Effect of ultrasonic nanocrystal surface modification on the fatigue behaviors of plasma-nitrided S45C steel

► UNSM was applied to plasma nitrided S45C specimens to enhance fatigue properties. ► Smaller grain size can be obtained on 48 h specimens' surface than 8 h. ► Decline of surface residual stress as increasing strike number can be observed. ► UNSM improved fatigue limit of nitriding 8 h specimens and no impact on 48 h. ► Fish-eye position was deepened for 48 h specimens and no fish-eye crack for 8 h.