High-temperature Resistant Property And Microstructure Characterization Of T92 Boiler Steel

The T92 steel operated at a steam temperature of 620℃in power generating plants was developed by Japan in 80's of 20th centery. The research on this steel has become more and more matured during the of 20 years of its application.As expected, the operated steam temperature will reach 650-700℃in 2020. Now new boiler steels are mainly studied and developed in foreign countries. The boiler steel made by China can not meet the requirement due to the lack of well understanding about relationship among microstructure, heat treatment and property.The T92 steel sample under this investigation is a heat-resistant ferritic steel with a composition of 9Cr-0.5Mo-1.8W added by the microalloying elements V and Nb. The T92 steel is mainly used for superheater and reheater pipes in power plants. It is inevitable that the mechanical property would degenerate and microstructure degrade during service. In order to explain and illustrate the microstructure evolution of microstructure and the mechenical property change of of T92 steel samples under high temperature and stress, high temperature ageing treatments and creep-rupture strength tests were performed at a constant temperature of 650℃. The microstructure evolution of the T92 steel during ageing and creep test was investigated using X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and EDS analysis.It is shown that the basic phase constitution of the T92 steel isα-Fe. The size of precipitates increases, whereas the number density of precipitates decrease with increasing the ageing and creep time. Precipitates that has been observed include the M23C6 carbides, MX carbonitrides and laves phase particles(Fe2W/Mo). The coarsening rate of Fe₂(Mo,W) is much larger than that of M₂₃C₆ carbides and MX carbonitrides. In addition to the precipitates, the microstructure morphologys of the steel samples also demonstrates some changes during the long term creep test: (1) A large amount of martensite laths were deformed and recovered under high temperature and stress; (2) The dislocation number was reduced. The interaction between dislocations and precipitates are also observed in the crept T92 steel samples. Hardness is a comprehensive criterion at the mechanical property of materials. We characterized the mechanical property change by microhardness measurements, which validate the correctness of our microstructure analysis for different steel samples.