High temperature oxidation behavior of chromium-rich alloys containing high carbides fractions. Part I: Nickel-base alloys

Six wear-resistant alloys based on nickel, containing 30wt.% Cr and from 2.5 to 5.0wt.% C, were elaborated by foundry and subjected to oxidation by air at 1000, 1100, and 1200 degrees C, for evaluating the oxidation behavior of hard bulk alloys. Their microstructures are rich in chromium carbides, eutectic, or pro-eutectic, and they also contain graphite for the highest carbon contents of interest. All the studied alloys obviously display a chromia-forming behavior, despite the initial low chromium content in the matrix. During oxidation, carbides disappear over an increasing distance from the oxidation front, with the consequence of the enrichment of the neighbor matrix in chromium. The minimal chromium content on the surface after oxidation decreases when the alloy is richer in carbon and increases with the oxidation temperature. The carbide-free zone tends to be deeper when the oxidation temperature increases, and also when the alloy's carbon content increases, in contrast with low-C Ni-30Cr alloys previously studied. The disappearance of carbides means a carbon loss as gaseous oxidized species. This probably disturbs the oxide scales growing on the external surface and may influence the oxidation behavior of the alloys. When present, graphite does not deteriorate dramatically the oxidation resistance of the alloys. The hardness of the alloys are lowered by the exposures to high temperature and by the presence of graphite.