Interrelationship between hydrogen and α′-martensite of SUS 304 austenitic stainless steel revealed by tensile tests

Abstract The hydrogen embrittlement (HE) behavior of SUS 304 austenitic stainless steel (ASS) was investigated by in-situ scanning electron microscope（SEM）observations combined with electron backscatter diffraction (EBSD), slow strain rate tensile (SSRT) tests and nanoindentation techniques. The specimens for in-situ tensile tests were cathodic pre-charged with hydrogen for 0 and 96 hours. The in-situ EBSD analysis and SEM observation indicated that α′-martensite was generated along the twin boundaries during the tensile process of 304 ASS, and the pre-charged hydrogen promoted the α′-martensite transformation. In turn, the tensile sample was first stretched by 20% to produce α′-martensite and then hydrogen charged, and the SSRT tests showed that the pre-existing α′-martensite increased the hydrogen diffusion depth and HE sensitivity of the experimental steels. The results of nanoindentation experiments showed that hydrogen was captured by the grain boundary as hydrogen trap, which slightly reduced the average hardness of the material owing to the “elastic shielding effect” of hydrogen, but because of the general solid solution hardening mechanisms and the martensitic transformation promoted by hydrogen, the role of hydrogen in hardness reducing cannot be overestimated. The presence of α′-martensite in 304 ASS determined the fracture mechanism during the subsequent tensile process. If the sample had no α′-martensite at the time of hydrogen charging, the fracture mode was transgranular fracture with the cracks initiating and propagating along the twin boundaries. But if the sample contained a certain amount of α′-martensite when hydrogen charging was carried out, the fracture mode was intergranular fracture with cracks initiating along the twin boundaries but propagating along the grain boundaries.