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 of heat input and temperature on the crack-tip opening displacement (CTOD) in the coarse-grained heat-affected zones (CG HAZs) that are generally considered to be the weakest sections of fusion welded structures. The study focuses primarily on the fracture mechanisms in CG HAZs in submerged-arc-welds (SAWs) between A707 steel base metal. The first section of the article explores the effects of heat input on the fracture toughness (CTOD) of the CG HAZs. Following a description of the microstructures in HAZs and inclusions 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 elucidated via detailed scanning electron microscopy (SEM) analysis. The results of multiple CTOD tests at 4.4 °C (in the transition temperature regime) are analyzed within a statistical framework. The measured distributions of CTOD data are shown to be well described by Weibull statistics. The second section of the article focuses on the effects of temperature on the CTOD. The study of fracture in the lower-shelf and transition-temperature regimes indicates that the ductile-to-brittle transition occurs between −10 and 23 °C. Fracture initiation in both the lower-shelf and transition-temperature regimes is shown to occur by ductile tearing, prior to a transition to cleavage fracture. The dimple size variations in the stable tearing regime are shown to be well described by Weibull statistics. The implications of the results are discussed for analysis of CG HAZ fracture.