Correlation of Effect of Plasma-Nitridingon Fatigue Life of Low-Alloy Steel with Surface Stresses Estimated using FEM Analysis

:Surface treatments affect fatigue life of rotating components due to stresses induced on surface. The fatigue life can be correlated with the stresses induced. However, to measure stresses in actual practice is time-consuming as well as costly process. Alternatively, FEA studies are carried-out to determine the stresses induced. 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 leads to 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. This paper discussesthe effect of thickness of white (compound layer) on fatigue life of component. The stresses induced due to plasma nitriding were determined by Finite Element Analysis (FEA)on the plasma nitrided samples with respect to thickness of compound layer for the En-24 steel. The rotating bending fatigue test on plasma nitrided samples was conducted at various stress levels to plot S-N curves. The experimental data indicates that the fatigue life of samples increases up to a certain thickness of compound layer and then decreases. This finding has been correlated with the stresses induced using FEA.