Impacts of adding nitrogen to aluminum-alloyed stainless steels through additive manufacturing and solid-state nitriding

在第一个项目期间，在附加制造和常规铸造条件下研究了铝含量高达6wt.%的轻质不锈钢。一期工程设计的Al含量近4.5wt.%的合金具有奥氏体铁素体和B_2强化铁素体的双重组织。由于该合金密度低，强度和塑性结合良好，因此被推荐为二期工程的候选合金。考虑到不锈钢的耐蚀性、固溶强化和奥氏体相作为损伤容忍相的促进作用，为了利用向不锈钢中添加氮的有利效果，第二个项目阶段旨在研究向候选合金中添加氮的前景。氮还可以通过形成氮化物，特别是AlN来提高耐磨性。氮加成将在添加剂制造的加工链的不同阶段以固体和液体两种状态发生。原位液态氮化包括在熔炼过程中通过增加氮气分压将氮气引入初始铸锭，在气体雾化过程中对合金熔滴进行氮化，以及在激光粉末床熔化过程中进行氮化(PBF-LB/M)。在精确控制的气体环境中进行研究，能够评估铝合金不锈钢原位氮化的机会、挑战和限制，同时利用添加剂制造的常规过程。另一方面，固态渗氮将通过高温溶液渗氮(HTSN)进行，以从加速扩散中受益，同时抑制有害相的形成，否则将需要后续的固溶热处理。通过比较材料在常规铸态和PBF-LB/M条件下的HTSN行为，可以明确PBF-LB/M固有的微结构缺陷对合金表面氮化反应的影响。此外，氮气浓度的梯度和表面附近的显微组织可以与液态氮化条件下的组织相关联。氮化合金中AlN的存在，尤其是在固态氮化条件下的外表面附近，有望提高合金的耐磨性。因此，工作计划包括摩擦学测试，以评估耐磨性。耐腐蚀性作为不锈钢的另一项重要性能要求，也将通过浸泡试验和组合腐蚀试验技术进行研究。

In the first project period, lightweight stainless steels containing up to 6 wt.% aluminum were studied in additively manufactured and conventionally cast conditions. One of the alloys designed in the first project period with nearly 4.5 wt.% Al exhibited a duplex matrix microstructure of austenite and B2-strengthened ferrite. Due to its low density and adequate combination of strength and ductility, this alloy is proposed as the candidate alloy for the second project period. To take advantage of the beneficial effects of nitrogen addition to stainless steels in view of the corrosion resistance, solid-solution strengthening and promotion of austenite as a damage-tolerant phase, the second project period aims at investigating the prospects of nitrogen addition to the candidate alloy. Nitrogen can additionally enhance the wear resistance by forming nitrides, especially AlN. Nitrogen addition will take place in both solid and liquid states in different stages of the processing chain of additive manufacturing. In-situ liquid-state nitriding comprises introducing nitrogen into the starting cast ingot by increasing the partial pressure of nitrogen during melting, nitriding of molten alloy droplets during gas atomization, and nitriding during laser-based powder bed fusion (PBF-LB/M). Investigations in precisely controlled gaseous environments enable to assess opportunities, challenges, and restrictions of in-situ nitriding of aluminum-alloyed stainless steels while taking advantage of the routine course of additive manufacturing. Solid-state nitriding, on the other hand, will be conducted by high-temperature solution nitriding (HTSN) to benefit from an accelerated diffusion, yet suppress the formation of detrimental phases which would otherwise necessitate a subsequent solution heat treatment. A comparison of the HTSN behavior for materials in conventional as-cast and PBF-LB/M conditions allows to clarify the impact of microstructural defects inherent to PBF-LB/M on the surface nitriding response of the alloy. Besides, the gradient of nitrogen concentration and microstructure near the surface will enable a correlation with the microstructure of liquid-state-nitrided conditions. The occurrence of AlN in nitrided alloys, especially near the outer surface of solid-state-nitrided conditions, is expected to enhance the wear resistance. Accordingly, the work program involves tribological tests to evaluate the wear resistance. Corrosion resistance, as another important performance-related requirement for stainless steels, will also be studied by immersion tests and combined corrosion testing techniques.