Effect of phase in surface layers on rotating-bending fatigue strength of SCM415 steel after austenitic nitriding

Abstract

The rotating bending fatigue strength of JIS-SCM415 steels after austenitic nitriding was evaluated to find the surface phase that gave the highest strength. Nitriding was performed at 903 K under conditions of KN = 0.15, 0.25 and 0.35 atm-1/2. In addition, specimens which were tempered at 523 K for 7.2 ks after nitridings with KN = 0.15 and 0.25 atm-1/2 were prepared. A specimen prepared by nitrocarburizing at 853 K was also evaluated as a comparison. Specimens nitrided at 903 K at KN = 0.15 atm-1/2 showed the highest fatigue limit because the 10 μm thick surface layer was a dual phase of austenite and martensite that might have a fine microstructure which inhibited initial crack generation and growth. Specimens nitrided at 903 K at KN = 0.35 atm-1/2 showed the second highest fatigue limit because the ε compound layer that was 20 μm thick had a high compressive residual stress. Specimens nitrides at 903 K at KN = 0.25 atm-1/2 showed the third highest fatigue limit because of the formation of a 6 μm thick compound layer which was a dual phase of ε and γ’ that had few voids and a high toughness (low hardness) in addition to its compressive residual stress. Tempered specimens showed a lower fatigue limit than that of non-tempered specimens. This was because the surface layer became brittle in tempered specimens. Specimens other than the tempered specimen in KN = 0.25 atm-1/2showed a fatigue strength higher than that of a sample nitrocarburized at 853 K.