Study on the effect of surface rolling process on fatigue behavior and surface integrity of 300M steel thread root

In this work, 300 M steel threads were treated by surface rolling process (SRP) with varying rolling forces. The enhancement mechanism of surface integrity on fatigue behavior were discussed in terms of surface geometric state, microstructure, surface hardness, and compressive residual stress (CRS). The SRP-1 sample with low rolling force had the lowest surface roughness with Sa 0.11 μm and Sq 0.145 μm, but the improvement effects of grain refinement and CRS were relatively low. The SRP-2 sample had the largest CRS, and the surface roughness and gradient grain were also synergistically improved. Consequently, SRP-2 sample exhibited the largest fatigue life, with a 74.6 % enhancement compared to B-SRP sample. The SRP-3 sample with largest rolling force achieved the finest and deepest gradient nanostructured layer, but the severe work-hardening introduced surface damage, which in turn reduced the strengthening effect. The fatigue behavior for different surface integrity demonstrates that low surface roughness contributes positively in inhibiting crack initiation, but not the critical factor for enhancing the fatigue life. In addition, gradient grains has a relatively limited effect on enhancing fatigue life even though it can delay the crack propagation rate. In the fatigue enhancement mechanism of 300 M steel threads, the contributions from each factor and their competitive effects collectively impact the fatigue behavior. CRS may have a critical impact on enhancing fatigue life compared to surface roughness and gradient microstructure, but the synergistic effects of work-hardening, CRS field, and surface geometric state cannot be neglected.