Paraequilibrium Carburization of Duplex and Ferritic Stainless Steels

AISI 301 and E-BRITE stainless steels were subjected to low-temperature (743 K) carburization experiments using a commercial technology developed for carburization of 316 austenitic stainless steels. The AISI 301 steel contained similar to 40 vol pct ferrite before carburization but had a fully austenitic hardened case, similar to 20-mu m thick, and a surface carbon concentration of similar to 8 at. pct after treatment; this "colossal" paraequilibrium carbon supersaturation caused an increase in lattice parameter of similar to 3 pct. The E-BRITE also developed a hardened case, 12- to 18-mu m thick, but underwent a more modest (similar to 0.3 pct) increase in lattice parameter; the surface carbon concentration was similar to 10 at. pct. While the hardened case on the AISI 301 stainless steel appeared to be single-phase austenite, evidence for carbide formation was apparent in X-ray diffractometer (XRD) scans of the E-BRITE. Paraequilibrium phase diagrams were calculated for both AISI 301 and E-BRITE stainless steels using a CALPHAD compound energy-based interstitial solid solution model. In the low-temperature regime of interest, and based upon measured paraequilibrium carbon solubilities, more negative Cr-carbon interaction parameters for austenite than those in the current CALPHAD data base may be appropriate. A sensitivity analysis involving Cr-carbon interaction parameters for ferrite found a strong dependence of carbon solubility on relatively small changes in the magnitude of these parameters.