Improved surface integrity and fatigue property of FV520B steel via surface mechanical rolling treatment process

In this paper, fatigue specimens prepared by the finish turning (FT) process were strengthened through surface mechanical rolling treatment (SMRT) process, and the effect of the SMRT process on the surface integrity and fatigue life of FV520B steel was investigated. The SMRT process significantly improves surface integrity, including surface roughness, microhardness, residual stress and gradient microstructure surface (GMS) layer. The fatigue life first increases and then decreases sharply with increasing pressure, while the fatigue life decreases slightly with increasing feed rate. Compared to FT specimen, the highest fatigue life was improved by 14.12 times after SMRT process. The fatigue fracture of FT specimen presents a continuous fatigue crack initiation site, while the fatigue fracture of SMRT specimens contains multi-point fatigue crack initiation sites or a single fatigue crack initiation site. In addition, the microstructure evolution mechanism of the SMRT specimens was analyzed. At the pressure of 18 MPa, EBSD results showed that the grain orientation was uniformly distributed, and the dislocation density and the number of grain boundaries were significantly increased on the surface. Meanwhile, TEM proves that a gradient nanostructured surface (GNS) layer was prepared, with an equiaxed nanocrystalline size of the topmost surface measuring approximately 34.9 nm. The improvement of fatigue life mainly relies on the synergistic effects of smooth surface, residual compressive stress and GNS layer after SMRT process.