High-cycle fatigue behavior of 18Cr-8Ni austenitic stainless steels with grains ranging from nano/ultrafine-size to coarse

The effect of grain size on high-cycle fatigue behavior under axial forces in the 18Cr-8Ni austenitic stainless steel was investigated. The fatigue strength of the austenitic stainless steel, obtained by the phase reversion process, with grain sizes of 400 nm, 1.4 µm, and 12 µm were 811 MPa, 568 MPa, and 501 MPa, respectively. After cyclic deformation, remarkable slip bands were formed on the surfaces of coarse-grained (CG) and fine grained (FG) samples, whereas, no slip bands or other deformation marks were observed on the surface of the nanograined/ultrafine-grained (NG/UFG) sample. Randomly oriented cracks of a few micrometers long appeared on the CG sample, while the cracks on NG/UFG were larger and the propagation direction was perpendicular to the loading direction. The increase in hardness after cycling was inversely proportional to the increase in strain-induced martensite among the NG/UFG, FG, and CG samples. Strain-induced martensite was produced primarily at the grain boundaries or slip bands in CG steels; in contrast, martensite was concentrated in the vicinity of nanograins in FG and NG/UFG steels. After fatigue, in the CG sample, a small amount of martensite, deformation bands, and large number of dislocations were formed, while in the NG/UFG sample a very small amount of ε-martensite, strain-induced martensite, and dislocations were formed.