On the influence of residual stresses in determining the micro-yield stress profile in a nitrided steel by nano-indentation

A new method has recently been developed for obtaining the local micro-plasticity properties of materials from nano-indentation measurements. This method is based upon the measurement and analysis of remanent displacement er versus the maximum load W, produced during successive nano-indentation loading-unloading cycles at an increasing W. The present paper is concerned with the influence of residual stresses in determining the local micro-yield stress using this method. First, the influence of residual stresses on er is evidenced experimentally on nitrided 32CrMoV13 samples by studying the effect of the redistribution of residual stress that occurs when the initial component is sectioned in different ways. Second, the actual residual stress at any measurement point is calculated for every sample configuration. This is achieved by identifying the volume dilatation strain profile in the nitrided layer. Third, the influence of residual stresses on er is simulated using a three-dimensional elasto-plastic contact code. In this way, we obtain the remanent displacement that would be measured if the material were free of residual stress. We then propose a new method for obtaining the local micro-yield stress in the presence of residual stresses. This method is then applied to the determination of the intrinsic micro-yield stress profile of nitrided 32CrMoV13 steel. The results show that in some specific configurations the micro-yield stress may be overestimated by some 20%.