Precipitation In Advanced Heat-resistant Austenitic Steel HR3C

Energy conservation and environmental protection are regarded worldwide as highly important issues. Generation of electric power by thermal power was widely utilized to generate electricity in China. Coal, fossil oil, and natural gas, which are unrenewable resources, are the major fuel of thermal power generation. So the new power plant design has sought lower fuel costs and emissions through further improvements in efficiency. Supercritical(SC) and ultra supercritical(USC) boilers with elevated steam conditions are being built and used in China.The heat-resistant steel is a major factor developing the generating unit. The supercritical(SC) and ultra supercritical(USC) boilers require steels with excellent high temperature strength, excellent oxidation, and corrosion resistance. Advanced materials such as TP347HFG, Super304H, NF709, and HR3C have been developed and successfully applied for these high temperature and pressure USC boilers. After long-term service in high temperatures and pressures, the creep rupture properties of these steels are greatly decreased due to microstructure degradation. Researches on the change of microstructure of these advanced materials are being done.HR3C steel (25Cr-20Ni-Nb-N) is a new type of heat-resistant austenitic steel which has been widely used for super-heater and re-heater tubes in the ultra supercritical pressure (USC) boiler. A large number of researches focused on the oxidation and corrosion resistance, weldability, and the mechanical property of HR3C. These researches indicated that precipitates had greatly influence on the high temperature strength, oxidation resistance, and corrosion resistance. The precipitation of HR3C steel is complicated due to a great deal of alloy elements been involved. So it needs lots of researches to reveal HR3C steel's precipitates.This work was focused on the change of microstructure and the precipitation of HR3C steel after thermal aging. Metallographic microscope, X-ray diffraction analysis, and transmission electron microscope had been used in this research. The results indicated that the major precipitates in intragranular were M23C6 carbides, NbCrN nitrides, and Nb(C,N). Cr3Ni2SiC carbides had been found in intragranular after long term aging. M23C6 carbides precipitated on grain boundaries and NbCrN nitrides had also been found on grain boundaries after long term aging. The precipitate ofσphase had been made sure by two methods. Influence of precipitates on mechanical property had been indicated by hardness tests and high temperature short time tensile tests. The results showed that the hardness and yield strength of HR3C were greatly improved after thermal aging.