Design and Characterization of a Heat-Resistant Ferritic Steel Strengthened by MX Precipitates
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INTRODUCTION
ABOUT 80 pct of the electricity generated worldwide today is produced by steam turbines. There has been a push in the past few decades to increase the operating temperature of steam generators to the ultra-supercritical (USC) regime, i.e., above 873 K, for improved thermal efficiency. More advanced USC power plants target temperatures at or above 923 K.[1–4] This requires creep-resistant alloys that can operate at these temperatures for at least 100,000 hours. T/P91 and T/P92 steels are considered commercial standards for use in supercritical and USC steam generator applications. They rely on dispersion strengthening by MX precipitates,
YAO DU, YIP-WAH CHUNG, and SEMYON VAYNMAN are with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208. XIAOLIN LI is with the Department of Materials Science and Engineering, Northwestern University and also with State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China. XU ZHANG is with the Theoretical and Applied Mechanics Program, Northwestern University. DIETER ISHEIM is with the Department of Materials Science and Engineering, Northwestern University and also with Northwestern University Center for Atom-Probe Tomography, Northwestern University. Contact e-mail: [email protected] Manuscript submitted March 21, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
where M = (V,Nb,Ti) and X = (C,N), M23C6 precipitates, and Laves phase Fe2(W,Mo) and solid-solution strengthening by W or Mo.[5–12] When exposed to these elevated temperatures for extended periods, there are concerns about microstructural degradation due to the metastability of MX, coarsening of M23C6 and the Laves phase, as well as Z phase formation (Cr(Nb,V)N).[13–24] In particular, the Z phase has been held responsible for the reduction of creep strength and failure of these steels after extended operation, as its precipitation results in the dissolution of finely dispersed MX.[19,25] It has been suggested that reduction of Cr and N concentration can effectively restrict Z phase formation.[23] Coarsening of M23C6 also causes decrease of creep strength due to the loss of effective grain boundary pinning. The coarsening resistance of MX vs M23C6 is illustrated by the work of Rojas et al.[26] M23C6 precipitates grow from 78 to 103 nm after creep deformation at 923 K for about 8000 hours, while MX precipitates start at about 30 nm and remain almost constant after the same creep deformation. Addition of alloying elements such as W, Co, and B has been used to improve the creep properties of ferritic steels.[27–33] For example, boron addition at certain alloy compositions can stabilize M23C6.[27–29] Low-carbon concentrations are known to suppress or eliminate M23C6 formation for improved microstructural stability.[34–38] For instance, Taneike and coworkers[34]
studied the effect of C concentration and found that lower C concentration gives rise to longer rupture time. Another approach is the use of coherent B2-NiAl precipitates f
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