Hydrogen Effect on Microstructural Alteration before Fatigue Fracture on Austenitic Stainless Steels

Type 304 stainless steel is typical austenitic one where fatigue life is deteriorated in hydrogen environments. There are few studies for fatigue crack initiation process compared to those of fatigue crack propagation. In this study, the authors experimentally investigated influence of hydrogen on dislocation structures and phase distribution before the fatigue crack initiation. A solution heat treated Type 304 plate was used as a sample, and round-bar fatigue specimens with a notch were machined. Fatigue tests were performed for the hydrogen-charged or -free specimens in fully reversed loading conditions at room temperature. The test was terminated before the crack initiation. Then, the dislocation structures and the phase distribution underneath the notch root were analyzed by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). For the hydrogen-charged specimen, planar dislocations were observed in the TEM images. With increase in fatigue cycles, Area of stacking faults (SFs) increased and ε martensite (εM) appeared on the (111)γ planes. α’ martensite (α’M) were observed at crossover sites of the εM phases on different (111)γ planes. In EBSD analysis, the α’M was often observed in a plate form parallel to the (111)γ planes. For the hydrogen-free specimens, on the other hand, dislocations cell structures and massive α’ M were observed in the TEM images. Neither SFs nor εM were formed. Thus, hydrogen increases dislocation planarity and changes martensitic transformation from γ to α’M, or γ to α’M through ε, resulting in different α’ M morphologies.