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Effect of Plasma Nitriding Treatment on Fatigue Life of En-24 steel

U. N. Puntambekar, G. S. Grewal, P. B. Joshi and P. Sampathkumaran

In this investigation plasma nitriding has been carried-out to enhance surface related properties of steel components. The hardened case in these components comprise of a compound (white) layer on top followed by a diffusion zone underneath it. The increase in hardness and strength at the surface, yield improvement in fatigue life. However, increase in thickness of the compound layer beyond a certain critical reduces the fatigue life of the base material. Hence, it is desirable to have close control on the process to limit the formation of excessively thick compound layer. The data related to increased fatigue life of various low alloy steels is available in the published literature, but the effect of thickness of compound layer on fatigue life could not be cited in literature. Hence, the present work focuses on this issue. This paper discusses the effect of plasma nitriding and the thickness of compound layer in particular, for the En-24 steel. The experiments were carried-out on steel rod samples which were plasma nitrided with formation of varying amount of thickness of compound white layer and maintaining diffusion layer depth the same. The characterization of samples was done in respect of chemical composition, microstructure & micro-hardness. The rotating bending fatigue tests on plasma nitrided samples were conducted at various stress levels to plot S-N curves. Based on the fatigue test results, modification factors for stress & life-cycles were established. The data indicate that the fatigue life of samples increases up to a certain thickness of compound layer and then decreases.