Effect of nitriding on low-cycle fatigue properties

Nitriding is a thermochemical surface treatment that is often utilized to improve the fatigue strength of mechanical elements. In fact it is well known that, after the treatment, owing to the formation of a nitrided external layer and a field of residual stresses that are compressive on the surface, the high-cycle strength is strongly increased. Treatment characteristics include temperature and duration, whose values are generally decided by the operator. In this study, experimental axial tests were conducted on smooth specimen groups, which were each treated by using different durations. The results show the strong influence of treatment time on the elastic modulus values: in particular, if the duration increases, the elastic modulus increases too. A simple model allowed us to evaluate the distribution of the stresses in the case and in the core, and it was found that by increasing the treatment time the external layer is more stressed with respect to the core. Low-cycle fatigue was investigated, and the epsilon(a)-N-f curves obtained by means of the differently treated specimens were compared. The results show that increasing the duration decreases the fatigue behaviour for high applied strains, and only one overload may be sufficient to fracture the external layer. Copyright (C) 1997 Elsevier Science Limited.