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：

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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 下老化过程中晶间 M6C 碳化物的产生，实现了卓越的热稳定性。相比之下，无氮钢在 783 K 时效过程中由于 M6C + 铁素体相变γ →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 M6C 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，因为微裂纹往往在高应变浓度的铁素体域和 M6C/铁素体界面处形成。值得注意的是，当在 923 K 时效时，N 掺杂钢表现出由 M23C6 和 M6C 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。