SUPER304H (18Cr-9Ni-3Cu-Nb-N, ASME CC2328) and TP347HFG (18Cr-12Ni-Nb, ASME SA213), which are applied fine-grained microstructure, have been developed for high strength and high steam oxidation resistant steel tubes. The longest creep rupture tests at 600°C for 85,426h of SUPER304H and at 700°C for 55,858h of TP347HFG prove to keep stable strength and microstructure with very few amount of σ phase and no other brittle phases when compared with conventional austenitic stainless steels, HR3C (25Cr-20Ni-Nb-N, ASME CC2115) have been developed for high strength and high corrosion resistant steel tube used in severe corrosion parts of recent USC boilers with steam temperature around 600°C. The longest creep test at 700°C and 69MPa for 88,362h of HR3C confirms high enough and stable creep strengths and microstructure at 600-800°C. These steel tubes have been installed in Eddystone No.3 USC power plant as superheater and reheater tubes since 1991, and have been removed and investigated microstructure after long-term service exposure. This paper describes up-dated long-term creep rupture properties of the steels and microstructural changes after long-term creep rupture and aging. Three steel tubes have been successfully applied as standard materials for superheater and reheater tubes in newly built USC boilers in the world.
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