Protection of cobalt-based refractory alloys by chromium deposition on surface: Part I: Sub-surface enrichment in chromium by pack-cementation and diffusion |
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Authors: | Grégory MichelPatrice Berthod Michel VilasiStéphane Mathieu Pierre Steinmetz |
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Affiliation: | Institut Jean Lamour, Department Chemistry and Physics of Solids and Surfaces, Team 206, Faculty of Sciences and Technologies, University Henri Poincaré Nancy 1, B.P. 70239, 54506 Vandoeuvre-lès-Nancy, France |
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Abstract: | The feasibility of surface chromium enrichment by pack-cementation was assessed for different low chromium-containing cobalt alloys, in order to improve their resistance against high temperature oxidation. A binary Co-10Cr alloy, two ternary Co-10Cr-0.5C and Co-10Cr-1.0C alloys and two TaC-containing Co-10Cr-based alloys were elaborated by foundry for the study. 7.5 h-long and 15 h-long cementations at 1050 °C, followed or not by a 75 h-long heat treatment at 1200 °C were performed on these alloys. Microstructure examinations performed using a Scanning Electron Microscope and concentration profiles using Electron Probe Micro Analysis-Wavelength Dispersion Spectrometry were realized in order to analyze the level of Cr-enrichment of the sub-surface region, with as studied criteria: the nature of the external Cr-enriched zone, the maximal chromium content on surface and the depth of chromium enrichment. The Cr-enrichment of the sub-surface succeeded for the Co-10Cr alloy and for the two tantalum-containing alloys, with the formation of an external metallic zone containing around 30 wt.% Cr. In contrast the chromium carbides-containing alloys were effectively enriched in chromium in surface but in the form of a continuous chromium carbide layer which can induce other problems such as spallation and then possible fast oxidation of the denuded alloy. Finally it appeared that only the carbon-free alloys, and the alloys reinforced by carbides more stable than chromium carbides, are potentially able to be successful enriched in chromium in their sub-surface by pack-cementation. |
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Keywords: | Cobalt-based alloys Pack-cementation Chromium Carbides |
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