Microstructure evolution of reactor pressure vessel steel during high-temperature tempering

The electron microscopy techniques were employed to analyze the microstructures during high-temperature tempering of nuclear reactor pressure vessel (RPV) steel. It has been noticed that microstructures changed with tempering. For instance, carbon enriched M/A constituents decomposed into ferrite laths and accumulated carbides during initial stage of tempering. Thereafter, tiny carbides grew up and stringy carbides spheroidized, and the small laths of martensite combined together and recrystallized at subsequent tempering process. Simultaneously, the carbon atoms in the constituents diffused into the matrix continuously. With further prolonging of tempering, the recrystallized process of ferrite laths completed and the carbides accumulation phenomenon was significantly improved after tempering at 650 ℃ for 5 h. Further, Mo₂C and Fe₃C carbides were found to be distributeduniformly in bainitic ferrite. In case of longer tempering, baintic ferrite would combine and broaden, and grain boundary carbides grew up sequentially and coarsened. Although the quasi equilibrium thermodynamic calculation indicates that austenitic transition temperature (Ac₁) of the investigated steel is 660 ℃, the experimental result shows that the Ac₁ is time-related. The newly formed austenite was detected during tempering at 660 ℃ for 5 h, and at 650 ℃ for 50 h, which may be related to the component fluctuation of M/A constituents and segregation of carbides. (authors)