Carburization of stainless steels at low temperatures, 450-475oC, occurs under paraequilibrium conditions i.e. at temperatures where substitutional solutes are effectively immobile but where interstitial solutes have reasonable mobilities, Under these conditions, the low-cost conformal atmospheric-pressure carburization process under study produces cases with extraordinary hardness, 1200-1400 HV, without formation of carbides and with no loss of ductility. The maximum carbon level in solid solution in a 316L austenitic steel is ~ 15 at%, as has been demonstrated by 3D atom probe field ion microscopy. The increased hardness leads to major improvements in wear resistance in finished components. Given that the hardened case is constrained by the uncarburized core, the large lattice expansion associated with the "colossal" supersaturation of carbon interstitials leads to surface compressive stresses >2 GPa, which greatly enhances the fatigue resistance. Finally, and most surprisingly, the good corrosion resistance of these corrosion-resistant materials is significantly enhanced.
While the process has been developed for a conventional 316L austenitic stainless steel, we have extended it to superaustenitic stainless steels, precipitation-hardened martensitic stainless steels, duplex and ferritic stainless steels, and Ni- and Co-base alloys.
