in a given environment. Corrosion resistance
and oxidation resistance are chemical properties that
determine how a material will withstand the effects of
the environment. Corrosion resistance is the ability of
a material to withstand corrosive attack. Acids and
saltwater are both corrosive. A material resistant to
attack by an acid, however, may not be resistant to the
effects of salt water. Oxidation resistance is the ability of a material to resist the formation of oxides. Metal
oxides occur when oxygen combines with the metal.
Mechanical Properties
Mechanical properties determine how a material
reacts under applied loads or forces. Testing procedures for mechanical properties are discussed in
Chapter 34.
Strength is the ability of a material to withstand
applied loads without failing.
Tensile strength is the ability of a material to resist
pulling forces.
Compressive strength is the ability of a material to
resist pressing or crushing forces. Tensile strength and
compressive strength are exact opposites, with respect
to the direction of the applied load. Figure 3-3 shows
the difference between tensile and compressive forces
acting on a material.
Ductility is the ability of a material to stretch or
bend without breaking.
Brittleness is the inability of a material to resist
fracturing. Brittleness is the exact opposite of ductility.
A material that is brittle has very low ductility and
breaks easily.
Toughness is the ability of a material to resist cracking and prevent a crack from progressing. Materials
with a high amount of ductility are usually very tough.
Hardness is the ability of a material to resist indentation or scratching. Hardness testing is done by forcing a steel ball or pointed diamond into the surface of
a material. Hardness of some steels can be improved
through various processes, such as quenching and
tempering. Quenching and tempering are discussed
later in the chapter.
Chapter 3 The Physics of Welding
Expansion and Contraction of Metal
Original
shape
Applied
Load
Applied
Load
A
Shape after load
is applied
Applied
Load
Applied
Load
B
Original
shape
Figure 3-3. A—When testing tensile strength, the thickness of
the material is reduced and length is increased. B—When testing compressive strength, the thickness of material is increased
and length is decreased.
When heat is applied to metal, the metal expands
(increases in size). When heat is removed, the metal
cools and contracts (reduces in size). Expansion and
contraction create stress in the metal. The metal may
relieve the stress by changing shape (warping). In
many welding applications, deformation or movement
is not acceptable. Welding jigs or fixtures are commonly
used to keep parts from moving. However, while being
welded, stress remains in parts that are clamped.
When welding a butt joint, the root opening may
be reduced in size toward the end of the weld. The
root opening is the space between the pieces to be
welded. This occurs because the weld metal contracts
and pulls the pieces together, as in Figure 3-4A. To
prevent this, the pieces should be tack welded as
shown in Figure 3-4B. The outer edges of the base
metal may also bend toward the weld bead. To prevent this, the pieces may be clamped into a welding
•
Align the parts to allow for contraction during
welding.
•
•
•
•
Use welding jigs or fixtures.
31
Heat-treat the welded parts.
Use the proper welding procedure.
The principles of expansion and contraction can
be used to straighten parts. If a flat plate or weldment
is bent or warped, heat can be used to straighten the
piece. An oxyfuel gas torch is used to locally heat the
metal, causing it to expand. To be effective, this heating must be done on the correct side of the metal and
in the correct location. This metal will not contract to
Weld
bead
Effects of Welding
The chemical and mechanical properties of a
metal are often affected when the metal is welded.
Heat produced by welding creates stress in the metal.
Heat also affects the ductility and toughness of the
metal. If the metal was heat-treated before being
welded, the effects of heat-treating are lost in the area
around the weld.
A weld that is made correctly is usually stronger
than the base metal. An incorrect welding procedure
may result in serious problems. For example, a stainless steel pipe that is corrosion-resistant to certain
chemicals may lose its resistance at an improperly
welded joint. Chemicals within the pipe may attack
the weld area, causing the pipe to leak.
Another example where the correct welding procedure is critical is armor plate for the military. Armor
plate is very tough. It resists cracks and prevents
cracks that do start from getting larger. If an improper welding procedure is used, the metal near the weld
area may lose its toughness. If a shell hits the armor, a
crack may develop at the weld and continue along the
weld joint.
Preheat the parts.
Straightening
Root opening
decreases as
weld progresses
Grain size is important in determining the
mechanical properties of a material. Large or coarsegrained materials have a high amount of brittleness
and a low amount of ductility. Fine-grained materials
have a low amount of brittleness and a high amount
of ductility.
fixture or positioned with a reverse angle to compensate for the movement. Figure 3-5 shows the effects of
welding an unrestrained butt joint and a method used
to compensate for the movement.
When welding a T-joint, the vertical piece may be
pulled toward the weld bead. To prevent this, place
the pieces in a welding fixture. Another solution is to
tack weld the vertical piece a few degrees from the
perpendicular, as shown in Figure 3-6.
In general, six techniques may be used to reduce
movement and/or stress when welding.
• Tack weld the parts.
Weld
bead
Original
root opening
A
Root opening
Tack
weld
A
Base metal moves
in the direction of
the weld bead
Base metal moves
in the direction of
the weld bead
Top View
B
End View
Figure 3-4. A—When a butt joint is not tack welded, the root
opening decreases as the weld progresses. In extreme cases, the
pieces may overlap. B—Notice the two views of a V-groove butt
joint that is tack welded. The root opening is uniform and will
be held uniform by the tack welds during the rest of the weld.
32
Welding Technology Fundamentals
B
Weld
bead
Position
after
welding
Position
prior to
welding
Figure 3-5. A—When the pieces in a butt joint are unrestrained, the outer edges bend toward the weld bead. B—When
the pieces in a butt joint are tack welded with a reverse angle,
the offset can compensate for the distortion. The amount of offset is determined by experimentation.
This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved.
Vertical
plate pulls
toward
the weld
Weld
bead
Interpass Heating
Interpass heating is a method of heating metal while
it is being welded or between weld passes. Interpass
heating is commonly used on thick plates and pipes.
Preheating is used to heat the metal to the desired temperature. Interpass heating is used to maintain an elevated temperature. Preheating and interpass heating
are both used to reduce or minimize the amount of
expansion, contraction, and stress resulting from welding.
Annealing and Normalizing
A
Position prior
to welding
Position
after
welding
Vertical plate pulls
toward weld bead
Weld
bead
B
Figure 3-6. A—With unrestrained pieces in a T-joint, the vertical piece is pulled toward the weld bead. B—This T-joint is
tack welded a few degrees from vertical. The offset compensates
for the movement of the vertical piece.
its original shape or position when it cools. Although
straightening a part is not easily performed, it is a skill
that can be acquired through practice.
Heat-Treating
Various heat-treating processes are used in industry.
Different heat-treating processes are used to accomplish
different purposes. Typical processes include annealing,
stress relieving, and quenching and tempering. In addition to heat-treating a part after welding, heat may be
applied before or during the welding process. Preheating
is done just before the welding operation. Interpass
heating is done while the weld is being completed.
Annealing and normalizing are heat-treating
processes in which a metal is heated and allowed to
cool slowly. The result is a decrease in the hardness
and an increase in the ductility of the metal, making
the material easier to bend and machine. The material is also less likely to crack and become distorted.
Normalizing involves heating the metal to a very
high temperature in an oven or furnace and then
removing the heated material from the oven and allowing it to cool to room temperature. Although annealing
is a process similar to normalizing, it requires a more
carefully controlled and much slower cooling process.
After the metal has been heated to the proper temperature for the correct amount of time, it is kept in an
annealing oven where the temperature is slowly lowered in small increments.
Stress-Relieving
Stress-relieving is similar to annealing, except that
lower temperatures are used. For example, to relieve
stress, mild steel is heated to only 1200°F (650°C) and
is kept at that temperature for a few hours. The metal
is then allowed to air cool.
Quenching and Tempering
Quenching and tempering are processes used to
harden steel and steel alloys. In the quenching process,
metal is heated to a fairly high temperature. The temperature is maintained for a given period of time. The
metal is then cooled quickly by immersing it in a bath
of water, oil, or other liquid. This produces a very
hard and brittle metal. The metal is then tempered by
reheating it to several hundred degrees and cooling it.
After tempering, the metal is less hard and no longer
brittle. A tempered metal has good toughness.
Preheating
Preheating is used to raise the temperature of the
metal before welding. Preheating causes less local
expansion of the part during welding. When preheating, the entire part is heated rather than one specific
area. When the part cools, less contraction occurs, and
less stress is developed.
Chapter 3 The Physics of Welding
Electrical Principles
Electricity is produced by the movement of electrons within a circuit. Electricity is measured in terms
of voltage and current. Voltage is the force that causes
electrons to flow through a circuit. Voltage is measured in volts. It can be compared to water pressure.
33
When your kitchen sink faucet is turned off, water
pressure is still present. See Figure 3-7A. When the
faucet is opened, water begins to flow because the
pressure is forcing the water out of the faucet.
Likewise, voltage is always present in an electrical circuit. Although an arc may not be struck (started)
between the base metal and electrode, voltage is still
present, Figure 3-7B. This is known as open circuit
voltage (OCV). When an arc is struck, voltage forces
electrons across the arc.
The air gap between the electrode and base metal
offers resistance to the flow of electrons. Resistance is
the opposition to the flow of electrons. Resistance is
measured in ohms. A higher voltage setting on electric
arc welding equipment allows the arc length to be
longer. The arc stops if the arc length is longer than
the voltage allows.
Current is the flow of electrons in an electrical circuit. Current is measured in amperes, or amps. Current
can be compared to the flow of water from a faucet.
When the faucet is turned off, water is unable to flow.
When the faucet is opened, water begins to flow, as in
Figure 3-7C. Likewise, when there is no arc, there is no
current flowing. When an arc is struck, current is produced as electrons flow across the arc. See Figure 3-7D.
Sometimes, when you turn a faucet all the way on,
water flows very fast with a great deal of force. This is
due to high water pressure and a high flow rate. If you
turn the faucet halfway off, the same pressure is there,
but the flow rate is reduced. At other times, when a
faucet is turned all the way on, only a small amount of
water flows, and it does not have much force. This is
due to low water pressure and a low flow rate. If you
place your thumb over part of the faucet opening, water
is delivered with much greater force. The pressure is
higher, although the flow rate has not changed. Similar
principles apply to voltage and current in an electrical
circuit. In some welding applications, a higher voltage
and/or current is required. In other applications, a
lower voltage and/or current may be required.
In later chapters, instructions will be given on the
setup of different types of welding machines. Most
machines require that the welder set the voltage
and/or the current that will be used.
A
Electrode
Base metal
B
Water flow
C
D
Welding arc
Figure 3-7. A—The faucet is off. Water does not flow, but
water pressure exists in the pipe. B—A welding arc has not
been struck. Electrons do not flow, but voltage exists in the circuit. C—The faucet is on. Water flows, and the water pressure
drops slightly. D—An arc is struck. Electrons flow, and the
voltage drops slightly.
Units of Measurement
There are two different measurement systems in
common use—the US conventional system and the SI
metric system. The US conventional system is used
primarily in the United States. The SI metric system is
the standard in much of the rest of the world and is
also used in the United States. Some of the basic units
used in the US conventional system are inches, feet,
gallons, pounds, and pounds per square inch. Some of
the basic units in the SI metric system are millimeters,
meters, liters, kilograms, and pascals.
34
In the US conventional system, the basic unit of
measurement is commonly converted and renamed.
For example, 12 inches are converted to 1 foot, 3 feet
are converted to 1 yard, and 1760 yards are converted
to 1 mile. Figure 3-8 lists common conversions in the
US conventional system.
The SI metric system adds a prefix to the basic
unit of measurement to increase or decrease the value.
For example, there are 1000 millimeters in 1 meter,
Welding Technology Fundamentals
This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved.
Common Conversions: US Conventional System
Linear Measurement
12 inches
3 feet
36 inches
1760 yards
5280 feet
=
=
=
=
=
1 foot
1 yard
1 yard
1 mile
1 mile
and 1000 meters in 1 kilometer. In these examples,
meter is the basic unit of measurement. The prefixes
milli and kilo are added to it to change the value. See
Figure 3-9 for a list of common prefixes used in the SI
metric system.
Conversions between the US conventional system
and the SI metric system may need to be made occasionally. Common conversions are listed in Figure 3-10.
Summary
Heat is used to create welds in many welding processes. Filler material is commonly used when welding with heat only.
Some welding processes use both heat and pressure. Filler material is generally not used in these
processes.
Liquid and Dry Measurement
2 pints = 1 quart
4 quarts = 1 gallon
31.5 gallons = 1 barrel
Property
Square Measurement
Area
144 square inches = 1 square foot
9 square feet = 1 square yard
1296 square inches = 1 square yard
Cubic Measurement
7.48 gallons
1728 cubic inches
27 cubic feet
202 gallons
=
=
=
=
1 cubic foot
1 cubic foot
1 cubic yard
1 cubic yard
Figure 3-8. This table displays common conversions in the
US conventional system.
Common SI Metric Prefixes
micro
milli
centi
deci
kilo
mega
=
=
=
=
=
=
1/1000000 or .000001
1/1 000 or .001
1/100 or .01
1/10 or .1
1 000
1 000 000
The unit of length is the METER
Examples of prefixes:
millimeter (mm) = .001 or 1/1000 of a meter
centimeter (cm) = .01 or 1/100 of a meter
kilometer (km) = 1000 meters
To Convert
From
To
Multiply By
Current
density
in.²
in.²
ft.²
ft.²
A/in.²
A/mm²
mm²
m²
mm²
m²
A/mm²
A/in.²
645.16
.00064516
92903
.092903
.00155
645.16
Deposition
rate
lb./h
kg/h
kg/h
lb./h
.045
2.2
Electrode
force
lb. (force)
kg (force)
N
N
N
lb. (force)
4.4482
9.8067
.22481
Flow rate
cfh
gallons/h
gallons/min.
Lpm
Lpm
Lpm
Lpm
cfh
.47195
.06309
3.7854
2.1188
Heat input
J/in.
J/m
J/m
J/in.
39.37
.0254
Length
in.
in.
ft.
ft.
mm
mm
lb.
psi
psi
N/mm²
kPa
kPa
kPa
psi
N/mm²
MPa
mm
m
mm
m
in.
ft.
kg
Pa
kPa
Pa
psi
lb/ft²
N/mm²
kPa
MPa
psi
25.4
.0254
304.8
.3048
.03937
.0032808
.45359
6894.8
6.8948
1,000,000
.14504
20.885
.0001
6.8948
1.000
145.04
Torque
in.·lb.
in.·lb.
N·m
N·m
.11298
1.3558
Travel
speed
in./min.
mm/s
mm/s
in./min.
.42333
2.3622
Volume
in.³
in.³
ft.³
ft.³
in.³
ft.³
gallons
mm³
m³
mm³
m³
L
L
L
16387
.000016387
28316850
.028317
.016387
28.317
3.7854
Mass
Pressure
(gas and
liquid)
Tensile
strength
The unit of weight is the GRAM
Examples of prefixes:
milligram (mg) = .001 or 1/1 000 or a gram
centimeter (cm) = 1000 grams
megagram (Mg) = 1000000 grams
The unit of volume is the LITER
Examples of prefixes:
milliliter (mL) = .001 or 1/1000 of a liter
kiloliter (kL) = 1000 liters
The unit of time is the SECOND
Examples of prefixes:
millisecond = .001 or 1/1 000 sec.
microsecond = .000001 or 1/1000000 sec
Figure 3-9. It is important to be familiar with common prefixes
used in the SI metric system. Examples of basic units of
measurement and prefixes are also shown.
Chapter 3 The Physics of Welding
Metal or other material can be welded together using pressure alone. Heat and filler material are not
required.
The properties of a metal determine how it is used. Properties can be grouped into three categories:
physical, chemical, and mechanical. These properties are largely determined by the chemical composition of a metal.
The chemical and mechanical properties of a metal are often affected when the metal is welded. A weld
that is made correctly is usually stronger than the base metal.
Electricity is produced by the movement of electrons within a circuit. Electricity is measured in terms
of voltage and current.
There are two different measurement systems in common use—the US conventional system and the SI
metric system. In the US conventional system, the basic unit of measurement is commonly converted
and renamed. In the SI metric system, a prefix is added to the basic unit of measurement to increase or
decrease the value.
To convert from °F to °C, use °C = (°F - 32) ÷ 1.8.
To convert from °C to °F, use °F = (1.8 × °C) + 32.
Figure 3-10. This table shows many common conversions for
US conventional and SI metric systems. To convert from units
in Column 2 to Column 3, multiply by the value in Column 4.
To convert from units in Column 3 to Column 2, divide by the
value in Column 4.
35
Review
?
Questions
Write your answers on a separate sheet of paper. Please do not write in this book.
1. A weld can be achieved through what three methods?
2. Briefly define the following terms.
A. Tensile strength
B. Ductility
C. Toughness
D. Hardness
3. True or False? Welding changes the properties of the metal being welded.
4. What happens to the size of base metal when it is heated?
5. What can happen when welding a T-joint that is not tack welded or clamped in a welding jig or fixture?
6. List four ways to reduce stress in a weld.
7. What type of heat-treatment process is used to harden steel and steel alloys?
8. Define the following terms.
A. Voltage
B. Current
C. Resistance
9. What are the two measurement systems in common use?
10. List the unit from the other measurement system that corresponds to the given unit of measurement.
Length: inch _____
Mass: pound _____
Electrode force: pound _____
Volume: liter _____
Tensile strength: pounds per square inch (psi) _____
36
Welding Technology Fundamentals
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

advertising