Alumina-Forming Austenitic Stainless Steels Strengthened by Laves Phase and MC Carbide Precipitates
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TRODUCTION
THE efficiency of energy conversion systems such as boiler/steam turbine power plants is a strong function of steam temperature and pressure.[1] Consequently, it is attractive to operate such plants at higher temperatures and pressures to increase their energy efficiency. However, under such conditions, high-temperature creep strength is a major issue for hot components in advanced fossil energy conversion and combustion systems. For example, current goals for advanced fossil energy power plants call for ferritic steels capable of operation above 600 C, and austenitic steels capable of operation at greater than 700 C. Components of Y. YAMAMOTO, Postdoctoral Research Fellow, M.P. BRADY, Senior Research and Development Staff Member, P.J. MAZIASZ, Distinguished Research and Development Staff Member, and B.A. PINT, Senior Research and Development Staff Member, are with the Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115, USA. Contact e-mail: [email protected] Z.P. LU, Research and Development Staff Member, formerly with the Materials Science and Technology Division, Oak Ridge National Laboratory, is now Professor with the State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, P.R. China 100083. C.T. LIU, Distinguished Research Professor, is with the Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996-2200, USA. M. TAKEYAMA, Associate Professor, is with the Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo 152-8552, Japan. Manuscript submitted March 29, 2007. Article published online September 20, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
interest range from super heater tubes to industrial gas turbine components. A key need is the concurrent development of strength and oxidation resistance, especially in oxidizing environments also involving exposure to sulfidation, carburization, or water vapor species. High nickel austenitic alloys and Ni-base superalloys, which meet many of these needs from a property performance perspective, are available, although they are far too costly to be used due to the high levels of nickel.[2] Advanced heat-resistant austenitic stainless steels typically rely on MC-type carbide precipitates, such as NbC, TiC, or V4C3, to achieve creep resistance at elevated temperatures.[3,4] The present work seeks to explore the potential to further improve creep resistance in these alloys through the use of intermetallic Laves phase precipitates as strengtheners. Previous work indicated good creep strengths could be obtained in Cr2Ta Laves phase reinforced Cr and Cr(Fe) matrix alloys.[5,6] Takeyama et al.[7] recently demonstrated that fine Fe2Nb Laves phase dispersions could be formed within the austenitic Fe matrix in Fe-20Cr-(25–35)Ni2Nb at. pct alloys. Fe2Nb Laves phase is thermally stable up to its melting point of 1627 C in the binary Fe-Nb system,[8] and it is also expected to equilibrate with NbC and c-Fe (f
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