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Microstructure and hardness of a low-chromiumcobalt-based alloy reinforced by tantalumcarbides destined to chromiumpack-cementation

Author(s): Patrice Berthod, Elodie Conrath

Lowchromiumcontents allow obtaining improved refractoriness in alloys, the chromia-forming behaviour of which being kept thanks to later Cr packcementation treatment on surface. The purpose of this work is to explore the microstructure of a {tantalumcarbides}-reinforced cobalt-based alloys the bulk chromium content of which is particularly low to maximize its refractoriness and then its potential of creep-resistance. Thermodynamic calculations were first performed to study the theoretic microstructures at high temperature and to assess the gain in solidus temperature which can be expected by lowering the chromiumcontent.Areal alloywith 0.3 wt.%C, 5 wt.%Ta only 5 wt.%Cr was then elaborated by induction foundry in inert atmosphere, for examining its microstructure and to evaluating its machinability by measuring its macro- and micro-hardness. The as-cast microstructure shows a double-phased state: matrix and eutectic tantalum carbides, the later ones having kept a script-like morphology known to be favourable to high strength at high temperature. A rather high level of hardness is also kept, but a little decreased by comparison with same earlier studied cobalt alloys with 30 wt.%Cr.No chromiumcarbides able to perturb the pack-cementation were seen in the microstructure.

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