From microhardness to fatigue life: a review of predictive approaches for surface-hardened mechanical components

This review paper aims to systematically examine the effects that the most common thermal and thermochemical surface hardening treatments - specifically surface hardening, carburising, and nitriding - have on the fatigue strength of mechanical components, which has historically been the primary cause of failure during service. Following a brief description of the heat treatments and the mechanical and metallurgical effects they produce on components, a predictive model of the fatigue strength of surface-hardened mechanical components subjected to bending stresses is presented, using the local fatigue limit approach. The parameters used in the model are the microhardness profile of the hardened zone, the distribution of residual stresses in the hardened layer, the fatigue strength of the base material, and the stress state induced by external forces. The proposed model was validated for three real-world cases: surface hardened specimen with a notch, smooth carburised specimen and smooth nitrided specimen. The results provide important insights from both an academic and industrial perspective. In fact, they allow for the determination of which parameters must be controlled to increase the fatigue strength of surface-treated mechanical components.