Investigation on hydrogen-induced cracks of Q500 steel under different strain rates

In this study, hydrogen microprinting was employed to characterize the distribution of hydrogen in the Q500 experimental steel. Slow strain rate tensile (SSRT) tests were also conducted at various strain rates on the hydrogen-charged steel to examine fracture morphology and crack propagation behavior, aiming to clarify the influence of strain rate on hydrogen-induced crack nucleation and growth. The results show that hydrogen atoms preferentially segregated at M/A islands and their boundaries, at inclusion-matrix interfaces, and at various grain boundaries, including prior austenite grain boundaries, interfaces between granular and lath bainitic ferrite, as well as packet and lath boundaries. After hydrogen charging, with decreasing strain rate, the yield strength, tensile strength, and elongation all decreased, whereas the hydrogen embrittlement index increased correspondingly. The microscopic fracture morphology at the tensile specimen edge gradually transitioned from dimples to river-like patterns. Meanwhile, the number of crack initiation sites increased, and the initiation locations shifted from M/A island boundaries (or M/A island boundaries plus packet boundaries) to M/A island boundaries and lath boundaries. Furthermore, the dominant crack propagation mode gradually changed from intergranular to transgranular fracture.