Experimental investigation on the initiation and propagation of corrosion fatigue cracks in EA4T axle steel treated by ultrasonic surface rolling

The effect of ultrasonic surface rolling process (USRP) on the growth of corrosion pits and the propagation of cracks during the corrosion fatigue (CF) process in EA4T axle steel are investigated in this study. The surface morphology, microstructure, microhardness, residual stress, high-cycle fatigue, crack growth rate, and surface damage evolution of the specimens were analyzed experimentally. The results indicate that USRP treatment reduced the surface roughness of the specimens and formed a work-hardened layer with a refined microstructure, higher microhardness, and compressive residual stress (CRS). According to the results of rotating bending fatigue tests, the air fatigue and CF performance of USRP-treated specimens were significantly improved compared to machined specimens. The corrosion mechanism of EA4T axle steel was identified as anodic preferential dissolution, leading to the formation of corrosion pits. The USRP treatment significantly increased the stress concentration factor at the corrosion pits, reducing crack initiation life. However, the introduction of CRS and grain refinement by USRP treatment led to a significant increase in crack propagation stage life, which is the fundamental reason for the enhanced CF life of EA4T axle steel.