Effect of corrosion on the fatigue damage behavior of M50 steel treated by ultrasonic surface rolling process

Ultrasonic surface rolling process (USRP) can improve the fatigue performance of M50 steel, but corrosion during service may affect the anti-fatigue performance. Microstructure and corrosion resistance of M50 steel treated by USRP is analyzed by field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and electrochemical impedance spectroscopy (EIS). Besides, the fatigue behavior of USRP sample under corrosion is analyzed by conducting fatigue tests after immersion corrosion tests. Results show that USRP treatment refines the surface grains to nanoscale (29.8 nm), introduces a certain residual compressive stress and decrease the precipitations of M50 steel, which improve the density of passive film and the resistance to local corrosion for M50 steel. Compared to M50 steel matrix, USRP sample has fewer and smaller corrosion pits at the same corrosion time, thereby reducing the risk of crack initiation. Meanwhile, the residual compressive stress value of USRP sample remains at a higher level. So the fatigue life of USRP sample is always higher than that of untreated sample under the same corrosion time. However, the corrosion process still leads to the destruction of surface integrity and the release of residual compressive stress, and the fatigue performance of USRP sample will gradually decrease with increasing the corrosion time.