Localized plasticity, transformation, and martensite cracking in hydrogen-charged metastable austenitic stainless steel studied by in situ electron channeling contrast imaging

In situ electron channeling contrast imaging was applied to investigate local deformation and microstructure evolution in an electrochemically hydrogen-pre-charged type 304 austenitic stainless steel. The imaging results revealed an acceleration of γ-ε-α' martensitic transformation on the surface by hydrogen; however, no cracking was observed immediately after the transformation. A plastic deformation over 10% induced stress concentration and localized plasticity near a grain boundary, which led to hydrogen-related intergranular cracking. A side of the grain boundary acting as the cracking site was composed of α' martensite; however, the other side neighboring an intergranular crack remained austenite. Interestingly, many intergranular cracks were terminated at the α'-martensite region, which indicated that the retained austenite played a significant role in hydrogen-related intergranular cracking. The retained austenite was suggested to result in a state of high hydrogen concentration at the prior austenite grain boundary. • In situ ECCI was applied to hydrogen embrittlement study. • Martensite cracking did not occur immediately after the transformation. • Plastic deformation over 10% induced localized plasticity near a grain boundary. • The locally deformed portion showed martensite and hydrogen-related intergranular cracking. • A side of the grain boundary acting as the cracking site was composed of α' martensite.