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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