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P92 ferritic steel is an improved version of 9 pct Cr-containing ferritic-martensitic steels developed for steam generator applications in the new-generation power plants such as ultrasuper critical (USC) power plants with increased operating steam temperatures and pressures to achieve higher efficiency and better environmental protection.[1] 9 pct Cr steels are also under active consideration as candidate materials for high temperature structural components of Generation IV nuclear power plants.[2] The choice of P92 steel for steam generator applications is based on the low thermal coefficient of expansion and high resistance to stress corrosion cracking in water–steam systems compared to austenitic stainless steels, in addition to better mechanical properties at elevated temperatures than the alternate P9 and P91 steels.[1,3] The improved version 9Cr-0.5Mo-1.8W-V-Nb steel, designated as P92 steel according to ASTM standards,[4] has been developed by the addition of 1.8 wt pct tungsten with reduced molybdenum in the P91 steel initially modified by the addition of strong carbide/nitride-forming elements such as niobium and vanadium along with controlled nitrogen.[1–3] Reduced Mo- and W-added P92 steel is also known as grade 92 steel and NF616 steel. The improved strength of the steel is derived mainly by the significantly higher contribution of solid solution strengthening due to the presence of tungsten in addition to stable precipitates and B.K. CHOUDHARY, Scientific Officer-H, E. ISAAC SAMUEL, Scientific Officer-E, G. SAINATH, Scientific Officer-C, J. CHRISTOPHER, Scientific Officer-D, and M.D. MATHEW, Scientific Officer-H, are with the Mechanical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India. Contact e-mail: [email protected]; [email protected] Manuscript submitted March 20, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A

high dislocation density in the normalized and tempered condition.[5–10] The strength due to precipitation is contributed by M23C6 carbides at the prior austenite grain, martensite lath and subgrain boundaries, and fine distribution of MX-type carbonitrides and nitrides in the matrix regions.[5–10] The MX precipitates have been observed to be Nb-rich carbonitride and V-rich carbonitride/nitride. It has been reported that the Laves phase, rich in tungsten with small amount of Mo, Fe2(W,Mo), forms in the vicinity of M23C6 carbides at grain and lath/ subgrain boundaries following thermal aging and creep exposure for longer durations.[5–10] Formation and growth of the Laves phase are considered to reduce the solid solution strengthening due to loss of W and Mo from the solid solution. However, the presence of the Laves phase at the grain and subgrain boundaries contributes to strengthening by inhibiting grain and subgrain deformation.[5] Further, the presence of tungsten also adds to the stability of the primary and secondary precipitates with comparatively slower growth rate, which in turn stabilizes the dislocation substructure in terms of dislocation cell