LONG-TERM CREEP PROPERTIES AND MICROSTRUCTURE OF SUPER304H, TP347HFG AND HR3C FOR ADVANCED USC BOILERS

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 degrees C for 85,426h of SUPER304H and at 700 degrees C for 55,858h of TP347HFG prove to keep stable strength and microstructure with very few amount of a 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 degrees C. The longest creep test at 700 degrees C and 69MPa for 88,362h of HR3C confirms high enough and stable creep strengths and microstructure at 600-800 degrees 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.