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Alloy design and creep strength of advanced 9%Cr USC boiler steels containing high concentration of boron

Abstract The research and development project of National Institute for Materials Science (NIMS) for advanced ferritic heat resistant steels for 650°C ultra super critical (USC) plants, revealed that the addition of >0·01 mass% boron to a 0·08C–9Cr–3W–3Co–V–Nb–,0·003N steel remarkably improves the long term creep strength. Boron enriched in M23C6 carbides near prior austenite grain boundaries suppresses the coarsening of carbides during creep deformation, leading to excellent microstructural stability and creep strength. If creep strength was further improved by the addition of nitrogen, it was found to enhance precipitation of fine MX. Addition of excess nitrogen to the high boron containing steel was found to reduce creep rupture lives and ductility. This results from a decrease in the amount of effective boron, which is dissolved in M23C6 and suppresses its coarsening, resulting from the formation of coarse BN at normalising temperature. The highest creep strength is obtained with steel of the following composition: 0·08C–9Cr–3W–3Co–0·2V–0·05Nb–0·008N–0·014B (mass%), which has an improved creep strength compared to P92. The 105 h extrapolated creep rupture strength at 650°C is ∼100 MPa. This steel also shows good creep ductility even in the long term. In conclusion, high boron bearing 9Cr–3W–3Co–V–Nb steel combined with the addition of 0·008 mass% nitrogen is a promising candidate for thick section components in the 650°C USC plants as it shows superior creep strength without impaired creep ductility.