Carbon supersaturation due to paraequilibrium carburization: Stainless steels with greatly improved mechanical properties

Low-temperature gas-phase carburization has been used to generate very high surface interstitial carbon contents, up to ∼12 at.%, in a 316L austenitic stainless steel. The high interstitial content leads to substantial surface hardening (Vickers hardness of ∼12 GPa, equivalent to Rockwell C of ∼71.5) with essentially no loss of ductility and with no carbide formation. Residual compressive stresses accompanying the low-temperature carburization enhance the high-cycle fatigue resistance, while the hardening enhances the wear resistance. These remarkable improvements in mechanical properties arise because the carburization is carried out at temperatures where “paraequilibrium”, rather than conventional thermodynamic equilibrium, determines the phase composition; paraequilibrium can be realized under conditions where substitutional solutes such as Cr and Ni are immobile whereas interstitial solutes such as carbon are not.