Investigation of the Combined Effects of Stress Concentrations and Plasma Nitriding Parameters on the Fatigue Performance of AISI 4140 Low Alloy Steel

Abstract

The fatigue performance of machine parts under cyclic loads is significantly reduced by notches, cracks and geometric irregularities on their surfaces. Therefore, it is necessary to eliminate the notch effect, which is effective on the formation of fatigue cracks. In this study, it is aimed to enhance the fatigue strength of notched parts via plasma nitriding. Four various radii (r = 1, 2, 4 and 8 mm) variations were created from AISI 4140 steel with theoretical stress concentration factors (Kt) of 1.63, 1.41, 1.27 and 1.19, respectively. Then, the samples were plasma nitrided at 480 °C for 2 h and 9 h. The morphological, structural and mechanical properties of the samples were characterized by SEM, XRD and microhardness tester. The fatigue tests were performed using a rotating bending fatigue test device. All plasma nitrided parts exhibited higher fatigue strength compared to untreated samples and the level of this increase decreased with increasing Kt. The fatigue properties of plasma nitrided parts improved in comparison to untreated parts, depending on the residual stresses, microhardness and diffusion layer thickness. Consequently, it was found that the fatigue limit improvement up to 72% was achieved in the samples with the lowest Kt and nitrided for long periods. Graphical Abstract: (Figure presented.) © ASM International 2024.