WELDING AND CUTTING
INTRODUCTION
Welded connections and assemblies represent a very large group of
fabricated steel components, and only a portion of the aspects of their
design and fabrication is treated here. The welding process itself is
complex, involving heat and liquid-metal transfer, chemical reactions,
and the gradual formation of the welded joint through liquid-metal deposition
and subsequent cooling into the solid state, with attendant metallurgical
transformations. Some of these items are treated in greater
detail in the references and other extensive professional literature, as
well as in Secs. 6.2, 6.3, and 13.1.
The material in this section will provide the engineer with an overview
of the most important aspects of welded design. In order that the
resulting welded fabrication be of adequate strength, stiffness, and utility,
the designer will often collaborate with engineers who are experts in
the broad area of design and fabrication of weldments.
ARC WELDING
Arc welding is one of several fusion processes for joining metal. By the
generation of intense heat, the juncture of two metal pieces is melted
and mixed—directly or, more often, with an intermediate molten filler
metal. Upon cooling and solidification, the resulting welded joint metallurgically
bonds the former separate pieces into a continuous structural
assembly (a weldment) whose strength properties are basically those of
the individual pieces before welding.
In arc welding, the intense heat needed to melt metal is produced by
an electric arc. The arc forms between the workpieces and an electrode
that is either manually or mechanically moved along the joint; conversely,
the work may be moved under a stationary electrode. The electrode
generally is a specially prepared rod or wire that not only conducts
electric current and sustains the arc, but also melts and supplies filler
metal to the joint; this constitutes a consumable electrode. Carbon or tungsten
electrodes may be used, in which case the electrode serves only to
conduct electric current and to sustain the arc between tip and workpiece,
and it is not consumed; with these electrodes, any filler metal
required is supplied by rod or wire introduced into the region of the arc
and melted there. Filler metal applied separately, rather than via a consumable
electrode, does not carry electric current.
Most steel welding operations are performed with consumable electrodes.
Welding Process Fundamentals
Heat and Filler Metal An ac or dc power source fitted with necessary
controls is connected by a work cable to the workpiece and by a ‘‘hot’’
cable to an electrode holder of some type, which, in turn, is electrically
connected to the welding electrode (Fig. 13.3.1). When the circuit is
energized, the flow of electric current through the electrode heats the
electrode by virtue of its electric resistance. When the electrode tip is
touched to the workpiece and then withdrawn to leave a gap between the
electrode and workpiece, the arc jumping the short gap presents a further
path of high electric resistance, resulting in the generation of an
extremely high temperature in the region of the sustained arc. The temperature
reaches about 6,500°F, which is more than adequate to melt
most metals. The heat of the arc melts both base and filler metals, the
latter being supplied via a consumable electrode or separately. The
puddle of molten metal produced is called a weld pool, which solidifies as the electrode and arc move along the joint being welded. The resulting
weldment is metallurgically bonded as the liquid metal cools, fuses,
solidifies, and cools. In addition to serving its main function of supplying
heat, the arc is subject to adjustment and/or control to vary the
proper transfer of molten metal to the weld pool, remove surface films
in the weld region, and foster gas-slag reactions or other beneficial
metallurgical changes.
Welding machine. AC or DC
power source and controls
Electrode holder
Electrode
Arc
Work
Work cable
Electrode (“hot”) cable
Fig. 13.3.1 Typical welding circuit.
Filler metal composition is generally different from that of the weld
metal, which is composed of the solidified mix of both filler and base
metals.
Shielding and Fluxing High-temperature molten metal in the weld
pool will react with oxygen and nitrogen in ambient air. These gases
will remain dissolved in the liquid metal, but their solubility significantly
decreases as the metal cools and solidifies. The decreased solubility
causes the gases to come out of solution, and if they are trapped in
the metal as it solidifies, cavities, termed porosities, are left behind. This
is always undesirable, but it can be acceptable to a limited degree depending
on the specification governing the welding.
Smaller amounts of these gases, particularly nitrogen, may remain
dissolved in the weld metal, resulting in drastic reduction in the physical
properties of otherwise excellent weld metal. Notch toughness is seriously
degraded by nitrogen inclusions. Accordingly, the molten metal
must be shielded from harmful atmospheric gas contaminants. This is
accomplished by gas shielding or slag shielding or both.
Gas shielding is provided either by an external supply of gas, such as
carbon dioxide, or by gas generated when the electrode flux heats up.
Slag shielding results when the flux ingredients are melted and leave
behind a slag to cover the weld pool, to act as a barrier to contact
between the weld pool and ambient air. At times, both types of shielding
are utilized.
In addition to its primary purpose to protect the molten metal, the
shielding gas will significantly affect arc behavior. The shielding gas
may be mixed with small amounts of other gases (as many as three
others) to improve arc stability, puddle (weld pool) fluidity, and other
welding operating characteristics.
In the case of shielded-metal arc welding (SMAW), the ‘‘stick’’
electrode is covered with an extruded coating of flux. The arc heat melts
the flux and generates a gaseous shield to keep air away from the molten
metal, and at the same time the flux ingredients react with deleterious
substances, such as surface oxides on the base metal, and chemically
combine with those contaminants, creating a slag which floats to the
surface of the weld pool. That slag crusts over the newly solidified hot
metal, minimizes contact between air and hot metal while the metal
cools, and thereby inhibits the formation of surface oxides on the newly
deposited weld metal, or weld bead. When the temperature of the weld
bead decreases, the slag, which has a glassy consistency, is chipped off
to reveal the bright surface of the newly deposited metal. Minimal surface
oxidation will take place at lower temperatures, inasmuch as oxidation
rates are greatly diminished as ambient conditions are approached.
Fluxing action also aids in wetting the interface between the base
metal and the molten metal in the weld pool edge, thereby enhancing
uniformity and appearance of the weld bead.
Process Selection Criteria
Economic factors generally dictate which welding process to use for a
particular application. It is impossible to state which process will
always deliver the most economical welds, because the variables involved
are significant in both number and diversity. The variables include,
but are not limited to, steel (or other base metal) type, joint type,
section thickness, production quantity, joint access, position in which
the welding is to be performed, equipment availability, availability of
qualified and skilled welders, and whether the welding will be done in
the field or in the shop.
Shielded Metal Arc Welding
The SMAW process (Fig. 13.3.2), commonly known as stick welding, or
manual welding, is the most popular and widespread welding process. It
is versatile, relatively simple to do, and very flexible in being applied.
To those casually acquainted with welding, arc welding usually means
shielded-metal arc welding. SMAWis used in the shop and in the field for
fabrication, erection, maintenance, and repairs. Because of the relative
inefficiency of the process, it is seldom used for fabrication of major
structures. SMAW has earned a reputation for providing high-quality
welds in a dependable fashion. It is, however, inherently slower and
more costly than other methods of welding.
Electrode
Gaseous shield
Arc stream
Base metal
Extruded
covering
Molten pool
Slag
Fig. 13.3.2 SMAW process.
SMAW may utilize either direct current (dc) or alternating current
(ac). Generally speaking, direct current is used for smaller electrodes,
usually less than 3⁄16-in diameter. Larger electrodes utilize alternating
current to eliminate undesirable arc blow conditions.
Electrodes used with alternating current must be designed specifically
to operate in this mode, in which current changes direction 120
times per second with 60-Hz power. All ac electrodes will operate acceptably
on direct current. The opposite is not always true.
Flux Cored Arc Welding (FCAW)
In FCAW, the arc is maintained between a continuous tubular metal
electrode and the weld pool. The tubular electrode is filled with flux and
a combination of materials that may include metallic powder(s). FCAW
may be done automatically or semiautomatically. FCAW has become
the workhorse in fabrication shops practicing semiautomatic welding.
Production welds that are short, change direction, are difficult to access,
must be done out of position (e.g., vertical or overhead), or are part of a
short production run generally will be made with semiautomatic
FCAW.
When the application lends itself to automatic welding, most fabricators
will select the submerged arc process (see material under ‘‘SAW’’). Flux cored arc welding may be used in the automatic mode,
but the intensity of arc rays from a high-current flux cored arc, as well as
a significant volume of smoke, makes alternatives such as submerged
arc more desirable.
Advantages of FCAW FCAW offers two distinct advantages over
SMAW. First, the electrode is continuous and eliminates the built-in
starts and stops that are inevitable with SMAW using stick electrodes.
An economic advantage accrues from the increased operating factor; in
addition, the reduced number of arc starts and stops largely eliminates
potential sources of weld discontinuities. Second, increased amperages
can be used with FCAW. With SMAW, there is a practical limit to the
amount of current that can be used. The covered electrodes are 9 to
18 in long, and if the current is too high, electric resistance heating
within the unused length of electrode will become so great that the
coating ingredients may overheat and ‘‘break down.’’ With continuous
flux cored electrodes, the tubular electrode is passed through a contact
tip, where electric current is transferred to the electrode. The short distance
from the contact tip to the end of the electrode, known as electrode
extension or ‘‘stickout,’’ inhibits heat buildup due to electric resistance.
This electrode extension distance is typically 1 in for flux cored electrodes,
although it may be as much as 2 or 3 in in some circumstances.
Smaller-diameter flux cored electrodes are suitable for all-position
welding. Larger electrodes, using higher electric currents, usually are
restricted to use in the flat and horizontal positions. Although the equipment
required for FCAW is more expensive and more complicated than
that for SMAW, most fabricators find FCAW much more economical
than SMAW.
FCAW Equipment and Procedures Like all wire-fed welding processes,
FCAW requires a power source, wire feeder, and gun and cable
assembly (Fig. 13.3.3). The power supply is a dc source, although either
electrode positive or electrode negative polarity may be used. The four
primary variables used to determine welding procedures are voltage,
wire feed speed, electrode extension, and travel speed. For a given wire
feed speed and electrode extension, a specified amperage will be delivered
to maintain stable welding conditions.
Fig. 13.3.3 FCAW and GMAW equipment.
As wire feed speed is increased, amperage will be increased. On
some equipment, the wire feed speed control is called the amperage
control, which, despite its name, is just a rheostat that regulates the
speed of the dc motor driving the electrode through the gun. The most
accurate way, however, to establish welding procedures is to refer to the
wire feed speed (WFS), since electrode extension, polarity, and electrode
diameter will also affect amperage. For a fixed wire feed speed, a
shorter electrical stick-out will result in higher amperages. If procedures
are set based on the wire feed speed, the resulting amperage verifies that
proper electrode extensions are being used. If amperage is used to set
welding procedures, an inaccurate electrode extension may go undetected.
Self-Shielded and Gas-Shielded FCAW Within the category of
FCAW, there are two specific subsets: self-shielded flux core arc welding
(FCAW-ss) (Fig. 13.3.4) and gas-shielded flux core arc welding (FCAW-g)
(Fig. 13.3.5). Self-shielded flux cored electrodes require no external
shielding gas. The entire shielding system results from the flux ingredients
contained in the tubular electrode. The gas-shielded variety of
flux cored electrode utilizes, in addition to the flux core, an externally
supplied shielding gas. Often, CO2 is used, although other mixtures may
be used.
Slag
Contact tip
Tubular electrode
Flux core
Fig. 13.3.4 Self shielded FCAW.
Both these subsets of FCAW are capable of delivering weld deposits
featuring consistency, high quality, and excellent mechanical properties.
Self-shielded flux cored electrodes are ideal for field welding operations,
for since no externally supplied shielding gas is required, the
process may be used in high winds without adversely affecting the
Fig. 13.3.5 Gas shielded FCAW.
quality of the weld metal deposited. With any gas-shielded processes,
wind shields must be erected to preclude wind interference with the gas
shield. Many fabricators with large shops have found that self-shielded
flux core welding offers advantages when the shop door can be left open
or fans are used to improve ventilation.
Gas-shielded flux cored electrodes tend to be more versatile than
self-shielded flux cored electrodes and, in general, provide better arc
action. Operator acceptance is usually higher. The gas shield must be
protected from winds and drafts, but this is not difficult for most shop
fabrication. Weld appearance is very good, and quality is outstanding.
Higher-strength gas-shielded FCAW electrodes are available, but
current practice limits self-shielded FCAW deposits to a strength of 80
ksi or less.
Submerged arc welding differs from other arc welding processes in that a
blanket of fusible granular flux is used to shield the arc and molten
metal (Fig. 13.3.6). The arc is struck between the workpiece and a
bare-wire electrode, the tip of which is submerged in the flux. The arc is
completely covered by the flux and it is not visible; thus the weld is
made without the flash, spatter, and sparks that characterize the openarc
processes. The flux used develops very little smoke or visible fumes.
Fig. 13.3.6 SAW process.
Typically, the process is operated fully automatically, although semiautomatic
operation is possible. The electrode is fed mechanically to the
welding gun, head, or heads. In semiautomatic welding, the welder
moves the gun, usually equipped with a flux-feeding device, along the
joint.
Flux may be fed by gravity flow from a small hopper atop the torch
and then through a nozzle concentric with the electrode, or through a
nozzle tube connected to an air-pressurized flux tank. Flux may also be
applied in advance of the welding operation or ahead of the arc from a
hopper run along the joint. Many fully automatic installations are
equipped with a vacuum system to capture unfused flux left after welding;
the captured, unused flux is recycled for use once more.
During welding, arc heat melts some of the flux along with the tip of
the electrode. The electrode tip and the welding zone are always
shielded by molten flux and a cover layer of unfused flux. The electrode
is kept a short distance above the workpiece. As the electrode progresses
along the joint, the lighter molten flux rises above the molten
metal to form slag. The weld metal, having a higher melting (freezing)
point, solidifies while the slag above it is still molten. The slag then
freezes over the newly solidified weld metal, continuing to protect the
metal from contamination while it is very hot and reactive with atmospheric
oxygen and nitrogen. Upon cooling and removal of any unmelted
flux, the slag is removed from the weld.
Advantages of SAW High currents can be used in SAW, and extremely
high heat input can be developed. Because the current is applied
to the electrode a short distance above the arc, relatively high amperages
can be used on small-diameter electrodes. The resulting extremely high
current densities on relatively small-cross-section electrodes permit
high rates of metal deposition.
The insulating flux blanket above the arc prevents rapid escape of
heat and concentrates it in the welding zone. Not only are the electrode
and base metal melted rapidly, but also fusion is deep into the base
metal. Deep penetration allows the use of small welding grooves, thus
minimizing the amount of filler metal to be deposited and permitting
fast welding speeds. Fast welding, in turn, minimizes the total heat input
to the assembly and thus tends to limit problems of heat distortion. Even
relatively thick joints can be welded in one pass with SAW.
Versatility of SAW SAW can be applied in more ways than other arc
welding processes. A single electrode may be used, as is done with other
wire feed processes, but it is possible to use two or more electrodes in
submerged arc welding. Two electrodes may be used in parallel, sometimes
called twin arc welding, employing a single power source and one
wire drive. In multiple-electrode SAW, up to five electrodes can be used
thus, but most often, two or three arc sources are used with separate
power supplies and wire drives. In this case, the lead electrode usually
operates on direct current while the trailing electrodes operate on alternating
current.
Gas Metal Arc Welding (GMAW)
Gas metal arc welding utilizes the same equipment as FCAW (Figs.
13.3.3 and 13.3.7); indeed, the two are similar. The major differences
are: (1) GMAW uses a solid or metal cored electrode, and (2) GMAW
leaves no residual slag.
GMAW may be referred to as metal inert gas (MIG), solid wire and
gas, miniwire or microwire welding. The shielding gas may be carbon
Fig. 13.3.7 GMAW welding process.
dioxide or blends of argon with CO2 or oxygen, or both. GMAW is
usually applied in one of four ways: short arc transfer, globular transfer,
spray arc transfer, and pulsed arc transfer.
Short arc transfer is ideal for welding thin-gage materials, but generally
is unsuitable for bridge fabrication purposes. In this mode of
transfer, a small electrode, usually of 0.035- to 0.045-in diameter, is fed
at a moderate wire feed speed at relatively low voltages. The electrode
contacts the workpiece, resulting in a short circuit. The arc is actually
quenched at this point, and very high current will flow through the
electrode, causing it to heat and melt. A small amount of filler metal is
transferred to the welding done at this time.
The cycle will repeat itself when the electrode short-circuits to the
work again; this occurs between 60 and 200 times per second, creating a
characteristic buzz. This mode of transfer is ideal for sheet metal, but
results in significant fusion problems if applied to thick sections, when
cold lap or cold casting results from failure of the filler metal to fuse to the
base metal. This is unacceptable since the welded connection will have
virtually no strength. Caution must be exercised if the short arc transfer
mode is applied to thick sections.
Spray arc transfer is characterized by high wire feed speeds at relatively
high voltages. A fine spray of molten filler metal drops, all
smaller in diameter than the electrode, is ejected from the electrode
toward the work. Unlike with short arc transfer, the arc in spray transfer
is maintained continuously. High-quality welds with particularly good
appearance are obtained. The shielding gas used in spray arc transfer is composed of at least 80 percent argon, with the balance either carbon
dioxide or oxygen. Typical mixtures would include 90-10 argon-CO2,
and 95-5 argon-oxygen. Relatively high arc voltages are used with
spray arc transfer. Gas metal spray arc transfer welds have excellent
appearance and evidence good fusion. However, due to the intensity of
the arc, spray arc transfer is restricted to applications in the flat and
horizontal positions.
Globular transfer is a mode of gas metal arc welding that results when
high concentrations of carbon dioxide are used. Carbon dioxide is not an
inert gas; rather, it is active. Therefore, GMAW that uses CO2 may be
referred to as MAG, for metal active gas. With high concentrations of
CO2 in the shielding gas, the arc no longer behaves in a spraylike fashion,
but ejects large globs of metal from the end of the electrode. This
mode of transfer, while resulting in deep penetration, generates relatively
high levels of spatter, and weld appearance can be poor. Like the
spray mode, it is restricted to the flat and horizontal positions. Globular
transfer may be preferred over spray arc transfer because of the low cost
of CO2 shielding gas and the lower level of heat experienced by the
operator.
Pulsed arc transfer is a relatively new development in GMAW. In this
mode, a background current is applied continuously to the electrode. A
pulsing peak current is applied at a rate proportional to the wire feed
speed. With this mode of transfer, the power supply delivers a pulse of
current which, ideally, ejects a single droplet of metal from the electrode.
The power supply then returns to a lower background current to
maintain the arc. This occurs between 100 and 400 times per second.
One advantage of pulsed arc transfer is that it can be used out of position.
For flat and horizontal work, it will not be as fast as spray arc
transfer. However, when it is used out of position, it is free of the
problems associated with gas metal arc short-circuiting mode. Weld
appearance is good, and quality can be excellent. The disadvantages of
pulsed arc transfer are that the equipment is slightly more complex and
is more costly.
Metal cored electrodes comprise another new development in GMAW.
This process is similar to FCAW in that the electrode is tubular, but the
core material does not contain slag-forming ingredients. Rather, a variety
of metallic powders are contained in the core, resulting in exceptional
alloy control. The resulting weld is slag-free, as are other forms of
GMAW.
The use of metal cored electrodes offers many fabrication advantages.
Compared to spray arc transfer, metal cored electrodes require
less amperage to obtain the same deposition rates. They are better able
to handle mill scale and other surface contaminants. When used out-ofposition,
they offer greater resistance to the cold lapping phenomenon
so common with short arc transfer. Finally, metal cored electrodes permit
the use of amperages higher than may be practical with solid electrodes,
resulting in higher metal deposition rates.
The weld properties obtained from metal cored electrode deposits can
be excellent, and their appearance is very good. Filler metal manufacturers
are able to control the composition of the core ingredients, so that
mechanical properties obtained from metal cored deposits can be more
consistent than those obtained with solid electrodes.
Electroslag/Electrogas Welding (ESW/EGW)
Electroslag and electrogas welding (Figs. 13.3.8 and 13.3.9) are closely
related processes that allow high deposition welding in the vertical
plane. Properly applied, these processes offer tremendous savings over
alternative, out-of-position methods and, in many cases, savings over
flat-position welding. Although the two processes have similar applications
and mechanical setup, there are fundamental differences in the arc
characteristics.
Electroslag and electrogas are mechanically similar in that both utilize
copper dams, or shoes, that are applied to either side of a squareedged
butt joint. An electrode or multiple electrodes are fed into the
joint. Usually, a starting sump is applied for the beginning of the weld.
As the electrode is fed into the joint, a puddle is established that progresses
vertically. The water-cooled copper dams chill the weld metal
and prevent its escape from the joint. The weld is completed in one pass.
Fig. 13.3.8 ESW process.
Fig. 13.3.9 EGW process.
Gas Tungsten Arc Welding (GTAW)
The gas tungsten arc welding process (Fig. 13.3.10), colloquially called
TIG welding, uses a nonconsumable tungsten electrode. An arc is established
between the tungsten electrode and the workpiece, resulting in
heating of the base metal. If required, a filler metal is used. The weld
area is shielded with an inert gas, usually argon or helium. GTAW is
ideally suited to weld nonferrous materials such as stainless steel and
aluminum, and is very effective for joining thin sections.
Highly skilled welders are required for GTAW, but the resulting weld
quality can be excellent. The process is often used to weld exotic materials.
Critical repair welds as well as root passes in pressure piping are
typical applications.
Externally applied
filler (optional)
Tungsten
electrode
Gas orifice
Shielding gas
Work
Power
connection
Gas
inlet
Cup
Fig. 13.3.10 Gas tungsten arc welding (GTAW

Additional comments :