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Welded Joint Design and Production

                    174   Chapter Three

                    circumstances, it is advisable to proceed with caution, reviewing the
                    principles outlined in this chapter, and, when necessary, to contact a
                    welding engineer with expertise in metallurgy to address the
                    specifics of the situation.
                      For most applications, however, modern welding codes such as the
                    American Welding Society’s AWS D1.1 Structural Welding Code—Steel
                    and the American Institute of Steel Construction’s Steel Construction
                    Manual list weldable steels suitable for construction. These materials
                    have a long history of satisfactory performance, and the codes supply
                    appropriate guidelines as to what precautions or techniques are appro-
                    priate for certain materials. For example, AWS D1.1 lists “prequalified
                    steels” that may be used in conjunction with a prequalified welding
                    procedure. The code requirements for the fabrication of these steels are
                    sufficiently justified that the contractor is not required to qualify the
                    welding procedures by test when using this particular material, provided
                    that all the other prequalified requirements were met.
                      Codes, however, do not necessarily include new developments from
                    the steel producers. An inevitable characteristic of codes is that they
                    will always lag behind industry. Once a particular steel has an accept-
                    able history of performance, it may be incorporated into the applicable
                    specifications. Until that time, the engineer must rely upon research
                    data to determine the suitability of the part for a specific application.
                      A variety of tests have been devised over the years, each capable of
                    measuring specific aspects of the weldability of the material under
                    different conditions. Some tests measure the heat-affected zone prop-
                    erties, whereas others are more sensitive to weld-metal cracking ten-
                    dencies. Unproven materials should be carefully reviewed by a com-
                    petent engineer before being used in actual applications, and actual
                    consideration of approximate weldability tests is recommended.
                      Listed in the following section are typical steels that are used for
                    welded construction today.

                    3.1.2 Modern base metals for welding

                    The carbon steels. Classification of the carbon steels is based principally
                    on carbon content. The groups are low carbon (to 0.30% carbon), medium
                    carbon (0.30 to 0.45%), and high carbon (more than 0.45%). Mechanical
                    properties of hot finished steels are influenced principally by chemical
                    composition (particularly carbon content), but other factors—finishing
                    temperature, section size, and the presence of residual elements—also
                                      3
                    affect properties. A ⁄4-in plate, for example, has higher tensile properties
                    and lower elongation than a 1 ⁄2-in plate of the same composition, resulting
                                              1
                                                              3
                    primarily from the higher rate of cooling of the  ⁄ 4 -in plate from the rolling
                    temperature. Medium- and high-carbon steels are not typically used for
                    structural operations and therefore will not be discussed further.


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