Influence of ion nitriding on fatigue strength of low-alloy (42CrMo4) steel: Experimental characterization and predictive approach

In this research the structure and fatigue properties of ion nitrided 42CrMo4 steel were evaluated and compared with a quenched and tempered structure. The fatigue limit were obtained by means of three-point bending fatigue tests for stress ratio (R = 0.1). The micro-hardness profiles were measured and used to demonstrate the case depth. The structure of surface layers and diffusion zone were examined by optical and scanning electron microscopy (SEM) and the in-depth residual stresses have been determined by the hole drilling technique. The improvement fatigue strength at 106 cycles was 35%. Explanation of the improvement was based on stabilized gradients of properties in treated layer occurred after cyclic relaxation. It results in a redistribution of hardness and compressive residual stresses that are favourable to the fatigue life. The high over-load sensitivity of the compound layer constitutes a serious limitation of the nitriding treatment in the low cycle fatigue domain. Based on multi-axial HCF criterion of Crossland and taking into account the different surface properties, which are: compressive residual stresses and superficial work-hardening, a local predictive approach was developed. Good agreement has been observed between the predicted results and those obtained from experimental investigations.