Influence of austenite grain boundary misorientation on hydrogen-induced intergranular crack propagation in a medium carbon martensitic steel

This study investigated the influence of austenite misorientation (θγ) on hydrogen-induced intergranular cracking along the prior austenite grain boundary (PAGB) in a medium C tempered martensitic steel. Utilizing EBSD analysis and prior austenite orientation reconstruction, distributions of martensite misorientation (θα′) and prior austenite misorientation (θγ) across PAGBs were compared in both fractured and total PAGBs. The comparison suggests that presence of low misorientation martensite segments (θα′ < 10o) at PAGB was beneficial to suppress PAGB cracking in accordance with the previous study. This low misorientation martensite segments are more likely to be achieved when θγ is either less than 10o (L-PAGB), or over 45o (H-PAGB) with a specific condition. The requirements for H-PAGB to achieve low misorientation martensite segment were investigated by applying interrupt quenching treatment, which can capture the initial nucleation of martensite. The result suggests that the nucleated martensite should hold near Kurdjumov-Sachs orientation relationship (KS OR) not only with austenite into which it grows, but also with opposite austenite sharing the grain boundary (double KS OR) to achieve low misorientation martensite segments. The double KS OR is satisfied in most of martensite nucleated in the early stage, indicating the importance of the variant selection to achieve low misorientation martensite segments at PAGB.