Fatigue behaviors of 40Cr steel bars after nitriding-ultrasonic impacting compound strengthening treatment

In this study, 40Cr steel specimens were subjected to a combination of nitriding and ultrasonic impacting compound strengthening treatments under different impact forces (50, 70, and 90 kg). Consequently, the fatigue life and failure mechanisms of the treated specimens were investigated. The results indicate that the fatigue life of nitride subjected specimens improved compared with the specimens without the nitride treatment. Further, the fatigue life of nitride specimens following the ultrasonic impacting treatment first increased and then decreased with increasing the impact force. The nitriding-ultrasonic impacting compound strengthened specimens with the impact force of 50 kg exhibited the highest fatigue life. In addition, a critical impact force was obtained within the range of 70-90 kg. Fatigue cracks were initiated on the surface for an impact force of less than 70 kg, and they were initiated around the inclusion of Al2O3-CaCO3 on the fracture as the impact force was greater than 90 kg. Finally, the fatigue crack initiation mechanisms of the nitriding-ultrasonic impacting compound strengthened specimens were elucidated in terms of the surface crack, surface roughness, hardness, and residual stress. This study is expected to provide new clues for the optimization of surface strengthening processes of mechanical workpieces.