The Effect of the Transformation of ε-Fe2-3N into γ′-Fe4N Phase on the Fatigue Strength of Gas-Nitrided Pure Iron

The effect of the iron nitride phases, epsilon-Fe2-3N and gamma '-Fe4N, on the fatigue strength was investigated. Pure iron was used to observe only the effect of nitride, excluding the effects of factors, such as residual stress, depending on the alloy composition and microstructural change according to working on the fatigue strength. In this work, epsilon and gamma ' phases were respectively grown at a time on the surface of the pure iron specimens using the appropriate nitriding potential K-N, the mixture rates of ammonia and hydrogen gases, at same temperature of 570 degrees C according to the Fe-N Lehrer diagram. Another gamma ' phase was prepared by first growing the epsilon phase and then transformed from epsilon phase into gamma ' phase by changing the K(N)at the same temperature of 570 degrees C in the 2-stage gas nitriding. The fatigue strengths of the iron nitride consisted of epsilon and gamma ' phases, gamma ' phase, and gamma ' phase grown by the 2-stage gas nitriding were evaluated, respectively. As a result, first, it can be seen that the diffusion layer of epsilon phase was deeper than gamma ' phase, but fatigue strength was lower. On the other hand, fatigue strength of both the gamma ' phases are higher than that of the epsilon, and the fatigue strength of gamma ' phase nitride grown by 2-stage gas nitriding was almost similar to that of gamma ' phase nitride grown at a time, i.e., fatigue strength was not significantly related to diffusion depth and depended on nitride phases in this study. Secondly, we cannot clearly conclude that there was the difference in fatigue strength according to the thickness of nitride layer consisted of gamma ' phase. However, it is clear that when epsilon phase was transformed to gamma ' phase, fatigue strength had the same level as gamma ' phase formed at one time.