Cobalt-Rich Phases in 11Cr-3W-3CoVNbTaNd Ferritic/Martensitic Steel
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pct Cr ferritic/martensitic (F/M) steels have been used in ultra-supercritical power plants with operating steam temperatures up to about 873 K to 883 K (600 °C to 610 °C).[1] Since 9 pct Cr F/M steels have their significant limitations on microstructural stability, long-term creep rupture strength, and oxidation resistance at temperatures higher than about 873 K (600 °C), advanced 9-12 pct Cr F/M steels with target operating temperatures up to 923 K (650 °C) and higher are now being developed in order to further increase steam parameters for an increase in thermal efficiency and hence the reduction of coal consumption and air pollution. Current design of advanced F/M steels is mainly focused on the following methods: increasing Cr content from 9 pct up to 12 pct to improve oxidation resistance; increasing W and decreasing Mo to improve high temperature creep strength because of good solid solution strengthening effect of W compared to Mo; the addition of Co to suppress d-ferrite formation during
YIN ZHONG SHEN, Professor, BO JI, formerly Graduate Student, and JIA RUI LU, Graduate Student, are with the School of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Minhang District, Shanghai 200240, China. Contact e-mail: [email protected] XIAO LING ZHOU, formerly Undergraduate Student with the School of Mechanical Engineering, Shanghai Jiao Tong University, is now Graduate Student with the ShanghaiTech University, and also with the Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, China. ZHONG XIA SHANG, Graduate Student, is with the School of Materials Science and Engineering, Shanghai Jiao Tong University. Manuscript submitted November 5, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
high temperature normalizing process[2,3] and improve creep properties at elevated temperatures.[3] Co is known as an austenitizing element. It has been reported that adding 2-3 pct Co drastically improves short-term creep strength, while long-term creep properties tend to deteriorate around 923 K (650 °C).[4,5] The mechanism that Co bearing in high-Cr ferritic steels affects creep properties at a high temperature is not yet clear because most of added Co presents as solid solution in matrix even after prolonged creep testing.[3] Therefore, it is very important to investigate the existing state of added Co in high-Cr F/M steels for understanding the mechanism that Co affects high-temperature creep properties of the steels, and optimizing the design of advanced F/M steels. It has been reported that in tempered high-Cr F/M steels containing 1-5 pct Co the content of Co in precipitates consisting of M23C6 and MX was very limited,[3,6] i.e., the concentration of Co in precipitates was very low, only around 0.007 mass pct of Co as carbide forming element was detected; it remains almost entirely in the matrix.[6] The main purpose of this paper is to report several kinds of Co-rich phases in a tempered 11Cr F/M steel containing 3 pct Co. The initial material used in this investigation was an
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