Modelling of hydrogen distribution at the fatigue crack tip of austenitic stainless steels in internal and external hydrogen tests

A hydrogen diffusion model was developed, which accounts realistically for the kinetics of hydrogen adsorption, absorption and their reverse processes at the gas-solid interface. Using the model, the hydrogen distribution at the fatigue crack tip of austenitic stainless steels (304, 316 L) in internal and external hydrogen tests were analyzed. Results indicate that for stable austenitic stainless steel, the hydrogen concentration at the fatigue crack tip is much higher in internal hydrogen test than that in external hydrogen test, which is attributed to the extremely low adsorption, desorption and diffusion rates of γ phase. For metastable austenitic stainless steel, hydrogen rapidly enters into strain-induced α′ martensite around the fatigue crack tip, forming an enrichment peak at the α′/γ interface in external hydrogen test. While significant hydrogen outgassing occurs in α′ phase in internal hydrogen test, leading to a much lower crack tip concentration compared with external hydrogen test.