Factors controlling transformation-induced hydrogen desorption and hydrogen-reduced transformability in austenitic steels clarified via dynamic hydrogen detection during cryogenic deformation

The dynamic detection of hydrogen was performed during cryogenic deformation. The experiment at -60 °C with type304 showed α′-martensite-related hydrogen desorption and a nearly steady hydrogen desorption rate. The steady hydrogen desorption during cryogenic deformation resulted from hydrogen supersaturation. However, the continuous increase in the hydrogen desorption rate with deformation, which was observed in the ambient temperature test, was primarily attributable to a change in hydrogen diffusivity. For type316L, the austenite is metastable at -60 °C. The hydrogen desorption during deformation at -60 °C was triggered by α′-martensite cracking that increased the surface α′-martensite fraction, as the surface region without cracking remained austenite, even after the cryogenic deformation owing to hydrogen-reduced transformability. We also clarified that α′-martensite formation during cryogenic deformation in type316L requires hydrogen desorption from the surface. Thus, hydrogen promotes γ–ε martensitic transformation, and subsequent hydrogen desorption enables α′-martensite formation, which increases the total fraction of α′-martensite.