Effect of high-pressure torsion processing and annealing on hydrogen embrittlement of type 304 metastable Austenitic stainless steel

The effect of high-pressure torsion (HPT) and annealing on hydrogen embrittlement (HE) of a type 304 stainless steel was studied by metallographic characterization and tensile test after hydrogen gas charging. A volume fraction of ∼78 pct of the austenite transformed to α′ martensite by the HPT processing at an equivalent strain of ∼30. Annealing the HPT-processed specimen at a temperature of 873 K (600 °C) for 0.5 hours decreased the α′ martensite to ∼31 pct with the average grain size reduced to ∼0.43 μm through the reverse austenitic transformation. Hydrogen charge into the HPT-processed and the HPT+annealed specimens in the hydrogen content of ∼10 to 20 ppm led to no severe HE but appeared in the solution-treated specimen. Especially the 873 K (600 °C) annealed specimen had the ∼1.4 GPa tensile strength and the ∼50 pct reduction of area (RA) despite the hydrogenation.