Fatigue life evaluation of 42CrMo4 nitrided steel by local approach: Equivalent strain-life-time

In this paper, the fatigue resistance of 42CrMo4 steel in his untreated and nitrided state was evaluated, using both experimental and numerical approaches. The experimental assessment was conducted using three points fatigue flexion tests on notched specimens at R = 0.1. Microstructure analysis, micro-Vickers hardness test, and scanning electron microscope observation were carried out for evaluating experiments. In results, the fatigue cracks of nitrided specimens were initiated at the surface. The fatigue life of nitrided specimens was prolonged compared to that of the untreated. The numerical method used in this study to predict the nucleation fatigue life was developed on the basis of a local approach, which took into account the applied stresses and stabilized residual stresses during the cyclic loading and the low cyclic fatigue characteristics. The propagation fatigue life was calculated using fracture mechanics concepts. It was found that the numerical results were well correlated with the experimental ones.

► Ion nitriding treatment of 42CrMo4 steel improves their fatigue strength by 32% as compared with the untreated state.
► This improvement is the result of the beneficial effects of the superficial work- hardening and of the stabilized compressive residual stress.
► The notch region is found to be the fatigue crack nucleation site resulting from a stress concentration (Kt = 1.6).
► The local equivalent strain-fatigue life method was found to be an interesting predictive fatigue life method for nitrided parts.