To analyze the effect of the surface hardening and residual stress on the fatigue strength and behavior of crack nucleation, fatigue test were performed on SACM 645 steels nitrided using the Nitemper method. The gas nitriding was performed for 9 hours at 540°C and 8 hours at 570°C to obtain different depths of hardening, and some of the specimens were subjected to stress relief (SR) treatment to clarify effect of residual stress on the fatigue characteristics. After nitriding, the fatigue strength of specimens was 40% higher than that of base materials. It was also found that the increment in the fatigue characteristics was due to the compressive residual stresses that formed on the surface of specimens and produced subsurface crack nucleation during fatigue.
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Hwang et al. (2003) studied this question.
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