Plain fatigue and fretting fatigue behaviors of 17-4PH steel subjected to ultrasonic surface rolling process: A comparative study

The effect of ultrasonic surface rolling process (USRP) on surface integrity, plain fatigue (PF) and fretting fatigue (FF) behaviors of 17-4PH steel was contrastively studied, and their internal mechanisms were discussed. The results indicated that USRP processing times had a significant influence on the PF and FF properties of 17-4PH steel. Improved PF life resulting from USRP was much higher than that of FF life due to the differences of PF and FF mechanisms. When the processing times were three cycles (USRP-3), its PF life or/and PF limit was the highest; while six subsequent USRP processing times (USRP-6) led to the maximum FF life. Gradient microstructure, large residual compressive stress and high microhardness were introduced by USRP technology, which contributed to improve PF and FF lives of 17-4PH steel. Additionally, the relatively smooth surface (Ra 0.116 mu m) and nanocrystalline structure layer of USRP-3 sample contributed to inhibit crack initiation and propagation, while surface fish-like scale or folded micro-topography (Ra 0.618 mu m) helped to relieve fretting damage for USRP-6 sample.