Effects of high-pressure hydrogen charging on the structure of austenitic stainless steels

The effects of high-pressure hydrogen and deuterium charging on the structure of AISI type 304 and AISI type 310 austenitic stainless steels have been investigated by neutron and X-ray diffraction. Rietveld analyses of the neutron diffraction data revealed that hydrogen atoms occupy exclusively the octahedral interstitial sites in both steels. No phase transformations have been observed in 310 stainless steel within the whole range of hydrogen-to-metal atomic ratios H/Me up to ≈ 1. In 304 stainless steel, the formation of ɛ-martensite was observed not only after hydrogenation at 3.0 GPa (H/Me = 0.56), but also after applying a pressure of 4.0 GPa without hydrogen. The results differ significantly from published studies on cathodically hydrogenated samples, where high amounts of ɛ-martensite were observed in both steels. High-pressure hydrogenation and cathodic hydrogen charging result in different phase transformation behaviour. The discrepancies can be explained by different hydrogen distributions resulting in quite different stress states.