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Influence of the chromium content on the as-cast microstructures of ternary alloys very rich in carbon part I: Cobalt-based alloys with 3, 4 or 5 wt.%C

Patrice Berthod, Ophelie Hestin, Elise Souaillat, Pascal Villeger

Three ternary alloys based on cobalt and containing chromium(12, 15.5 and 23wt.% Cr) and very high amounts of carbon (3, 4 and 5wt.% C) were first subjects of preliminary thermodynamic calculations, and second they were really elaborated by foundry under inert atmosphere. The obtained ingots weremetallographically prepared then characterized by electronmicroscopy, X-ray diffraction and surface fractions measurements. The calculations and metallographic results were compared to the ones previously obtained for ternary cobalt alloys with the same carbon contents but all containing 30wt.%Cr. In some case the position of the new alloys with regard to the eutectic composition is modified consecutively to the change in chromium content. Theirmicrostructures showed especially high quantities of lamellar graphite, obviously formed during solidification, together with the expected high fractions of carbides. As the presence of such graphite was not in conform with the first calculated results, new calculations were performed after having forbidden the appearance of cementite. The new calculated results were much closer to the microstructures experimentally obtained. Furthermore there was a rather good agreement between the predicted mass fractions of carbides and graphite and the ones deduced from the measured surface fractions of these phases in the real alloys.