Page 48 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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FABRICATION AND ERECTION*

2.10 CHAPTER TWO

source may be portable, but the need for moving it is minimized by connecting it to the electrode
holder with relatively long cables.
The size of electrode (core wire diameter) depends primarily on joint detail and welding position.
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Electrode sizes of / 8, / 32, / 16, / 32, / 4, and / 16 in are commonly used. Small-size electrodes are 14 in
long, and the larger sizes are 18 in long. Deposition rate of the weld metal depends primarily on
welding current. Hence, use of the largest electrode and welding current consistent with good prac-
tice is advantageous.
About 57% to 68% of the gross weight of the welding electrodes results in weld metal. The
remainder is attributed to spatter, coating, and stub-end losses.
Shielded metal arc welding is widely used for manual welding of low-carbon steels, such as A36,
and HSLA steels, such as A572 and A588. Though stainless steels, high-alloy steels, and nonferrous
metals can be welded with this process, they are more readily welded with the gas metal arc process.
Submerged arc welding (SAW) produces coalescence by the heat of an electric arc struck
between a bare metal electrode and the base metal. The weld is shielded by flux, a blanket of gran-
ular fusible material placed over the joint. Pressure is not used on the parts to be joined. Filler metal
is obtained either from the electrode or from a supplementary welding rod.
The electrode is pushed through the flux to strike an arc. The heat produced by the arc melts
adjoining base metal and flux. As welding progresses, the molten flux forms a protective shield
above the molten metal. On cooling, this flux solidifies under the unfused flux as a brittle slag that
can be removed easily. Unfused flux is recovered for future use. About 1.5 lb of flux is used for each
pound of weld wire melted.
Submerged arc welding requires high currents. The current for a given cross-sectional area of
electrode often is as much as 10 times as great as that used for manual welding. Consequently, the
deposition rate and welding speeds are greater than for manual welding. Also, deep weld penetration
results. Consequently, less edge preparation of the material to be joined is required for submerged-
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arc welding than for manual welding. For example, material up to / 8 in thick can be groove-welded,
without any preparation or root opening, with two passes, one from each side of the joint. Complete
fusion of the joint results.
Submerged arc welding may be done with direct or alternating current. Conventional welding
power units are used but with larger capacity than those used for manual welding. Equipment with
current ratings up to 4000 A is used.
The process may be completely automatic or semiautomatic. In the semiautomatic process, the
arc is moved manually. One-, two-, or three-wire electrodes can be used in automatic operation, two
being the most common. Only one electrode is used in semiautomatic operation.
Submerged arc welding is widely used for welding low-carbon steels and HSLA steels. Though
stainless steels, high-alloy steels, and nonferrous metals can be welded with this process, they are
generally more readily welded with the gas-shielded metal-arc process.
Gas metal arc welding (GMAW) produces coalescence by the heat of an electric arc struck
between a filler-metal electrode and base metal. Shielding is obtained from a gas or gas mixture
(which may contain an inert gas) or a mixture of a gas and flux.
This process is used with direct or alternating current. Either straight or reverse polarity may be
employed with direct current. Operation may be automatic or semiautomatic. In the semiautomatic
process, the arc is moved manually.
As in the submerged arc process, high current densities are used, and deep weld penetration
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results. Electrodes range from 0.020 to / 8 in diameter, with corresponding welding currents of about
75 to 650 A.
Practically all metals can be welded with this process. It is superior to other presently available
processes for welding stainless steels and nonferrous metals. For these metals, argon, helium, or a
mixture of the two gases is generally used for the shielding gas. For welding of carbon steels, the
shielding gas may be argon, argon with oxygen, or carbon dioxide. Gas flow is regulated by a
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flowmeter. A rate of 25 to 50 ft /h of arc time is normally used.
Flux-cored arc welding (FCAW) is similar to the GMAW process except that a flux-containing
tubular wire is used instead of a solid wire. The process is classified into two subprocesses, self-
shielded and gas-shielded. Shielding is provided by decomposition of the flux material in the wire.

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