An investigation of the fracture behavior of coarse-grained heat-affected zones in a707 steel welds

This article presents the results of an experimental study of the effects o f heat input and temperature on the crack-tip opening displacement (CTOD) i n the coarse-grained heat-affected zones (CG HAZs) that are generally consi dered to be the weakest sections of fusion welded structures. The study foc uses primarily on the fracture mechanisms in CG HAZs in submerged-arc-welds ' (SAWs) between A707 steel base metal. The first section of the article ex plores the effects of heat input on the fracture toughness (CTOD) of the CG HAZs. Following a description of the microstructures in HAZs and inclusion s in the CG HAZs, the article summarizes the effects of heat input (1.2, 1. 8, and 2.6 MJ/ m) on the CTOD. The underlying fracture mechanisms are eluci dated via detailed scanning electron microscopy (SEM) analysis. The results of multiple CTOD tests at 4.4 degreesC (in the transition temperature regi me) are analyzed within a statistical framework. The measured distributions of CTOD data are shown to be well described by Weibull statistics. The sec ond section of the article focuses on the effects of temperature on the CTO D. The study of fracture in the lower-shelf and transition-temperature regi mes indicates that the ductile-to-brittle transition occurs between -10 and 23 degreesC. Fracture initiation in both the lower-shelf and transition-te mperature regimes is shown to occur by ductile tearing, prior to a transiti on to cleavage fracture. The dimple size variations in the stable tearing r egime are shown to be well described by Weibull statistics. The implication s of the results are discussed for analysis of CG HAZ fracture.