Nitrogen enhances microstructural thermal stability of Si-modified Fe-Cr-Ni austenitic stainless steel

Abstract

High-temperature long-term microstructural instability is an urgent problem to be solved for high-silicon Fe-Cr-Ni austenitic stainless steel. In this study, we propose a novel strategy to improve the microstructural thermal stability of Si-modified Fe-Cr-Ni austenitic steels via N doping. The microstructural evolution behaviors of N-free and N-doping steels were systematically investigated during aging at 783–923 K. The findings indicate that N doping results in substantial grain refinement and improves the strength of the steel. Importantly, it is found that N doping inhibits the premature segregation of Ni, Cr, Si, and Mo at grain boundaries by reducing their diffusion coefficients, thereby suppressing the generation of intergranular M₆C carbides during aging at 783 K, achieving superior thermal stability. In contrast, N-free steel exhibits microstructural instability due to the γ → M₆C + ferrite transformation during aging at 783 K. At 823 and 873 K, it is concluded that the diffusion of alloying elements accelerates, resulting in the formation of M₆C and ferrite in N-doping steel and subsequent microstructural instability. It contributes to a decrease in impact toughness, as microcracks tend to form at the ferrite domain and M₆C/ferrite interface with high strain concentration. Notably, when aged at 923 K, N-doping steel exhibits a cellular structure composed of M₂₃C₆ and M₆C carbonitrides, with Nb(C, N) serving as the nucleation site within the grains. This differs from the intragranular χ-phase observed in N-free steel, as the nucleation driving force of the χ-phase decreases with an increasing N content. The study offers valuable insights for the development of fastener materials intended for utilization in lead-cooled fast reactors.