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A Study of Cobalt-Based Superalloys for High-Temperature Glass Vitrification Applications



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Moffat, James 


The contents of this thesis are concerned with the industrial manufacturing process of glass mineral wool insulation. The process converts the molten glass into a wool product by a spinning the glass at high temperatures using a critical spinner component. The component, manufactured from the cobalt-based superalloy RM4, is exposed to a complex environment of stresses, temperatures and oxidising/corrosive substances leading to its rapid degradation. The work in this thesis seeks to understand the individual contributions each of these environmental factors have on the microstructure and physical properties of RM4, whilst proposing additional processing steps to improve the performance of the component. The results from this thesis show that the microstructure of the RM4 spinner component contains a continuous skeletal network of chromium-based carbide phases throughout the microstructure of the alloy. The carbide network was vulnerable to selective oxidation and corrosion processes, critically leading to severe internal oxidation and chromium depletion by a volatilisation mechanism. To reduce the continuity of the carbide network, and thereby protecting the alloy from excess chromium depletion, a solution heat treatment regime was applied to redistribute the carbide network. Although these heat treatments proved unsuccessful in increasing the oxidation resistance of RM4, the heat treatment regime was effective at reducing the internal oxidation experienced in the Stellite-21 alloy. This was principally due to the lower carbon content of Stellite-21, avoiding the supersaturation issues RM4’s matrix phase exhibits during the heat treatment. However, the two-step heat treatment regime was found to produce a more stable distribution of precipitated carbides at high temperatures in the RM4 alloy. In large-scale spinner trials performed by Knauf Insulation, the heat-treated spinners were found to possess a superior creep resistance compared to the as-cast spinners. Consequently, Knauf Insulation are now in the preliminary steps of incorporating this heat treatment to the spinner manufacturing process, potentially increasing the service life, and thus the yield of the glass mineral wool produced. Investigations into the effects of refractory element additions into cobalt-based superalloys was performed on the Mar M-509 alloy, where it was found that higher concentrations of tantalum and titanium favoured the formation of script-like MC carbides. Since stable script-like carbides have been associated with enhanced creep resistance, these results are encouraging for the development of creep resistant and oxidation resistant cobalt-based superalloys for glass mineral wool production applications. The final package of work within this thesis focuses on the effects of trace amounts of antimony contamination on the mechanical and creep properties of RM4. At high concentrations, antimony was found to decrease the creep life of the alloy, particularly at higher stresses, although the specific mechanism leading to this degradation remains elusive. However, atom probe data has suggested that additional deleterious elements that are commonly found in the processing environment, particularly phosphorus, may segregate to the grain boundaries during service exposure, embrittling the alloy and accelerating creep.





Stone, Howard


Materials Science, Metallurgy, Oxidation, Corrosion, Cobalt-based Superalloy


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge