Ripening Kinetics of Bimodal-Sized Carbide via Boundary Diffusion in Coarse-Grained 9Cr Martensitic Steel

We investigated the ripening kinetics of M 23 C 6 carbide by both experiments and numerical modeling in the commercial fine-grained P92 martensite ferritic heat-resistant steel and our designed 9Cr and 9Cr1W steels that contain 9 wt pct Cr and have the coarse grains. All of them have the similar compositions except for W and Mo contents. Both the boundary diffusivities of solute-alloying elements of Cr and W and the interfacial energy ( σ ) of carbide and matrix have been tailored in the simulations to fit the measured coarsening kinetics of carbide during the aging. We found that the diffusivity of Cr and W enhanced by 6 to 7 times due to the boundary diffusion and the reduced σ due to the W alloying are both required for the good fitting. Different from the normal size distribution of carbide in fine-grained P92, the carbides in both the coarse-grained 9Cr and 9Cr1W steels have the bimodal size distribution after the tempering, i.e. , the coarse and fine M 23 C 6 particles are located at high-angle grain boundaries (HAGBs) and low-angle grain boundaries (LAGBs), respectively, and the size difference is enlarged after the subsequent aging. This is attributed to much fewer HAGBs in 9Cr and 9Cr1W steels after the tempering than those in P92 steel. The employed two-cell model can successfully reproduce the measured coarsening kinetics of these bimodal-sized particles during the aging for 3000 hours. The simulation results suggest that the ripening of fine M 23 C 6 particles at LAGBs is suppressed by the rapid ripening of the coarse ones at HAGBs due to the competition of solute atoms being partitioned.