Effect of the ultrasonic surface rolling process on the surface modified layer and fatigue performance of 18CrNiMo7-6 alloy steel

This study systematically examines the effects of ultrasonic surface rolling (USRP) on 18CrNiMo7-6 alloy steel’s surface integrity and fatigue performance. Results show USRP significantly improves surface characteristics: surface roughness (Ra) decreases by 73.4% (from 0.64 to 0.17 μm), stress concentration factor (Kst) reduces by 40.9% (from 1.81 to 1.07), and microhardness increases to 849 HV, peaking at 105 μm depth. The treatment generates a deep compressive residual stress field (− 1530 MPa at 165 μm) and refines surface grains to 0.32 μm. However, excessive processing (amplitude ≥ 24 μm) causes surface microcracks. Fatigue testing demonstrates USRP’s ability to shift crack initiation from surface to subsurface regions. The optimal treatment (USRP-7) enhances fatigue strength by 103.8% (from 633.7 to 1291.7 MPa) for Kt = 3 notched specimens. The multi-scale strengthening mechanism combines surface morphology, stress reduction, hardness gradient, residual stress, and microstructure refinement, offering valuable insights for designing fatigue-resistant high-stress components.Graphical Abstract