Internal-carburizing behavior of Ni-V, Ni-Cr, and Ni-3Nb alloys

Ni-2V, Ni-5V, Ni-12V, Ni-10Cr, Ni-20Cr, and Ni-3Nb alloys were carburized in 1.5 v/o C3H6 (bal. H-2) over the range 700-1000 degrees C. Carburization of Ni-SV, Ni-12V, Ni-20Cr, and Ni-3Nb obeyed the parabolic rate law. Ni-2V and Ni-10Cr, however, for med only thin carbide scales upon carburizing. Carburization rates decreased with increased vanadium content from Ni-5V to Ni-12V for all exposure conditions. V4C3 formed throughout the reaction zones of Ni-12V. Cr3C2 formed in the surface regions and Cr7C3 formed within the interior of Ni-20Cr. NbC precipitated in Ni-3Nb under all conditions. The precipitate morphology changed with temperature and distance from the gas-metal surface. V4C3 and Cr3C2 particles were generally small and spheroidal near the surface of Ni-12V and Ni-20Cr, respectively, increasing slightly in size with distance from the surface and with increasing temperature. The vanadium and chromium carbides formed intergranular networks toward the reaction fronts. The NbC precipitates were generally large and became Widmanstatten at increasing distance within the carburized zone of Ni-3Nb. Expressions for the diffusion coefficient of carbon in nickel from the measured permeabilities and carbon solubility data were determined. Solubility products were determined for all of the carbides formed and found to be large in comparison with the product of the activities of the precipitate elements. Wagner's theory of internal oxidation was shown to be an approximation to the carburization kinetics at temperatures of 900 degrees C or higher.