Microstructure Evolution of a Reactor Pressure Vessel Steel During High-Temperature Tempering

The electron microscopy techniques were employed to analyze the microstructure evolution during high-temperature tempering of a reactor pressure vessel steel. The results show that carbon enriched martensite/austenite (M/A) constituents decomposed into ferrite laths and accumulated carbides during initial stage of tempering. Simultaneously, the carbon atoms in the constituents diffused into the matrix continuously. With further prolonging of tempering, Mo₂C carbides were found to be distributed uniformly in bainitic ferrite. In case of longer tempering, bainitic ferrite would combine and broaden, and grain boundary carbides grew up sequentially and coarsened. The newly formed austenite was detected during tempering at 660 °C for 5 h, and at 650 °C for 100 h,which shown that the Ac₁ is time-related. This phenomenon may be depend on the component fluctuation of M/A constituents and segregation of carbides.