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.

中文翻译：

---

氮气增强了硅改性 Fe-Cr-Ni 奥氏体不锈钢的微观结构热稳定性


高温长期微观组织不稳定是高硅 Fe-Cr-Ni 奥氏体不锈钢亟待解决的问题。在这项研究中，我们提出了一种通过氮掺杂来提高 Si 改性 Fe-Cr-Ni 奥氏体钢微观组织热稳定性的新策略。系统研究了无 N 和 N 掺杂钢在 783-923 K 老化过程中的微观结构演变行为。研究结果表明，N 掺杂导致大量的晶粒细化并提高钢的强度。重要的是，研究发现 N 掺杂通过降低 Ni、Cr、Si 和 Mo 的扩散系数来抑制它们在晶界的过早偏析，从而抑制了在 783 K 时效过程中晶间 M ₆ C 碳化物的产生，实现了优异的热稳定性。相比之下，无氮钢在 783 K 时效过程中由于 γ → M ₆ C + 铁素体相变而表现出微观结构不稳定性。在 823 和 873 K 时，得出结论，合金元素的扩散加速，导致 N 掺杂钢中形成 M ₆ C 和铁素体，以及随后的微观结构不稳定。它有助于降低冲击韧性，因为在高应变浓度的铁素体域和 M ₆ C/铁素体界面处往往会形成微裂纹。值得注意的是，当在 923 K 时效时，N 掺杂钢表现出由 M ₂₃ C ₆ 和 M ₆ C 碳氮化物组成的蜂窝结构，其中 Nb（C， N） 作为晶粒内的成核位点。这与在无 N 钢中观察到的晶内 χ 相不同，因为 χ 相的成核驱动力随着 N 含量的增加而减小。 该研究为开发用于铅冷却快堆的紧固件材料提供了有价值的见解。