Overview of Strategies for High-Temperature Creep and Oxidation Resistance of Alumina-Forming Austenitic Stainless Steel
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I.
INTRODUCTION
THE efficiency of fossil-fired boiler/steam turbine power plants can be increased by increasing the operation temperature and pressure, with a further benefit of reduced greenhouse gas emissions.[1,2] Candidate structural materials of interest to achieve temperature/ pressure increases range, depending on component, from ferritic and austenitic stainless steels to Ni base alloys. Extensive efforts are underway worldwide to evaluate and further improve high-temperature strength, oxidation/corrosion resistance, and fabricability in these classes of materials, ideally at reduced or equivalent cost to currently used materials.[3,4] A family of alumina-forming austenitic (AFA) stainless steel alloys is currently under development by the authors.[5–15] These alloys are targeted for potential use as superheater/reheater tubes in fossil-fired steam plants, among other applications for chemical and petrochemical processing and energy production. The AFA alloys possess the ability to form an external, protective Al2O3 scale at 873 K to 1173 K (600 C to 900 C), which affords superior oxidation and corrosion protection to the Cr2O3 scales that grow on conventional stainless steel alloys in many industrially relevant environments. Y. YAMAMOTO, Research and Development Associate, M.P. BRADY and M.L. SANTELLA, Senior Research and Development Staff Members, H. BEI, Research and Development Staff Member, P.J. MAZIASZ, Distinguished Research and Development Staff Member, and B.A. PINT, Group Leader, are with the Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6083. Contact e-mail: [email protected] Author’s Note: Part of this overview article is based on an earlier review (Ref. 5 and findings first reported in Refs. 6–15). Manuscript submitted January 4, 2010. Article published online July 3, 2010 922—VOLUME 42A, APRIL 2011
Of particular interest to fossil-fired steam plant applications is the potential for superior oxidation resistance to water vapor/steam containing environments.[16–18] High-temperature creep strength is achieved in AFA alloys primarily via MC carbide precipitates. The alloys typically contain only 2.5 to 4 wt pct Al and less than 15 wt pct Cr in order to permit stabilization of an austenitic matrix for high-temperature strength at relatively low levels of Ni additions (20 to 25 wt pct). Development efforts for AFA alloys date back to at least the 1970s based on the recognition that alumina scales offer the potential for superior oxidation/corrosion resistance to chromia scales in many environments.[19–22] However, none have succeeded sufficiently in developing compositions that exhibit both creep and oxidation resistance. This is because of the strong bcc stabilizing effect of Al and Cr additions on Fe,[23] which makes the materials unacceptably weak at elevated temperatures if a single-phase austenitic matrix cannot be obtained. Due to these complications, many efforts have instead focused on alumina coatings or aluminizing surface treatments for austenitic stainl
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