Page 190 - Handbook of Structural Steel Connection Design and Details
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Welded Joint Design and Production
Welded Joint Design and Production 175
Low-carbon steels. In general, steels with carbon contents to 0.30%
are readily joined by all the common arc-welding processes. These
grades account for the greatest tonnage of steels used in welded
structures. Typical applications include structural assemblies, as well
as many other areas.
Steels with very low carbon content—to 0.13%—are generally good
welding steels, but they are not the best for high-speed production
welding. The low carbon content and the low manganese content
(to 0.30%) tend to produce internal porosity. This condition is usually
corrected by modifying the welding procedure slightly—usually by
using a slower speed. Steels with very low carbon content are more
ductile and easier to form than higher-carbon steels. They are used
for applications requiring considerable cold forming, such as stamp-
ings or rolled or formed shapes.
Steels with 0.15 to 0.20% carbon content generally have excellent
weldability, and they can be welded with all types of mild-steel elec-
trodes. These steels should be used for maximum production speed on
assemblies or structures that require extensive welding.
Steels at the upper end of the low carbon range—the 0.25 to 030%
carbon grades—generally have good weldability, but when one or
more of the elements is on the high side of permissible limits, crack-
ing can result, particularly in fillet welds. With slightly reduced
speeds and currents, any mild steel type of electrode can be used. In
5
thicknesses up to ⁄ 16 in, standard procedures apply.
If some of the elements—particularly carbon, silicon, or sulfur—are
on the high side of the limits, surface holes may form. Reducing cur-
rent and speed minimizes this problem.
Although for some welding applications these steels require little or
no preheating, heavy sections (2 in or more) and certain joint configu-
rations often require a preheat. In general, steels in the 0.25 to 0.30%
carbon range should be welded with low-hydrogen processes.
High-strength–low-alloy structural steels. Higher mechanical properties
and better corrosion resistance than that of structural carbon steels
are characteristics of the high-strength–low-alloy (HSLA) steels.
These improved properties are achieved by addition of small amounts
of alloying elements. Some of the HSLA types are carbon-manganese
steels; others contain different alloy additions, governed by require-
ments for weldability, formability, toughness, or economy. The
strength of these steels is generally between that of structural carbon
steels and that of high-strength quenched and tempered steels.
High-strength–low-alloy steels are usually used in the as-rolled
condition, although some are available that require heat treatment
after fabrication. These steels are produced to specific mechanical
property requirements rather than to chemical compositions.
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