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High temperature oxidation behaviour of chromium-rich cobalt-based alloys containing high carbides fractions

Patrice Berthod

Six wear-resistant {30wt.%Cr, 2.5 to 5.0wt.%C} - containing cobalt-based alloyswere cast and tested in oxidation by air at 1000, 1100 and 1200°C. The fractions of chromium carbides are especially high and graphite is present but only in the carbon-richest versions, in small quantities. Chromia always exists on the oxidized surfaces, but scales of other oxides, CoO and CoCr2O4, also appeared. Carbides have disappeared from surface over a depth increasing with the alloyÂ’s carbon content and with temperature. The carbide- free depths are higher and the chromiumcontents on surface are lower for these cobalt alloys than previously observed for similar nickel alloys with the same Cr and C contents. The quantities of chromiumand of carbon lost by the oxidized hard cobalt alloys are especially high. All the results show that the behaviours of these cobalt alloys in high temperature oxidation are worse than for the analogous nickel alloys: not really chromiaforming behaviour, faster oxidation, even catastrophic in some cases. The bulk microstructures were either not changed by the high temperature exposures, or onlymoderately at 1200°C. The hardness of the cobalt alloys is lowered by the exposures to high temperature but remains above the hardness of the similar hard nickel base alloys.