Fracture behavior of heat-affected zone in low alloy steels

Past elastic-plastic fracture studies for leak-before-break (LBB) assessment of low alloy steel pipings have been focused mostly on the behavior of base metals and their weld metals. In contrast, the heat-affected zone (HAZ) of a welded pipe has not been studied in detail primarily because the size of the HAZ is too small to make specimens for mechanical properties measurements. In this study, microstructural analyses, microhardness tests, tensile tests and J–R tests have been conducted as a function of distance from a fusion line and temperature for HAZ materials of SA106Gr.C low alloy piping steels. For the ferrite–pearlite steels such as SA106Gr.C, the HAZ specimens showed a higher yield strength and fracture toughness compared with those of its base metal. These characteristics, despite of grain coarsening, can be explained by cleaner microstructures of HAZ materials with a finer morphology of carbides compared with pearlitic–ferritic base metals. However, the situation can be reversed for a bainitic steel since its HAZ can develop an upper bainitic structure with a reduced fracture resistance and strength, warranting further studies.