Mo-rich Laves phase in a 9.5Cr-1.5MoCoVNbNB heat-resistant steel during long-term aging at 620 °C

Abstract

In this paper, precipitation and evolution of Mo-rich Laves phase were investigated during thermal aging at 620 °C in 9.5Cr-1.5MoCoVNbNB steel. Experimental results showed that Mo-rich Laves phase particles were observed at grain boundaries adjacent to M 23 C 6 after aging for 2000 h. Then, Laves phase particles were found at other sites such as packet boundaries, block boundaries, and lath boundaries. The analysis of EDS and results of APT revealed that the nucleation of Mo-rich Laves phase near the M 23 C 6 carbides should be attributed to the segregation of P and Si atoms at the M 23 C 6 carbides/martensite interfaces. It experimentally assessed the evolution of number, size, and area fraction of Mo-rich Laves phase increased simultaneously with aging time from 2000 h to 6000 h. The number of Mo-rich Laves phase decreased rapidly from 6000 h to 8000 h and then remained the same level up to 10,000 h. The size of the Mo-rich Laves phase increased continuously until the aging time of 10,000 h and reached 410 nm. The area fraction of the Mo-rich Laves phase dropped slightly from 6000 h to 10,000 h. Johnson-Mehl-Avrami indicated that the nucleation rate of the Mo-rich Laves phase decreases after aging for 6000 h. Ostwald ripening equations indicated that the Mo-rich Laves phase was controlled by the volume ( m = 3) and grain boundary diffusion ( m = 4). • Si and P atoms at the M 23 C 6 carbides/matrix interfaces may promote the nucleation of Mo-rich Laves phase. • The evolution of Mo-rich Laves phase during aging at 620 °C for 10,000 h were investigated. • The nucleation and growth mechanism of Mo-rich Laves phase in 9.5Cr-1.5MoCoVNbNB steel during aging is proposed.