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High Temperature Oxidation Behavior of Cast Cobalt-Based Superalloys versus the Nature of their Reinforcing Carbides

Patrice Berthod
Elodie Conrath
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Résumé

In some cases the compulsory resistance against high temperature corrosion by molten substances leads to choose chromia-forming alloys instead gamma/gamma’ nickel-based superalloys. Cast cobalt-based superalloys represent then an interesting alternative solution since they may benefit from hot oxidation and corrosion resistance as well as high mechanical resistance at elevated temperature. Some of them are even able to be used at temperatures as high as 1200°C, thanks to their interdendritic network of very refractory MC-type carbides. Indeed the presence of this type of mechanical reinforcement does not limit the content in chromium and, on the other hand, the script-like morphology of many of these MC carbides is particularly efficient for combatting creep. This is the case of the NbC, TaC and HfC, provided that these carbides are present in quantity high enough to really bring significant strength. For that carbon contents as high as 0.5 wt.% and M present with the same atomic content as carbon are required. But one can fear that the rather high resulting mass contents (from 3.9 wt.% for niobium to 7.5 wt.% for Ta, for instance) may significantly change the initial high temperature oxidation behavior of the alloy’s base. In this work three {Co-25wt.%Cr-0.5wt.%C}-based alloys containing 3.9 wt.% Nb, 7.5 wt.% Ta or 7.4 wt.% Hf, contents respecting the atomic equivalence between C and the M element, condition for promoting the formation of MC carbides exclusively. These alloys were elaborated by high frequency induction melting under inert atmospheres and submitted to thermogravimetry oxidation tests at 1200°C to specify their mass gain kinetics at this level of temperature of use to which they are devoted. Nb and Hf, elements able to form MC carbides well-known for their efficiency in combating creep at elevated temperatures induced obviously very low resistance against high temperature oxidation. If still considered as strengthening principle the use of these carbides supposes significant improvement in behavior of the obtained alloys: optimized chemical composition (as enrichment in chromium beyond 30 wt.% or use of protective coatings, for example). Currently, despite their moderate resistance to volume fraction lowering and to fragmentation, the carbides issued from tantalum appear as the best compromise between mechanical resistance and oxidation resistance at working temperatures as high as 1200°C.
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hal-03337098 , version 1 (07-09-2021)

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Mélissa Ritouet, Patrice Berthod, Elodie Conrath. High Temperature Oxidation Behavior of Cast Cobalt-Based Superalloys versus the Nature of their Reinforcing Carbides. 9th International Symposium on High-Temperature Corrosion and Protection of Materials (HTCPM 2016) – May 20-25, 2016 - Les Embiez (France), May 2016, Les Embiez, France. ⟨hal-03337098⟩
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