Surface Strengthening of 30CrMoA Gear Journal Shafts by Roller Burnishing: Residual Stress Field Characterization, Parametric Analysis, and Multi-Objective Optimization

Residual tensile stresses are readily introduced into the surface layers of gear-shaft journals during machining, promoting fatigue crack initiation and propagation and degrading service reliability. This study investigates mechanical rolling strengthening of the outer cylindrical surfaces of 30CrMoA gear-shaft journals to improve residual stress distribution. Finite element (FE) simulations and X-ray diffraction (XRD) measurements were combined to characterize the rolling-induced residual stress fields and to elucidate the effects of rotational speed, indentation depth, and feed rate on the residual compressive stress magnitude and strengthened-layer thickness. Response surface models based on a central composite design (CCD) were established, and multi-objective optimization was performed to identify optimal rolling parameters. The results reveal a gradient residual compressive stress field in the surface layer, with the maximum compressive stress at approximately 0.20 mm below the surface, whereas slight residual tensile stresses develop in the deeper substrate to maintain self-equilibrium of the residual stress field. Indentation depth exerts the most pronounced influence, significant parameter interactions are observed, and the strengthening responses saturate over the investigated ranges. Experimental validation confirms that the optimized parameters simultaneously enhance the surface residual compressive stress magnitude and strengthened-layer thickness, with relative errors between predictions and measurements below 8.0%. The proposed model accurately describes the depth-dependent residual stress distribution, providing a theoretical basis and practical guidance for parameter design and optimization of mechanical rolling strengthening of 30CrMoA gear-shaft journals.