Effect of hydrogen on martensite formation in austenitic stainless steels in high-pressure torsion

The martensitic transformation induced by high-pressure torsion (HPT) and hydrogen desorption in Type 304 and 316L austenitic stainless steels is reported in this paper. When Type 304 with only 2.2 mass ppm hydrogen was subjected to HPT, up to 95% of the austenite transformed to martensite. For Type 316L with 1.5 ppm hydrogen content, martensitic transformation during HPT was up to 50%. Martensitic transformation during HPT was suppressed by hydrogen pre-charging to <80% for Type 304 with 24.2 ppm hydrogen and <10% for Type 316L with 22.3 ppm hydrogen. This is presumed to be due to slip localization by hydrogen-enhanced slip planarity. Because of the significantly greater hydrogen solubility and lower diffusivity in austenite as compared to martensite, it is hypothesized that when hydrogen-containing austenite is transformed to martensite, excess hydrogen is released and then diffuses primarily through martensite.