Hardness profile analysis of elasto-plastic heat-treated steels with a gradient in yield strength

An elasto-plastic indentation study on materials with a yield strength gradient like steels which have undergone thermal hardening such as nitriding, is carried out using experimental and finite element methods. The analysis of the normalized mean contact pressure as a function of a dimensionless strain parameter for graded materials shows that the mean contact pressure presents the same tendency as equivalent homogeneous materials. A simple model for the average plastic zone radius of graded materials is given. A relation is developed to predict the hardness variation as a function of the indentation depth. Lastly, a theory based on the effective evolution of hardness is proposed to determine the yield stress and hardness profile for materials with a decreasing yield stress with depth. Results for carbo-nitriding steels obtained by the standard Vickers micro hardness techniques are compared with those obtained by the proposed method.