Precipitate evolution in the deposited metals of 9Cr2W steel with different carbon contents during the 550 °C thermal aging process

Abstract

Two kinds of 9Cr ferritic/martensitic (F/M) steel deposited metals with different carbon contents (0.04 wt% – 04C and 0.10 wt% - 10C) were prepared and aged at 550 °C for 500, 1000, 3000, and 10,000 h. Using scanning electron microscopy and transmission electron microscopy to characterize the evolution of the MX, M₂₃C_6, and Laves phases. The relationships between the average sizes of the precipitates and aging time were established. The results show that the MX phase is the most stable precipitate in the aging process. The 04C deposited metals have more finely dispersed MX phases, but their microstructural stability is worse than that of the 10C deposited metals. After aging for 500–1000 h, the M₂₃C₆ reaches the precipitation equilibrium approximately. Even with an aging of 10,000 h, the Laves phase does not reach the precipitation equilibrium. The segregation caused by rapid solidification in the deposited metals results in the Laves phase precipitating earlier, and M₂₃C₆ reaches precipitation equilibrium more quickly than the base metal. Thermodynamically, reducing carbon content limits the evolution of M₂₃C₆ while promoting the evolution of the Laves phase in the deposited metals. However, from the kinetic perspective, the evolution of M₂₃C₆ carbides in 04C deposited metals is faster owing to their fewer nucleation, and the evolution of the Laves phase in 10C deposited metals is faster because of the enwrapping growth mechanism. Reducing the carbon content increases the coarsening rate of M₂₃C₆ and limits the coarsening of the Laves phase. The M₂₃C₆ carbide is the most effective precipitate to pin the sub-grain boundaries. After aging at 550 °C for 10,000 h, the Laves phase still has an effective pinning effect on the deposited metals. The combined pinning effects of the M₂₃C₆ and Laves phases can effectively prevent sub-grain coarsening. During the 550 °C aging process, the higher content of M₂₃C₆ carbides with a size range of 100 nm to 200 nm in 10C deposited metals is the main reason for its superior microstructural stability compared to the 04C deposited metals.