Tensile Properties and Deformation Characteristics of a Ni-Fe-Base Superalloy for Steam Boiler Applications
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INTRODUCTION
THE development of advanced ultra-supercritical (A-USC) coal-fired power plants with steam conditions of up to 973 K (700 C) and 37.5 MPa requires the use of Ni-base superalloys in the hottest boiler and turbine sections. Ni-base alloys such as CCA 617,[1,2] Nimonic 263,[3,4] and Inconel 740[5,6] have been evaluated as candidate boiler materials for A-USC applications. However, those alloys are prohibitively expensive due to a high content of Co (10 to 20 wt pct) and/or Mo and W (6 to 8 wt pct). A low cost austenitic Ni-Fe-base alloy designated as GH2984 (developed by the Institute of Metal Research, China) has been used as superheater tube alloy in marine environments at temperatures between 923 K and 973 K (650 C and 700 C) for several years[7–9] and it has recently attracted much attention as a promising candidate alloy for use as boiler material in A-USC power plants. This alloy contains a high amount of Fe (33 wt pct), no Co, and low amount of Mo (2.2 wt pct). It is primarily strengthened through precipitation of homogeneously dispersed fine c¢ phase in c matrix. Some strength is also derived from the precipitation of MC and M23C6 carbides at both the grain boundary and grain interior.[10] Alloy GH2984 possesses good oxidation and hot corrosion resistance.[11,12] Also, this alloy has good workability which
makes it easy to form large and complex-shaped components.[7] The operation of heat-resistant structural components such as boilers in power plants requires good understanding of the overall mechanical properties and related deformation mechanisms in a wide range of conditions under the loads of tensile stress, creep, and fatigue.[13–21] However, a limited number of articles on the mechanical properties and deformation behavior of GH2984 alloy are available. Guo and Du[7] reported the tensile properties of GH2984 alloy at room temperature and at 973 K (700 C), but the properties of GH2984 alloy at other temperatures of interest have not been studied. In addition, deformation mechanisms of GH2984 alloy have not been reported yet. To evaluate the potential applications of GH2984 alloy for 973 K (700 C)-class A-USC boilers, this paper, one of a series of studies, investigated the tensile properties and determined the deformation mechanism of GH2984 alloy by means of tensile tests, scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), and transmission electron microscopy (TEM). Results are discussed in terms of fractography and evolution of dislocation substructures deformed from room temperature to 1073 K (800 C).
II. ZHIHONG ZHONG and ZHAN SHI, Postdoctoral Researchers, and YUEFENG GU, Senior Researcher, Group Leader, are with the High Temperature Materials Unit, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan. Contact e-mail: [email protected] YONG YUAN, Postdoctoral Researcher, is with the Environment and Energy Materials Division, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047
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