Mechanical characterisation of microstructural evolution in 304 stainless steel subjected to high-pressure torsion with and without hydrogen pre-charging

Micro-tensile tests were employed on a 304 metastable austenitic stainless steel to mechanically characterise the microstructures developed by processing through high-pressure torsion (HPT) with and without hydrogen pre-charging. The martensite formed by HPT processing of hydrogen-containing austenite exhibited low yield and tensile strengths but a high reduction of area compared to the one processed in the absence of hydrogen. This may be because dynamic martensite formed with hydrogen contains more retained austenite. Hydrogen charging into the austenite allowed the formation of ε-martensite, instead of deformation twinning, as an intermediate phase in the transformation to α′-martensite, which led to variation in the plastic behaviour. The inhomogeneity of the microstructure and the defects produced by deformation with hydrogen build a foundation but hardly play a crucial role in the hydrogen embrittlement (HE) of metastable austenitic steels. Excess hydrogen due to the dynamic martensitic transformation of hydrogen-containing austenite localises deformation in the retained austenite between the martensite regions formed, leading to the HE of metastable austenitic steels.