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Chapter 31:
Gas Flame and Arc Processes
DeGarmo’s Materials and Processes in
Manufacturing
31.1 Oxyfuel-Gas Welding
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Use the flame produced by the combustion of
a fuel gas and oxygen as the source of heat
Combustion of oxygen and acetylene (C2H2)
produces a temperature of about 3250℃
(5850oF) in a two-stage reaction.
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First stage:
C2H2 + O2  2CO + H2 + heat
Second stage:
2CO + O2  2CO2 + heat
H2 + 1/2O2  H2O + heat
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Oxyacetylene Welding Torch
FIGURE 31-1 Typical
oxyacetylene welding torch and
cross-sectional schematic.
(Courtesy of Victor Equipment
Company, Denton, TX)
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Oxyacetylene Flame Temperatures
Inner cone: the first
stage of combustion
is complete. Most
welding should be
performed with the
torch positioned this
point just above the
metal being welded.
FIGURE 31-2 Typical
oxyacetylene flame and the
associated temperature
distribution.
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Oxyfuel-gas Welding
FIGURE 31-3 Oxyfuel-gas
welding with a consumable
welding rod.
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Oxyfuel-Gas Welding Process
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Oxyfuel Application
Monel: Alloy of Nickel, Copper, Iron and Manganese, with antacidity
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31.2 Oxygen Torch Cutting
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Oxyfuel-gas cutting (OFC), commonly called
flame cutting, is the most common thermal
cutting process
When ferrous metal is cut, the process is
according to one or more reactions at high
temperature above 815℃(1500oF)
Fe + O  FeO + heat
3Fe + 2O2  Fe2O3 + heat
4Fe + 3O2  2Fe2O3 + heat
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Flame Cutting
FIGURE 31-4 Flame cutting of
a metal plate.
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Oxyacetylene Cutting Torch
Acetylene is by far the most common fuel used in oxyfuel-gas cutting,
the process is often referred to as oxyacelylene cutting (OFC-A)
Fuel gases: acetylene (OFC-A), natural gas (OFC-N), propane (OFC-P)
, and hydrogen (OFC-H)
FIGURE 31-5
Oxyacetylene cutting
torch and crosssectional schematic.
(Courtesy of Victor
Equipment
Company, Denton, TX)
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Underwater Cutting Torch
FIGURE 31-6 Underwater
cutting torch. Note the extra set
of gas openings in the nozzle to
permit the flow of compressed
air and the extra control valve.
(Courtesy of Bastian-Blessing
Company, Chicago, IL)
An auxiliary skirt surrounds the main tip, and a additional set of gas
passages conducts a flow of compressed air that provides secondary
oxygen for the oxyacetylene flame and expels water from the zone
where the burning of metal occurs
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31.3 Flame Straightening
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Use controlled, localized upsetting as a
means of straightening warped or buckled
materials
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Flame Straightening
FIGURE 31-7 Schematic
illustrating the theory
of flame straightening.
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31.4 Arc Welding
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An arc between two electrodes was a
concentrated heat souse that could approach
4000°C.
Current 1 to 4000 A (large)
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Typically range from 100 to 1000 A
Voltage 20 to 50 V (low)
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Arc Welding Schematic
FIGURE 31-8 The basic
electrical circuit for arc welding.
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Arc Welding Polarity
• DCEN: direct-current electrode-negative, or
SPDC -- direct current and straight polarity
• Fast melt of electrode, i.e., high metal deposition rate
• Shallow molten pool on the workpiece (weld penetration)
• DCEP: direct-current electrode-positive, or
RPDC -- direct current and reverse polarity
• low melt of electrode, i.e., low metal deposition rate
• Break up any oxide films and give deeper penetration
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Arc Welding Electrode
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Manual arc welding is almost always
performed with shielded (covered) electrodes.
Continuous bare-metal wire can be used as
the electrode in automatic or semiautomatic
arc welding, but this is always in conjunction
with some form of shielding and are–
stabilizing medium and automatic feedcontrol devices.
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Metal Transfer Modes
FIGURE 31-9 Three modes of metal transfer during arc welding. (Courtesy of
Republic Steel Corporation, Youngstown, OH)
As the electrode melts, the arc length and the electrical resistance of the arc
length vary. To maintain a stable and satisfactory welding conditions, the
electrode must be moved toward the work at a controlled rate.
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31.5 Consumable-Electrode Arc Welding
Shielded metal arc welding (SMAW)
 Flux-cored arc welding (FCAW)
 Gas metal arc welding (GMAW)
 MIG
 Submerged arc welding (SAW)
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Shielded Metal Arc Welding
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Low cost equipment
Finite-length electrode (1.5 to 6.5 mm in
diameter and 20 to 45 cm in length)
Characteristics of shielding materials
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Vaporize to provide a protective atmosphere
Provide ionizing elements to help stabilize the arc,
reduce weld metal spatter, and increase efficiency
of deposition
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Shielded Metal Arc Welding
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Act as a flux to deoxidize and remove impurities
from the molten metal
Provide a protective slag coating to accumulate
impurities, prevent oxidation, and slow the cooling
of the weld metal
Add alloying elements
Add additional filler metal
Affect arc penetration
Influence the shape of the weld bead
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Welding Electrode Designation
FIGURE 31-10 Designation system for arc-welding electrodes.
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Shielded Metal Arc Welding
FIGURE 31-11 A shielded
metal arc welding (SMAW)
system.
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Schematic of SMAW
FIGURE 31-12 Schematic diagram of shielded metal arc welding (SMAW).
(Courtesy of American Iron and Steel Institute, Washington, DC.)
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Process Summary of SMAW
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Flux-Cored Arc Welding
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Overcome some of the shielding metal arc
limitations by moving the powdered flux to the
interior of a continuous tubular electrode.
Compared to the stick electrodes of the
shielded metal arc process, the flux-cored
electrode is both continuous and less bulky,
since binders are no longer required to hold
the flux in place.
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Flux-Cored Arc Welding
FIGURE 31-13 The flux-cored
arc welding (FCAW) process.
(Courtesy of The American
Welding Society, New York.)
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Process Summary of FCAW
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Gas Metal Arc Welding
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Use of supplementary shielding gas, such
as CO2 flowing through the torch to protect
arc and molten metal. No longer a need for
the volatilizing flux.
continuous, solid, uncoated metal wire as
an electrode (or a continuous hollow tube
with powdered alloy additions in the center,
known as a metal-cored electrode), or
referred to as metal inert-gas welding (MIG).
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Schematic of Gas Metal Arc Welding
FIGURE 31-14 Schematic
diagram of gas metal arc
welding (GMAW). (Courtesy of
American Iron and Steel Institute,
Washington, DC.)
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Process Summary of GMAW
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Submerged Arc Welding
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No shielding gas is used. Instead, a thick
layer of granular flux is deposited just ahead
of a solid bare-wire consumable electrode.
Portion of flux melts and acts to remove
impurities from the rather large pool of molten
metal, while the unmelted excess provides
additional shielding.
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Schematic of Submerged Arc Welding
FIGURE 31-15 (Top) Basic
features of submerged arc
welding (SAW). (Courtesy of
Linde Division, Union Carbide
Corporation, Houston, TX)
(Bottom) Cutaway schematic of
submerged arc welding.
(Courtesy of American Iron and
Steel Institute, Washington, DC.)
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Process Summary of SAW
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Stud Welding
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Used to attach studs, screws, pins, or other
fasteners to a metal surface.
Inserted stud acts as an electrode and a dc
arc is established between the end of the
stud and the workpiece. After a small amount
of metal is melted, the two pieces are brought
together under light pressure and allowed to
solidify.
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Stud Welding Gun
FIGURE 31-16 Diagram of a
stud welding gun. (Courtesy of
American Machinist.)
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Stud Welding Examples
FIGURE 31-17 (Left) Types of studs used for stud welding. (Center) Stud and ceramic ferrule.
(Right) Stud after welding and a section through a welded stud. (Courtesy of Nelson Stud
Welding Co, Elyria, OH)
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31.6 Nonconsumable-Electrode Arc
Welding
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Gas tungsten arc welding
 TIG
 nonconsumable electrode
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Gas tungsten arc spot welding
Plasma arc welding
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Gas Tungsten Arc Welding Torch
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Known as tungsten inert-gas (TIG) welding or
Heliarc welding when helium was the shielding
gas.
Nonconsumable tungsten electrode provide the
arc but not the filler metal.
Tungsten electrode: alloyed with thorium oxide,
zirconium oxide, cerium oxide, or lanthanum
oxide to provide better current-carrying and
electron-emission characteristics and longer
electrode life.
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Gas Tungsten Arc Welding Torch
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Operating in an inert environment using inert
gas as shielding gas, such as argon, helium, or
mixture of them, NOT CO2.
Filler metal is optional. When filler metal is
required, it is generally selected to match the
chemistry and/or tensile strength of the metal
being welded.
Maximum penetration is obtained with direct
current electrode negative (DCEN) conditions.
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Gas Tungsten Arc Welding Torch
FIGURE 31-18 Welding torch used in nonconsumable electrode,
gas tungsten arc welding (GTAW), showing feed lines for power,
cooling water, and inert-gas flow. (Courtesy of Linde Division, Union
Carbide Corporation, Houston, TX)
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Schematic of GTAW
FIGURE 31-19 Diagram of gas
tungsten arc welding (GTAW).
(Courtesy of American Iron and
Steel Institute, Washington, DC.)
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Metal Deposition Rate Comparison
The hot-wire process
is not practical when
welding copper or
aluminum because it
is difficult to preheat
the low-resistivity
filler wire.
FIGURE 31-20 Comparison of the
metal deposition rates in GTAW with
cold, hot, and oscillating-hot filler wire.
(Courtesy of Welding Journal.)
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Process Summary of GTAW
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Gas Tungsten Arc Spot Welding
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Access it limited to one side of the joint or
where thin sheet is being attached to heavier
material.
Weld nugget:
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Arc spot welding – form at the surface where the
gun makes contact.
Resistance spot welding – form at the interface
between the two members.
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Schematic of Inert-Gas-Shielded Tungsten
Arc Welding
FIGURE 31-21 Process
schematic of spot welding by
the inert-gas-shielded tungsten
arc process.
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Example of GTAW
FIGURE 31-22 Making a spot
weld by the inert-gas-shielded
tungsten arc process. (Courtesy
of Air Reduction Company Inc.,
New York, NY)
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Plasma Arc Welding
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Arc is maintained between a nonconsumable
electrode and either the welding gun
(nontransferred arc) or the workpiece (transferred
arc).
Characterize as a high rate of heat input and
o
temperatures on the order of 16500℃ (30000 F).
Offer fast welding speeds, narrow welds with deep
penetration (a depth-to-width ratio of about 3),
narrow heat-affected zone, less distortion.
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Types of Plasma Arc Torches
FIGURE 31-23 Two types of
plasma arc torches. (Left)
Transferred arc; (right)
nontransferred arc.
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GTAW versus Plasma Arc Process
FIGURE 31-24 Comparison of the nonconstricted arc of
gas tungsten arc welding and the constricted arc
of the plasma arc process. Note the level and
distribution of temperature. (Courtesy ASM International,
Materials Park, OH.)
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Process Summary of PAW
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31.7 Welding Equipment
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Power source DC or AC
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Generally employ the “drooping voltage”
characteristics
Jigs or fixtures (also called positioners) are
frequently used to hold the work in production
welding.
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Voltage Characteristics
FIGURE 31-25 Drooping-voltage characteristics of
typical arc-welding power supplies. (Left) Direct
current; (right) alternating current.
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Welding Equipment
Most welding
uses solid-state
transformerrectifier
machines
FIGURE 31-26 Rectifier-type
AC and DC welding power
supply. (Courtesy of Lincoln
Electric Company, Cleveland, OH)
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31.8 Arc Cutting
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Carbon arc and shielded metal arc cutting
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Use arc from a carbon or shielded metal arc
electrode to melt the metal
Remove the molten metal from the cut by
gravity or the force of arc itself
Air carbon arc cutting
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Arc maintain between a carbon electrode and the
workpiece
Blow the molten metal from the cut by the air
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Arc Cutting
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Oxygen arc cutting
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Electric arc and a stream of oxygen are combined to
make the cut
Electrode is a coated ferrous-metal tube
Expel the molten metal from the cut by a highvelocity of jet of gas
Gas metal arc cutting
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Electrode penetrate completely through the
workpiece, cutting rather than welding occur.
Expel the molten metal from the cut by a highvelocity of jet of gas
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Arc Cutting
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Gas tungsten arc cutting
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Employ the same basic circuit and shielding gas as used in
gas tungsten arc welding
Expel the molten metal from the cut by a high-velocity of jet
of gas
Plasma arc cutting
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With the nontransferred-type of torch, temperature can
be about 16500℃
With the transferred-type of torch, temperature can be
as high as 33000℃
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Example of Plasma Torch
FIGURE 31-27 Cutting sheet
metal with a plasma torch.
(Courtesy of GTE Sylvania,
Danvers, MA)
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31.9 Metallurgical and Heat Effects in
Thermal Cutting
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Low rate of heat input, oxyacetylene cutting will
produce a rather large HAZ Plasma arc cutting
is so rapid, and the heat is so localized.
Imbalanced residual stress will induce distortion.
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Cutting Process Comparison
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Reference Problems
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Review Questions
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1 (3), 10, 18, 22, 35, 38, 43, 50, 64,
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