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.
Zou et al. (Fri,) studied this question.