Abstract: Microstructure transition temperatures of the P92 steel were determined by expansion method. After austenitizing, the steel was isothermal treated at different temperatures for different time. Microstructure evolution of the P92 steel isothermal treated at different temperatures for different time was analyzed by OM and SEM. Microhardness of the P92 steel under different states was measured by Vickers hardness tester. The results show that martensite transformation occurs at isothermal temperature of 370 ℃ and below for 1 h, and microstructure of the P92 steel at room temperature is martensitic. Bainite transformation occurs at isothermal temperature of 400-430 ℃ for 1 h. When isothermal treated at 520-620 ℃ for 1 h, the supercooled austenite is relatively stable and doesn't change in a short time. When isothermal treated at 650 ℃ and above for a long time (25 h), eutectoid decomposition of supercooled austenite occur, forming pearlitic structure. When isothermal treated at 680-770 ℃ for 25 h, there is proeutectoid ferrite precipitation, and with the increase of temperature, the proeutectoid phase increases. With the increase of isothermal temperature(160-770 ℃), the hardness of the P92 steel shows an overall downward trend, and gradually decreases from 516 HV to 163 HV.

Key words: P92 steel, isothermal temperature, microstructure evolution, hardness