Nanocrystallization of a quenched RAFM steel and microstructure evolution during annealing heat treatment

Nanocrystalline grains were produced on a quenched steel by means of surface mechanical attrition treatment (SMAT). Nano-grains in the topmost surface and depth-dependent microstructures were characterized by XRD, SEM and TEM. The average grain sizes after SMAT was 6.6 nm by statistical analysis from dark images of TEM. Rapid grain growth was observed after annealing for 5 min at 823 K, and the reasons for this were the release of residual stress and enhanced diffusion rates. The average grain sizes were 42.6 nm, 79.1 nm and 92.8 nm after annealing for 5 min, 30 min and 120 min at 823 K, respectively. The slow grain growth during annealing can contribute to the strong pinning force produced by the dispersive M(C,N) carbides and solute atoms in the present RAFM steel. Diffraction peaks of TaC carbides became weaker after annealing heat treatment from the XRD results, but no M23C6 type carbides were found even after annealing for 120 min.