Surface-layer microstructure control for metastable austenitic stainless steel to prevent hydrogen permeation

Solution nitriding was applied to type 304 stainless steel plate to make the austenite phase in the surface layer stable against cold rolling. Through this treatment, we could produce a structure-gradient stainless steel plate where an austenitic structure exists in the surface layer while a large fraction of deformation-induced martensitic structure is dispersed inside the plate. This surface-layer microstructure control significantly suppressed the permeation of hydrogen from the atomosphere owing to the low diffusivity of hydrogen in the stable austenitic surface layer.