An investigation of the fracture behavior of coarse-grained heat-affected zones in A707 steel welds
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NTRODUCTION
COARSE-GRAINED heat-affected zones (CG HAZs) in weldments, generally, are regarded as the weakest sections that control the failure of welded steel structures.[1] The degraded fracture performance in these regions, often, is associated with the formation of coarse-grained microstructures,[2] brittle-hard phases, such as martensite[3] and inclusions[4]. However, a fundamental understanding of the role of these metallurgical factors in the fracture processes that occur in the transition-temperature regime (between the lower- and upper-shelf regions) is yet to emerge. In the case of A707 steel,[5] which is used primarily in the pipeline applications in the oil and gas industry, there have been relatively few prior studies of the effects of the CG HAZ microstructures and inclusion distributions on the fracture mechanisms.[6] There is, therefore, a need for investigations of the effects of microstructure and inclusion distribution on the fracture mechanisms in the CG HAZs of fusion welded sections fabricated from A707 steel. A wide variety of process parameters are associated with the fusion welding processes[7] that, typically, are used in the fabrication of steel joints. These include controllable parameters, such as voltage, current, and weld speed. However, a large number of welding process parameters are difficult to control. For example, in multipass welding, the microstructures are inhomogeneous in the HAZs. This is due to the reheat process that occurs with each weld pass. W.O. SOBOYEJO, Professor, and J. ZHOU, Graduate Research Assistant, are with the Princeton Materials Institute and the Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544. J. CROMPTON, Manager, T. McGAUGHY, Project Manager, and F. ORTH, Business Manager, are with the Edison Welding Institute, Columbus, OH 43212. Manuscript submitted January 7, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
Nevertheless, some prior studies[8,9] have identified the welding heat input as the critical process parameter that controls the fracture toughness levels in the CG HAZs. It is, therefore, of practical interest to develop a fundamental understanding of the effects of heat input on the microstructure and fracture toughness of the CG HAZs in A707 steel welds. In the transition-temperature regime, the fracture processes in steels depend on the outcome of a competition between brittle and ductile fracture processes.[10] The transition-temperature regime in steels, also, has been shown to be sensitive to alloy composition. Since the fracture toughness levels in the CG HAZs, generally, are lower than those in other regions, it is of practical importance to develop a fundamental understanding of the effects of temperature on the fracture mechanisms in the CG HAZs of A707 steel weldments. Several studies[11–17] have been carried out on the fracture mechanisms in the HAZs of steel welds. However, a fundamental understanding of the fracture mechanisms in the CG HAZs is yet to emerge. Hence, there is a need for fu
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