Enhancing Surface Integrity and Fatigue Performance of 42CrMo Rolled Threads Through Localized Root Rolling Strengthening

To improve the fatigue life of high-strength bolts, this study builds upon conventional thread rolling by introducing a localized rolling reinforcement process specifically at the thread root. Experimental specimens were prepared from 42CrMo high-strength bolts using a combined manufacturing technique that integrates thread forming and root rolling. A comparative analysis was conducted to evaluate the fatigue performance of bolts with and without the root rolling reinforcement. The experimental results demonstrated that the thread root rolling treatment further refines the surface grains beyond the effects of standard thread rolling. At a rolling force of 2.5 kN, the surface microhardness increased from the original 500 HV0.2 to 540 HV0.2. The process also improved surface finish, reduced grain size, and increased dislocation density. The optimal enhancement was achieved at a rolling force of 3.5 kN, resulting in an approximately 11-fold improvement in fatigue life. Fractographic analysis via Scanning Electron Microscopy (SEM) indicated a reduced number of crack initiation sites. This study confirms the effectiveness of the proposed rolling reinforcement process, offering a viable technical pathway for optimizing the anti-fatigue manufacturing of high-strength bolts.