Carburization resistance of nickel-base, heat-resisting alloys

Cast heat-resisting alloys containing 45 or 60 wt% nickel were isothermally carburized in flowing gas mixtures of H-2-5CH(4) (volume percent) at temperatures of 900-1,100 degrees C, and their performance compared with that of standard commercial grades containing 30-35% nickel. Chromium-rich M7C3 and M23C6 precipitated internally in all materials, to depths which increased according to parabolic kinetics. Comparison of the rate constants with those predicted from Wagner's diffusion theory showed that carbon diffusion through a chromium-depleted alloy matrix was the rate-controlling process. The 45% nickel alloys carburized more slowly than the 30-35% nickel grades as a result of decreased carbon solubility and diffusivity at the higher nickel level. The 60% nickel alloys also contained aluminum. At higher temperatures, diffusion of aluminum to the surface led to A1(2)O(3) scale formation and enhanced carburization resistance. The degree of protection obtained depended on alloy aluminum content.