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Hyogo 660-0891 JapanFor the construction of ultra super critical (USC) power plant, 9Cr-3W base ferritic heat-resistant steels with relatively high B and no N have been investigated. Authors have been revealed in the previous report that the addition of i39ppm B significantly improves creep strength of the steels, whereas most of added B forms unidentified borides, which are deemed almost ineffective to creep strength. The effect of improved heat treatment on creep strength and distribution of B in precipitates is investigated to effectively utilize and decrease added B As a result of the analysis of the extracted residue and characterization of precipitates using field emission Auger eleotron spectroscopy (FE-AES), most of added B still forms borides in the 92ppm B added steel. These composites are almost dissolved and the B content in M23Ce carbides is significantly increased by normalizing at 142SK. It is also found by FE-AES analysis that B content in M23CG oarbides near prior-austenite grain boundaries is relatively higher than that inside grains. Creep strength at 923K for the 92ppm B added steel normalized at 1423K is not improved ~lt short times, but it is remarkably improved to almost the same level as the 139ppm B added steel at long times. This ex~Ilent creep strength is achieved resulting in improving microstructural stability through the effective utilization of added B by high-temperature normalizing.KEY WORDS: ferritic heat-resistant steel; creep; boron; FE-AES M23C6 oarblde mlcrostructure USC power plant.

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SUPER304H (18Cr-9Ni-3Cu-Nb-N; ASME CC2328) and TP347HFG (18Cr-12Ni-Nb; ASME SA213) have been developed for high strength oxidation resistant steel tubes to operate at high steam temperatures and pressures. The longest creep rupture tests performed to date (600uC for 85 426 h for SUPER304H; 700uC for 55 858 h for TP347HFG) showed that the stable strength and microstructure were retained, with very little formation of s-phase compared with conventional austenitic stainless steels and no other brittle phases. The alloy HR3C (25Cr-20Ni-Nb-N; ASME CC2115) has been developed for the high strength and high corrosion resistant steel tubes used in recent ultrasupercritical (USC) boilers with steam temperatures of ,600uC. The longest creep test conducted to date (700uC, 69 MPa for 88 362 h) confirmed a stable creep strength and microstructure at 600-800uC. Superheater and reheater tubes of these alloys installed in the Eddystone No.1 USC power plant since 1991 have been removed and investigated. Updated long term creep rupture properties of the steels and microstructural changes during service are reported. Three steel tubes have been successfully applied as standard materials for superheater and reheater tubes in newly built USC boilers.

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