Stabilities of the precipitates in CLAM steel during 30,000 h thermal aging

China Low Activation Martensitic steel (CLAM steel) was thermally aged for up to 30,000 h at 550 and 600 °C. Scanning electron microscopy and transmission electron microscopy were employed to characterize the evolutions of the Laves phase, the M23C6 phase and the MX phase. The relationships between the particle sizes of the three precipitation phases with time, and the Johnson-Mehl-Avrami (J-M-A) equation for the growth kinetics of the Laves phase during thermal aging at 600 °C were obtained. The results show that the relationship between the particle size of the Laves phase with time is different from those of the M23C6 phase and the MX phase; the Laves phase has the largest coarsening rate and the worst stability; the time exponent of the J-M-A equation of the Laves phase is large (n=2.8). In 9–12%Cr tempered ferritic/martensitic steels, the precipitation of the Laves phase tends to lag behind the M23C6 phase. The Laves phase has two nucleation modes: independent nucleation and dependent nucleation adhering to the M23C6 phase. There is no unified understanding of how the growths of the two phases in the assemblage affect each other after dependent nucleation. In this study, an enwrapping growth mechanism of the Laves phase was observed, which provides a new clue to this question. The dependent nucleation and enwrapping growth mechanism of the Laves phase is the reason for the linear growth of its particle size and the large time exponent.