Desulfurization Model Using Solid CaO in Molten Ni-Base Superalloys Containing Al
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UCTION
NI-BASE superalloys are heat-resistant materials commonly used for hot sections of aero engines and power-generating gas turbines. Improvement of their heat-resistance properties leads to a better thermal efficiency of the gas turbines.[1] Particularly, the heat resistance of the Ni-base single-crystal superalloys has been improved by adding rare metals such as Re, Ta, W,
YUKI KISHIMOTO and TAKETO IGUCHI are with the Waseda University: 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan and also with the National Institute for Materials Science (NIMS): 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan. Contact e-mail: [email protected] SATOSHI UTADA is with the Waseda University, and with the National Institute for Materials Science (NIMS), and also with the Institute Pprime and SAFRAN Aircraft Engines: Te´le´port 2, 1 Avenue Cle´ment Ader, 86360 Chasseneuil-du-Poitou, France. YUHI MORI is with the Waseda University, and also with the National Institute for Materials Science (NIMS). MAKOTO OSAWA, TADAHARU YOKOKAWA, TOSHIHARU KOBAYASHI, KYOKO KAWAGISHI, and HIROSHI HARADA are with the National Institute for Materials Science (NIMS). SHINSUKE SUZUKI is with the Waseda University. Toshiharu Kobayashi—deceased on January 27, 2019. Manuscript submitted April 3, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
and Ru.[1–4] However, the rising content of the rare metals is causing an increase in the material cost and an unstable supply system. Several methods have been studied to solve the cost and supply issues, for instance, remelting of the foundry scrap[5] and chemical extraction of some elements.[6] An effective solution is the ‘‘direct and complete recycling method’’ which has been studied recently.[7,8] The key processes in this method are the remelting of scrapped turbine blades, the refining of the melts using a CaO crucible, and the chemical composition adjustment by addition of raw materials in the remelting process. Sulfur which intrudes into the components during operation is known to reduce various high-temperature properties of Ni-base superalloys and oxidation including creep strength[9–11] [11–16] resistance. The desulfurization using CaO has been extensively studied and commercialized for steel manufacturing. The desulfurization reaction is generally promoted by the addition of Al into molten iron in a CaO crucible[17,18] or by Al2O3 coating on the surface of solid CaO to form calcium aluminate slag.[19,20] Degawa et al.[21] and Fruehan et al.[22] interpreted their observation results at the interface between molten Fe and solid CaO to be the slagging reaction of forming CaO and Al2O3 phases. In their studies, the whole reaction was explained as the reduction of CaO by Al followed by the
desulfurization by Ca. Saelim et al. measured the rates for desulfurizing molten Fe-O-S by a CaO crucible in an electric-resistance furnace, and their result indicates that the rates were controlled by diffusion of S in the melt.[23] If the melt is mechanically stirred during the reaction, the rate-limiting
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