Effect of pre-torsion and deep rolling composite strengthening on surface integrity and fatigue properties of 45CrNiMoVA steel

Aiming to improve the fatigue life of axle parts, the surface of 45CrNiMoVA steel was modified by using the composite strengthening method of pre-torsion - deep rolling (PT-DR), and the effects of different processes on the surface properties of the material were analyzed. The significant residual stress influence layer below 1.8 mm on the material surface was formed by the PT process, which is beneficial to the formation of slender twins. Through the DR process, a residual compressive stress of −1409 MPa is generated on the surface layer of the material. The plastic deformation is conducive to the formation of high-density dislocation cells on the surface layer. The residual compressive stress in the surface layer is released after the DR-PT process, and the texture orientation is parallel to the PT direction. The surface roughness of the PT-DR process is reduced from 0.526 μm to 0.252 μm, the surface microhardness reaches 630 HV, and the residual compressive stress of the subsurface layer is high. Fatigue testing showed that the optimum process method for strengthening corresponding to the longest life was PT-DR, which was 5 times the PT process and showed a double-helix fracture behavior. This is due to surface grain refinement and gradient nanostructures, as well as the effective inhibition of dislocation slip by residual compressive stresses.