EVALUATION OF THE RESIDUAL STRESSES IN A LOW TEMPERATURE CARBURIZED STAINLESS STEEL BY THE MICROHARDNESS MEASUREMENT

Many attempts were made in recent years to engineer the surface of austenitic stainless steels to improve their hardness and tribological properties, without deteriorating their corrosion resistance, with the secondary effect of improving the fatigue properties of those materials. Low temperature carburizing (LTC) treatment improves surface hardness and wear resistance of the austenitic stainless steels without reducing their corrosion resistance. Surface hardness over 1000 Vickers and compressive residual stresses whose modulus exceeds 1500 MPa are usually achieved in the carburized layer, having a thickness of some tens of microns, thanks to the formation of the so-called “S-phase”, a carbon-supersaturated austenite phase. Usually these quantities are measured independently by means of micro-hardness tests and X-ray diffraction, but since the mechanism that induces such high values of hardness and residual stresses is austenitic lattice expansion due to the carbon diffusion, without any other hardening mechanism, the micro-hardness increase should be a function of residual stresses only. In this paper the results obtained by two numerical models developed by the Ansys code are shown. The first model was used to calculate the residual stresses due to carbon supersaturation by means of a thermomechanical analogy, while the second to simulate the micro-hardness test on the material with the residual stresses field. By joining the two methods it was possible to evaluate the distribution of the residual stresses with the final aim to take them into account for the fatigue life prediction of specimens and components.