Hydrogen Embrittlement of Austenitic Stainless Steels with Different Surface Treatments

This study evaluated the effect of surface conditions on the hydrogen embrittlement (HE) characteristics of SUS304 austenitic stainless steel, as SUS304 is one of the candidate structural materials for hydrogen energy systems. Various machining processes, including milling (ML), shot peening (SP), and cold rolling (CR), were employed to modify the surface roughness, internal strain, and microstructural characteristics of the test sample. Namely, this approach was conducted as the surface-absorbed hydrogen is related to effective hydrogen: a high internal strain was obtained in the entire CR area along with a rough surface, while SP and ML samples displayed high strain levels near the surface. The strain value was reflected in the hardness level due to their work hardening and strain-induced martensite formation. Concerning this, the hardness values of CR and SP samples were higher than 6 and 2 times the as-received samples. The hydrogen content charged to the samples was contingent upon the strain level: higher strain corresponded to elevated hydrogen content, particularly in CR samples. Despite the notable high hydrogen content in CR samples, HE was not detected in the tensile test. Conversely, even with a low hydrogen content, severe HE was observed in all samples during the fatigue test. The susceptibility of stainless steel to HE proved sensitive to cyclic loading, wherein surface-absorbed hydrogen migrated to the crack tip during cyclic loading. Detailed discussions on the reasons for these observations are provided based on the experimental results.