The contribution of intragranular acicular ferrite microstructural constituent on impact toughness and impeding crack initiation and propagation in the heat-affected zone (HAZ) of low-carbon steels

We elucidate here the contribution of intragranular acicular ferrite (IAF) on impending crack initiation and propagation in the heat-affected-zone (HAZ) of low carbon steels based on Instrumented Charpy impact test. It was observed that both crack initiation energy and crack propagation energy of the steel in the presence of IAF were higher than steel that did not contain IAF, where the former situation was observed to have a large ductile fracture zone as compared to the latter. An analysis of the statistical distribution of voids in the cross-section of impact fracture indicated that IAFs increase the length of plastic zone at the tip of the main crack and impede crack initiation and enhance crack initial energy because of refined prior austenite grain and superior deformability. Furthermore, EBSD analysis of secondary cracks indicated that IAF impedes crack propagation and increases crack propagation energy by forming high angle grain boundaries with surrounding bainite and its own superior deformability.