Numericalanalysis of low-temperature surface carburization for 316L austenitic stainless steel

Low-temperature surface carburization has proven to be one of the most effective techniques for improving the mechanical properties of 316-type austenitic stainless steel (Fe-Cr-Ni alloy), including surface hardness, fatigue resistance and wear resistance. It is well known that carbon diffusion in austenitic stainless steel is a very complicated process and still not fully understood. So it is of great importance to figure out the carbon diffusion mechanism in steel and establish a model that can predict the carbon concentration along the depth direction in any given carburization conditions. Studies in recent years reveal that trapping effect should be considered in carbon diffusion in austenitic steels at low temperature. In this work, low-temperature surface carburization treatment was carried out with 316L austenitic stainless steel, and the carbon concentration along the depth direction was measured. A kinetic model based on the "trapping-detrapping" mass transport mechanism for simulating the carbon fraction- depth profile was developed. This model considered that the diffusion of carbon under the influence of trap sites formed by local chromium atoms. Then the calculated carbon concentration was compared to the experimental results in order to check the validity of the model. The results show as follow: (1) in low-temperature-carburized 316L austenitic stainless steel, the carbon fraction-depth profile exhibits plateau-type shape which is not consistent with the standard analytic solution of the diffusion equation (Fick's law of diffusion); (2) carbon fractiondepth profile based on "trapping-detrapping" model is in good agreement with experimental carbon fraction-depth profile, which indicates the trapping effect plays an important role in carbon diffusion; (3) carbon diffusivity decreases by the trapping effect of Cr atoms, and the detrapping energy of carbon deduced from fitting experimental data is 165 kJ/mol; (4) the proposed model can only be used to describe the carbon diffusion in austenitic stainless steel during low-temperature surface carburization without chromium carbide precipitation. In addition, the influence of stresses induced by incorporating the carbon into austenite lattice on the carbon transport mechanism is not included in the trapping-detrapping model.