Analysis of contact fatigue behavior of 18CrNiMo7-6 gears based on surface integrity and microstructure

This study aims to clarify how different initial surface states affect the micropitting and pitting behavior of carburized 18CrNiMo7-6 gears. Gears subjected to grinding, shot peening, and shot peening followed by barrel finishing were tested under the same contact fatigue conditions for a preset duration of 5.0 × 10 7 cycles. Case 1-SPC consisted of carburizing and grinding, followed by shot peening and barrel finishing. Case 2-SPS consisted of carburizing and grinding, followed by shot peening. Case 3-RC consisted of carburizing and grinding only. The results show that micropitting and pitting are governed by the coupled evolution of surface topography, hardness gradient, residual stress, and near-surface microstructure. All cases exhibited near-surface softening after cycling, while the redistribution of the hardness gradient strongly depended on the initial surface state. Shot peening introduced a high magnitude compressive residual stress field, and after cycling the maximum compressive residual stress in Case 1-SPC remained as high as 977.85 MPa, although this condition also showed the most pronounced stress relaxation. In contrast, barrel finishing produced a smoother and more surface oriented compressive stress distribution with better stability, and Case 1-SPC exhibited the best resistance to both micropitting and pitting. Surface observations showed a regional micropitting band in Case 1-SPC, denser micropitting in Case 2-SPS, and larger localized pitting in the ground condition. In addition, cyclic contact loading induced grain refinement, elevated local orientation gradients, and enhanced plastic heterogeneity in the near-surface layer. After cycling, Case 1-SPC showed the most pronounced microstructural reconstruction, with an average grain size refined to 0.57 μm and an average geometrically necessary dislocation (GND) density increased to 19.99 × 10 14 m -2 . Overall, shot peening followed by barrel finishing provided the most favorable balance among residual stress stability, surface integrity retention, and damage suppression, and was therefore the most effective route for improving the resistance of carburized 18CrNiMo7-6 gears to micropitting and pitting.