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59
Twin carbon arc welding (TCAW) in which the arc is established between
two carbon electrodes
Gas carbon arc welding (CAW-G) no longer has commercial significance
Electroslag welding is a highly productive welding process developed in the
United States during the 1930s. It involves the melting of the surfaces of the
metal workpieces and the filler metal with a molten slag to cause
coalescence. An electric arc is passed through the slag to heat it, but the arc
itself is extinguished by the slag. Electroslag welding is commonly used to
weld in a vertical orientation, and is particularly popular with steels. In the
1970s, it was used extensively in bridges, ships, and other large metal
structures. However, in 1977 the Federal Highway Administration banned its
use in welds for some structural members of bridges, due to concerns of weld
imperfections and poor properties. Benefits of the process include its high
metal deposition rates. Many welding processes require more than one pass
for welding thick workpieces, but often a single pass is sufficient for
electroslag welding. The process is also very efficient, since joint preparation
and materials handling are minimized while filler metal utilization is high.
The process is also safe and clean, with no arc flash and low weld splatter or
distortion.
Electrogas welding (EGW) is a continuous vertical position arc welding
process developed in 1961, in which an arc is struck between a consumable
electrode and the workpiece. A shielding gas is sometimes used, but pressure
is not applied. A major difference between EGW and its cousin electroslag
welding is that the arc in EGW is not extinguished, instead remaining struck
throughout the welding process. It is used to make square-groove welds for
butt and welding, especially in the shipbuilding industry and in the
construction of storage tanks. In EGW, the heat of the welding arc causes the
electrode and workpieces to melt and flow into the cavity between the parts
being welded. This molten metal solidifies from the bottom up, joining the
parts being welded together. The weld area is protected from atmospheric
contamination by a separate shielding gas, or by the gas produced by the
disintegration of a flux-cored electrode wire. The electrode is guided into the
weld area by either a consumable electrode guide tube, like the one used in
electroslag welding, or a moving head. When the consumable guide tube is
used, the weld pool is composed of molten metal coming from the parts being
welded, the electrode, and the guide tube. The moving head variation uses an
assembly of an electrode guide tube which travels upwards as the weld is
laid, keeping it from melting. Electrogas welding can be applied to most
steels, including low and medium carbon steels, low alloy high strength
steels, and some stainless steels. Quenched and tempered steels may also be
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