Surface modification and its effect on the tensile and fatigue properties of 300M steel subjected to ultrasonic surface rolling process

Surface modification of 300M steel induced by ultrasonic surface rolling process (USRP) was studied regarding surface topography, microstructure, the distribution of shear strain, microhardness, and residual stress profiles. The effects of USRP on the tensile and fatigue properties were finally discussed. The results indicated that one pass of USRP can reduce the surface roughness (Ra) by more than one order of magnitude (from 0.30 μm to 0.025 μm). Besides, a severe plastic deformation layer was noticed at the top surface, which led to an increase of surface microhardness by about 17% and an elevation of compressive residual stress by about 190%. In USRP, a high plastic shear strain of ~3 and shear strain rate of ~105 s−1 can be induced into the topmost surface, which motivates the dislocation activities in the form of dislocation tangles, dislocation walls, and dislocation cells. Dislocation slip supplemented by mechanical twinning dominates the microstructural evolution pattern of the USRP sample. The excellent surface finish, high microhardness, and high compressive residual stress result in an obvious increase in the resistance of high-cycle fatigue behavior. Besides, the tensile strength was improved by 3.8%, whereas the corresponding yield strength and elongation were reduced by 5.9% and 24.3%, respectively.