Residual-stress distribution in gear journal shafts induced by mechanical roll hardening

Residual tensile stresses in the surface layer of machined 30CrMoA gear journal shafts readily promote fatigue crack initiation and degrade in-service reliability. To mitigate these stresses, this study investigates a localized mechanical rolling-hardening process for the journal diameter. The residual-stress distribution in the rolled journal is characterized by combining finite-element simulation with X-ray diffraction residual-stress measurements, and the effects of key process parameters, namely rolling depth, workpiece rotational speed, and feed rate, on the magnitude of residual compressive stress and the depth of the strengthened layer are systematically examined. A response surface methodology based on central composite design is then employed to develop predictive models for the residual stress and layer depth, thereby enabling multi-objective optimization of the rolling process. The results demonstrate that mechanical rolling generates a stable, graded residual compressive-stress field within the localized surface layer, with the peak compressive stress occurring at approximately 0.20 mm beneath the surface, while the underlying matrix exhibits only weak equilibrating tensile stresses. Among the process parameters, rolling depth exerts the most significant influence, while pronounced coupling and saturation effects between parameters are also observed. An optimal parameter combination is determined and experimentally validated: under the optimized conditions, both the magnitude of the residual compressive stress and the depth of the strengthened layer are markedly increased, while the deviation between the simulated and experimental results remains below 8.0 %. The established parametric model accurately predicts the localized residual-stress state of rolled 30CrMoA journals, thereby providing theoretical guidance and technical support for the surface rolling strengthening of gear journals.