Fatigue behavior of ultrafine-grained and coarse-grained Cr–Ni austenitic stainless steels

High-cycle bending fatigue behavior of an ultrafine-grained Cr–Ni Type 301LN austenitic stainless steel, obtained by reversion annealing, was investigated and compared to that of the conventional coarse-grained counterpart. The fatigue limit was significantly increased from 350 MPa to 630 MPa, reaching 59% of the tensile strength, a behavior attributed to grain refinement. Fatigue cycling resulted in hardness increments that were very different between these structures; in the coarse-grained steel 47%, but only 6–10% in the ultrafine-grained steel. The fatigue damage was observed to occur by grain boundary cracking in the ultrafine-grained steel, while mostly by slip band formation and crack propagation along these slip bands and grain boundaries in the coarse-grained counterpart. Dislocation structures observed reflected these pronounced differences in the cyclic behavior.