Microstructural evolution and aging precipitation behavior of nuclear-grade N-strengthened austenitic stainless steel

Abstract

This study systematically investigates the degree of recrystallization, precipitation behavior, and their relationship with the hardening mechanisms in hot-rolled nuclear-grade N-strengthened austenitic stainless steel aged at various temperatures. The characterization techniques employed include microstructural analysis, thermodynamic phase diagram calculations, and hardness testing. The results indicated that both the rolled and aged states of the experimental steel comprised large deformed grains, recrystallized fine grains, and a limited number of precipitates. An increase in aging temperature resulted in a gradual rise in the proportion of high-angle grain boundaries and a decrease in the full width at half maximum (FWHM) of the (220) and (311) diffraction peaks, signifying enhanced recovery and recrystallization. Micron-scale precipitates in both states were predominantly strip-like Laves phase, while nano-precipitates primarily consisted of granular Z-phase, pearl-string Nb(C, N), and block M₂₃C₆. Laves phase, Z-phase, and Nb(C, N) precipitated both at grain boundaries and within grains. In contrast, M₂₃C₆ precipitated exclusively along grain boundaries at the 800 °C aging temperature. Aging at 650 °C increased the quantity of all types of precipitates. Within highly deformed grains, the size of Z-phase precipitates decreased, while within highly recrystallized grains, the sizes of both Nb(C, N) and Z-phase precipitates increased. As the aging temperature increased, the precipitation of the Laves phase gradually diminished, while the number of Z-phase precipitates increased. The number of Nb(C, N) precipitates initially increased before decreasing, and the sizes of both Z-phase and Nb(C, N) precipitates gradually increased. Additionally, the macro hardness of the experimental steel varied with aging temperature, peaking at 285 HV at 800 °C. The dominant hardening mechanism evolved with increasing aging temperature, transitioning from dislocation hardening to a combination of precipitation hardening and fine-grain hardening, and ultimately to fine-grain hardening. This study elucidates the aging precipitation behavior and hardening mechanisms in N-strengthened austenitic stainless steel, providing a theoretical foundation for optimizing the heat treatment process for steels within this compositional system.