Microstructural evolution and dynamic recrystallization mechanism of 347H heat-resistant steel

Abstract

The microstructural evolution of 347H heat-resistant steel during hot working directly affects its microstructural homogeneity and service performance; however, the synergistic roles of different dynamic recrystallization mechanisms and heterogeneous microstructural constituents in microstructural reconstruction remain insufficiently understood. In this study, hot compression tests were conducted to investigate the microstructural evolution of 347H steel at different true strains under deformation conditions of 1200 °C and 10 s⁻¹. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) were further employed to elucidate the underlying dynamic recrystallization mechanisms. The results show that dynamic softening is jointly governed by discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX), with a pronounced spatial partitioning of the two mechanisms. With increasing true strain, the microstructure evolves through three successive stages: localized nucleation, mixed-grain development, and microstructural homogenization. At ε = 0.2–0.3, dislocation accumulation and localized DRX nucleation dominate; at ε = 0.5, asynchronous nucleation and grain-boundary migration lead to the development of a mixed-grain structure; and at ε ≥ 0.7, sustained nucleation coupled with grain-boundary migration promotes microstructural homogenization, ultimately producing fine equiaxed recrystallized grains. Local strain concentrations arising from deformation incompatibility around residual δ/γ phase boundaries and coarse MX particles provide preferential sites for DRX nucleation. In contrast, fine second-phase particles dispersed within the matrix suppress grain-boundary migration through Zener pinning, thereby retarding grain growth and stabilizing the fine-grained structure at high strains.