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

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 K-N = 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 K-N = 0.15 atm(-1/2) showed the highest fatigue limit because the 10 m 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 K-N = 0.35 atm(-1/2) showed the second highest fatigue limit because the e compound layer that was 20 m thick had a high compressive residual stress. Specimens nitrides at 903 K at K-N = 0.25 atm(-1/2) showed the third highest fatigue limit because of the formation of a 6 mu m thick compound layer which was a dual phase of e and y' 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 K-N = 0.25 atm(-1/2) showed a fatigue strength higher than that of a sample nitrocarburized at 853 K. (C) 2020 The Author. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.orgilicenses/by-nc-nd/4.0/).