Effects of prior austenite grain size on hydrogen embrittlement behavior in high-strength martensitic steel

The present study investigates the effects of prior austenite grain (PAG) size on hydrogen embrittlement behavior in high-strength martensitic steel utilizing slow strain rate tensile tests integrated with digital image correlation (DIC) and X-ray computed tomography analyses. The results demonstrate that PAG refinement significantly improves the hydrogen embrittlement resistance under a similar hydrogen level. DIC analysis reveals that the susceptibility to hydrogen embrittlement originates from the plastic deformation related to the lath martensite microstructure. PAG refinement mitigates the microstructure-related strain localization, thereby enhancing the hydrogen embrittlement resistance. Additionally, PAG refinement increases the apparent fracture toughness by promoting greater plastic work and enhancing crack meandering/branching, though the latter effect peaks at a medium PAG size. The enhanced resistance to hydrogen embrittlement thus could be attributed to several factors: reduced strain localization, lower local hydrogen concentration per unit area of PAG boundaries, frequent deflection of intergranular cracks, and an increased density of high-angle boundaries that impede quasi-cleavage cracks—all facilitated by PAG refinement.