Thermo-ultrasonic crystal plasticity analysis of residual stress and surface integrity in vibration-assisted grinding of 12Cr2Ni4A gear steel

Ultrasonic vibration-assisted grinding (UVG) can improve the surface integrity of hardened gear steels, but the mesoscopic pathway from vibration-induced softening to residual-stress retention remains insufficiently resolved. This study develops a thermo-ultrasonic crystal plasticity finite element method (CPFEM) framework for 12Cr2Ni4A gear steel. Process-scale grinding force and heat flux are converted into equivalent moving mechanical and thermal boundary loads for polycrystalline submodels, while ultrasonic and thermal softening are introduced into a dislocation-density-based slip law as effective, experimentally constrained terms. The simulations show that ultrasonic vibration reduces the critical resolved shear stress (CRSS), promotes coordinated activation of multiple {110} slip systems, and increases dislocation storage in the near-surface layer. These changes provide a Type III -> Type II -> Type I stress-transfer route: local subgrain stress heterogeneity is redistributed at the grain scale and finally retained as a deeper macroscopic compressive residual-stress profile. Compared with conventional grinding (CG), the predicted peak compressive residual stress reaches -369.13 MPa under UVG, and the residual stress at 100 μm remains more compressive than that under CG (-49.76 MPa vs. -23.48 MPa). The framework is calibrated using hot-compression data and checked against UVG experiments, electron backscatter diffraction/KAM/GND characterization, and X-ray diffraction residual-stress measurements. By connecting process loading, crystallographic deformation, dislocation evolution, and retained stress, this work gives a materials-based basis for controlling surface integrity in high-strength steel grinding.