Building Design and Construction Handbook - Merritt - Ventech!

BUILDING MATERIALS 4.69
Shielded-metal-arc welding fuses parts to be joined by the heat of an electric
arc struck between a coated metal electrode and the material being joined, or base
metal. The electrode supplies filler material for making the weld, gas for shielding
the molten metal from the air, and flux for refining this metal.
Submerged-arc welding fuses the parts to be joined by the heat of an electric
arc struck between a bare metal electrode and base metal. The weld is shielded
from the air by flux. The electrode or a supplementary welding rod supplies metal
filler for making the weld.
Gas-metal-arc welding produces fusion by the heat of an electric arc struck
between a filler-metal electrode and base metal, while the molten metal is shielded
by a gas or mixture of gas and flux. For structural steels, the gas may be argon,
argon with oxygen, or carbon dioxide.
Electroslag welding uses a molten slag to melt filler metal and surfaces of the
base metal and thus make a weld. The slag, electrically conductive, is maintained
molten by its resistance to an electric current that flows between an electrode and
the base metal. The process is suitable only for welding in the vertical position.
Moving, water-cooled shoes are used to contain and shape the weld surface. The
slag shields the molten metal.
Electrogas welding is similar to the electroslag process. The electrogas process,
however, maintains an electric arc continuously, uses an inert gas for shielding, and
the electrode provides flux.
Stud welding is used to fuse metal studs or similar parts to other steel parts by
the heat of an electric arc. A welding gun is usually used to establish and control
the arc, and to apply pressure to the parts to be joined. At the end to be welded,
the stud is equipped with a ceramic ferrule, which contains flux and which also
partly shields the weld when molten.
Preheating before welding reduces the risk of brittle failure. Initially, its main
effect is to lower the temperature gradient between the weld and adjoining base
metal. This makes cracking during cooling less likely and gives entrapped hydrogen,
a possible source of embrittlement, a chance to escape. A later effect of preheating
is improved ductility and notch toughness of base and weld metals and
lower transition temperature of weld. When, however, welding processes that deposit
weld metal low in hydrogen are used and suitable moisture control is maintained,
the need for preheat can be eliminated. Such processes include use of lowhydrogen
electrodes and inert-arc and submerged-arc welding.
Rapid cooling of a weld can have an adverse effect. One reason that arc strikes
that do not deposit weld metal are dangerous is that the heated metal cools very
fast. This causes severe embrittlement. Such arc strikes should be completely removed.
The material should be preheated, to prevent local hardening, and weld
metal should be deposited to fill the depression.
Pronounced segregation in base metal may cause welds to crack under certain
fabricating conditions. These include use of high-heat-input electrodes, such as the
1 ⁄4-in E6020, and deposition of large beads at slow speeds, as in automatic welding.
Cracking due to segregation, however, is rare with the degree of segregation normally
occurring in hot-rolled carbon-steel plates.
Welds sometimes are peened to prevent cracking or distortion, though there are
better ways of achieving these objectives. Specifications often prohibit peening of
the first and last weld passes. Peening of the first pass may crack or punch through
the weld. Peening of the last pass makes inspection for cracks difficult. But peening
is undesirable because it considerably reduces toughness and impact properties of
the weld metal. (The adverse effects, however, are eliminated by a covering weld
layer.) The effectiveness of peening in preventing cracking is open to question. And