Stress Rupture Fracture Model and Microstructure Evolution for Waspaloy
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INTRODUCTION
WASPALOY * is a c¢ precipitation-hardened nickel*Waspaloy is a trademark of Special Metals Corporation, New Hartford, NY.
based superalloy, which owes its high strength to c¢ precipitates, less MC and M23C6 carbides, and solid solution strengthening due to additions of chromium, cobalt, and molybdenum. Carbides provide additional strength by pinning grain boundaries, which reduces GBS at elevated temperature. Due to a good match between strength and toughness, Waspaloy still has high tensile strength and a long stress rupture life at 1033 K (760 C) and a good oxidation resistance at 1143 K (870 C). Therefore, it has been widely employed in aerospace, oil and chemical industry equipment, and a variety of high-temperature components.[1–12] These applications include turbine and compressor disks, shafts, spacers, turbine cases, fasteners, and other miscellaneous hardware. In recent years, although new high-temperature and difficult-to-deform materials have been developed for the gas turbine, such as U720Li, "g742, "r79, Rene’95, and Rene’88DT,[13] Waspaloy is still considered as the main material for high-temperature components of high-power gas turbines due to its ZHIHAO YAO, Ph.D., Lecturer, MAICANG ZHANG, Associate Professor, and JIANXIN DONG, Professor, are with the Department of Materials Science, University of Science and Technology Beijing, Beijing, 100083, P.R. China. Contact e-mail: [email protected] Manuscript submitted January 27, 2011. Article published online February 23, 2013 3084—VOLUME 44A, JULY 2013
good hot workability and excellent integrated performance under long-term service conditions. Waspaloy is commonly produced by vacuum induction melting (VIM) followed by vacuum arc remelting (VAR) and/or electroslag remelting (ESR).[14] The conventional heat treatment for a Waspaloy turbine disk involves a three-step treatment consisting of 1269 K to 1311 K (996 C to 1038 C)/4 h/AC, 1116 K (843 C)/4 h/AC, and 1033 K (760 C)/16 h/ AC (where each step is given by the temperature, the time at this temperature, and the nature of subsequent cooling, with AC indicating air cooling). Its microstructure is mainly composed of a Ni-Cr-Co (c)-rich matrix, hardened by two kinds of fine Ni3 (Al, Ti) c¢ precipitates. Grain boundaries are decorated by discrete M23C6 carbides.[15] While the alloy is subjected to high temperature and stress over long periods, microstructures would be bound to change, including the coarsening of c¢ precipitates, the formation of continuous carbide network, MC degradation, and so on. The changes of microstructures may be harmful to tensile and creep/ stress rupture performance.[16–22] Stress rupture property is an important assessment parameter for high-temperature materials during hightemperature service-exposed operation.[20] There have been several studies of creep/stress rupture behavior and its life evaluation for Waspaloy.[23–27] Chang et al.[23] reported that the stress rupture life of Waspaloy significantly improved with the increase of c¢ precipitates. Bressers et a
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