STEEL - Materials for Civil and Construction Engineers - page 125

Welding

3.9

Many civil engineering structures, such as steel bridges, frames, and trusses
require welding during construction and repair. Welding is a technique for
joining two metal pieces by applying heat to fuse the pieces together. A filler
metal may be used to facilitate the process. The chemical properties of the
welding material must be carefully selected to be compatible with the mate-
rials being welded. A variety of welding methods are available, but the com-
mon types are arc welding and gas welding. Other types of welding include
flux-cored arc welding, self-shielded flux arc welding, and electroslag weld-
ing (Frank and Smith 1990).
Arc welding uses an arc between the electrode and the grounded base
metal to bring both the base metal and the electrode to their melting points.
The resulting deposited weld metal is a cast structure with a composition
dependent upon the base metal, electrode, and flux chemistry. Shielded
metal arc welding (stick welding) is the most common form of arc welding.
It is limited to short welds in bridge construction. A consumable electrode,
which is covered with flux, is used. The flux produces a shielding atmos-
phere at the arc to prevent oxidation of the molten metal. The flux is also
used to trap impurities in the molten weld pool. The solidified flux forms a
slag that covers the solidified weld, as shown in Figure 3.24. Submerged arc
welding is a semiautomatic or automatic arc welding process. In this process,
a bare wire electrode is automatically fed by the welding machine while a
granular flux is fed into the joint ahead of the electrode. The arc takes place
in the molten flux, which completely shields the weld pool from the atmos-
phere. The molten flux concentrates the arc heat, resulting in deep penetra-
tion into the base metal.
Gas welding (mig welding) is another type of welding in which no flux
is used. An external shielding gas is used, which shields the molten weld

Electrode covering

Core wire

Shielding atmosphere

Weld pool

Solidified slag

Metal and slag droplets

Weld metal

Penetration depth

Base metal

Direction of welding

FIGURE 3.24 Schematic drawing of arc welding.