Chapter 29 - Goodheart-Willcox

small production shops, body shops, home or farm
shops, or school shops. These special welding and
cutting processes are, however, being found in everincreasing
numbers in large manufacturing and
repair businesses. The terms and abbreviations used
in this chapter to describe these processes are those
approved by the American Welding Society in AWS
A3.0, Standard Welding Terms and Definitions.
Special Arc Welding Processes
There are many welding situations that require
development of a special form of welding. Welding
on metal over 1′ (300 mm) thick is very difficult to do
using SMAW, GMAW, or GTAW. Electrogas arc welding
and electroslag welding were developed to meet
this need.
When scientists created plasmas experimentally,
they discovered that extremely high temperatures
and heat outputs were also created. Welding engineers
developed the plasma arc welding and cutting
processes to take advantage of this new heat source.
Arc stud welding makes it possible to attach fastening
352
Flux-cored electrode
Guide rollers
Molten slag
Molten weld metal
Solidifying weld metal
Solidified metal
Plate 2
devices to steel structures very easily. Specially
designed equipment makes it possible to arc weld
under water.
Electrogas Welding (EGW)
The electrogas welding (EGW) process was developed
as a means of welding extremely thick metal.
Thick metal sections are hard to weld, since the weld
area must be must be kept near the melting point at all
times. This is difficult with regular welding equipment.
Metal of almost any thickness can be welded with
the electrogas process. See Figure 29-1. The weld is
always made in a vertical position. One or more consumable
electrodes are fed into the joint from above.
To make the weld, the welding arc is struck, and the
consumable electrode melts in the joint to form the
weld. Solid electrode wire or flux-cored wire may be
used. Two water-cooled copper shoes act as dams.
They contain the molten metal in the weld area and
cool the completed weld. As the copper shoes and
electrode move up, the welding continues to the top
of the joint.
Wire guide
Completed weld
Plate 1
Gas shielding
Water
circulation
Water
connections
Figure 29-1. A schematic drawing of the electrogas welding process. The shoes are water-cooled and move up along with the weld.
A shielding gas protects the weld and molten metal.
Welding Technology Fundamentals
The weld area is protected from contamination by
a shielding gas. The gas may be carbon dioxide, an
inert gas, or a combination of two gases. It is supplied
from above through openings in the shoes and covers
the molten metal.
Electroslag Welding (ESW)
The electroslag welding (ESW) process is similar to
electrogas welding. A thick layer of powdered flux is
placed in the joint before welding begins. Once welding
starts, the powdered flux melts. It forms a protective,
floating slag above the weld area as shown in
Figure 29-2. Impurities in the molten metal float to the
top and mix with the flux. The molten flux protects the
weld metal from impurities in the air above the weld.
Submerged Arc Welding (SAW)
Submerged arc welding (SAW) has several advantages
over other welding processes. The larger electrodes,
higher currents, and narrower groove angles
used in SAW permit faster welding. The following is
a list of the advantages of SAW:
• Faster than manual arc welding.
• No visible arc.
• No spatter.
• High-quality weld.
• The welding groove angle may be smaller.
Consumable
electrodes
Molten slag
Molten
weld metal
Solidified
weld metal
Thickness
of plate
Chapter 29 Special Welding and Cutting Processes
Wire feed rolls and oscillating
mechanism mounted on carriage
that rises automatically as
weld metal builds up
Face of
plate
#2
In SAW, as in GMAW, a consumable electrode is fed
into the weld joint. A thick layer of a powdered flux is
deposited ahead of the electrode, as shown in Figure 29-3.
The arc between the electrode and base metal occurs
beneath this thick flux layer. The flux covers the arc and
prevents any spatter. This is normally a fully automated
machine welding process, Figure 29-4. It may be done
semiautomatically, as well. In that case, the welder
guides the wire and flux-feeding mechanism.
Plasma Arc Welding (PAW)
Scientists say that there are four states (conditions)
of matter: solid, liquid, gas, and plasma.
Plasma is an ionized gas (a gas made up of atoms that
have lost or gained electrons). Ionized gas or plasma
is extremely hot—reaching temperatures as high as
43,000°F (24,000°C). The plasma arc is an excellent
heat source for welding or cutting. Cross sections of
the two types of arc plasma torches, transferred and
nontransferred, are shown in Figure 29-5.
An inert gas, such as helium, argon, or nitrogen, is
used to create the plasma. The gas, which flows
through the restricted nozzle, is called the orifice gas.
Inert shielding gas is passed around the restricted
nozzle to shield the weld area. A nonconsumable
tungsten electrode is used in the torch. The noise of the
plasma arc process can be harmful to one’s hearing;
therefore, industrial quality ear protection should be
worn when performing plasma arc welding.
Face of
plate
#1
Plates in vertical position
Water-cooled copper
slides confine molten slag
and weld metal
Completed weld
Figure 29-2. A schematic drawing of an electroslag weld in progress. Three consumable electrodes are used in this application.
The molten slag floating above the weld prevents oxidation.
353