Investigation on the effect of machined surface texture on surface integrity and torsional fatigue behavior of ultra-high strength steel treated by ultrasonic surface rolling process

This investigation conducted ultrasonic surface rolling processing (USRP) and torsional fatigue testing on 45CrNiMoVA ultra-high-strength steel with various hard turning (HT) surface textures. It investigated the influence of HT-machined surface topography on USRP-induced surface integrity and torsional fatigue behavior. An energy-based predictive model incorporating machined surface integrity was established for torsional fatigue life assessment. The results showed that post-USRP surface roughness consistently fell within the range of 0.4–0.8 μm across HT-processed surfaces with varying scallop heights. Surfaces featuring HT textures enabled USRP to achieve a maximum plastic deformation layer depth of 95 μm, with an average grain size refined to 0.52 μm. Distinct HT surface topographies significantly altered torsional fatigue life under USRP treatment. HT surfaces featuring a tool nose radius of 0.2 mm and scallop height of 60 μm achieved exceptional post-USRP torsional fatigue life exceeding 60,000 cycles. Fracture mode variations impacted the torsional fatigue life. Hybrid fracture modes involving combined normal stress, transverse shear stress, and longitudinal shear stress reduced crack propagation rates and enhanced torsional fatigue performance. Under torsional loading, 45CrNiMoVA exhibited cyclic softening behavior in its shear stress response. The torsional fatigue life prediction model considering surface integrity demonstrated minimum and mean prediction accuracies of 84.9 % and 93.7 %, surpassing the predictive capability of conventional energy-based models. This investigation provides critical insights into how machined surface textures influence post-enhanced surface integrity and torsional fatigue behavior, and demonstrates that the surface performance of components can also be improved through the rough machining-strengthening process without the finish machining.