Carbon paraequilibrium in austenitic stainless steel

Carburization of austenitic stainless steels under paraequilibrium conditions—i.e., at (low) temperatures where there is essentially no substitutional diffusion—leads to a family of steels with remarkable properties: enhanced hardness, resulting in improved wear behavior, enhanced fatigue, and corrosion resistance, and with essentially no loss in ductility. These enhanced properties arise from an enormous carbon solubility, which, absent carbide formation, is orders of magnitude greater than the equilibrium solubility. Using interaction parameters from the latest CALPHAD assessment of the Fe-Cr-Ni-carbon system, the authors have calculated the equilibrium and paraequilibrium carbon solubility in a model Fe-18Cr-12 Ni (wt pct) austenitic steel (essentially a model 316L composition), as well as the carbon solubility in this austenite when paraequilibrium carbide formation occurs (i.e., when carbides form in a partitionless manner). For temperatures in the range 725 to 750 K, the calculations predict a paraequilibrium carbon solubility of ～5.5 at. pct. Carburization of 316L stainless steel at these temperatures, however, results in significantly higher concentrations of carbon in solid solution—up to 12 at. pct. Much better agreement with experimental data is obtained by calculating the paraequilibrium carbon solubility using Wagner interaction parameters, taken from the most comprehensive experimental study of this system. The discrepancy between the two predicted solubilities arises because the CALPHAD Cr-carbon interaction parameters are not sufficiently exothermic at the low temperatures used for paraequilibrium carburization. After multiple paraequilibrium carburization cycles, carbide formation can occur. The carbides that form under these conditions do so in a near-partitionless manner (there is modest Ni rejection to the austenite/carbide interface) and have an unusual stoichiometry: M₅C₂ (the Hägg or η carbide).