Effect of ultrafine grain refinement on hydrogen embrittlement of metastable austenitic stainless steel

Micro-tensile tests were performed on high-pressure-torsion-processed specimens of type 304 steel with grain sizes in the range of 0.1-0.5 μm to clarify the effect of ultrafine grain refinement on the hydrogen embrittlement (HE) of metastable austenitic steel. The ultrafine-grained (UFG) specimens with average grain sizes < ∼0.4 μm exhibited a limited uniform elongation followed by a steady-stress regime in the stress-strain curves, which was attributed to a martensitic transformation. A high yield stress and a moderate elongation to failure were attained for the UFG specimens with an average grain size of ∼0.5 μm in the uncharged state. Hall-Petch relationships well hold between the yield stress and the average grain size for each uncharged and hydrogen-charged specimen. Hydrogen charging increased the friction stress by 40% but did not change the Hall-Petch coefficient. Hydrogen-induced ductility loss was mitigated by ultrafine grain refinement. Ductility loss due to hydrogen charging manifested in the local deformation after a martensitic transformation. This indicates that hydrogen does not significantly affect the martensitic transformation, but shortens the subsequent local deformation process.