The Role of Microstructure in Hydrogen-Induced Fatigue Failure of 304 Austenitic Stainless Steel

The effect of 104 mass ppm of hydrogen on the evolved microstructures associated with accelerated fatigue failure in type 304 austenitic stainless steel is reported. The fracture surface morphology changed from ductile striations to mixed mode that appeared "quasi-cleavage-like" and "flat." Detailed microstructural characterization determined that these fractures were along the austenite-martensite interfaces. The morphology and orientation of the strain-induced martensite were impacted by the presence of hydrogen. Hydrogen constrained the formation of alpha '-martensite into linear, planar bands in the grains nearest the fracture surface, and epsilon-martensite was formed between the alpha '-martensite bands. The dislocation structure generated by the cyclic loading and the restriction of the martensitic transformation to specific forms by hydrogen is explained through the hydrogen-enhanced localized plasticity mechanism.