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

在第一个项目期间，在增材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。在第一个项目阶段设计的合金中，Al含量接近4.5 wt.%，呈现出奥氏体和b2强化铁素体的双基体组织。由于该合金密度低，强度和延展性结合良好，因此被建议作为第二个项目阶段的候选合金。为了充分利用氮添加到不锈钢中的有益效果，考虑到抗腐蚀、固溶强化和促进奥氏体作为损伤容忍相，项目二期旨在研究候选合金中添加氮的前景。氮还可以通过形成氮化物，特别是氮化铝来提高耐磨性。在增材制造加工链的不同阶段，氮气的添加将以固态和液态两种状态进行。原位液态氮化包括在熔炼过程中通过提高氮气分压将氮气引入铸锭，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炼过程中进行氮化。在精确控制的气体环境中进行研究，可以评估铝合金不锈钢原位氮化的机遇、挑战和限制，同时利用增材制造的常规过程。另一方面，固态渗氮将通过高温固溶渗氮（HTSN）进行，以受益于加速扩散，但抑制有害相的形成，否则需要随后的固溶热处理。通过比较传统铸态和PBF-LB/M条件下材料的HTSN行为，可以明确PBF-LB/M固有的显微组织缺陷对合金表面氮化响应的影响。此外，氮浓度的梯度和表面附近的微观结构可以与液态氮化条件的微观结构相关联。氮化合金中AlN的存在，特别是在固态氮化状态的外表面附近，有望提高其耐磨性。因此，工作计划涉及到摩擦学测试，以评估耐磨性。耐腐蚀性，作为不锈钢的另一个重要的性能相关要求，也将通过浸泡试验和联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.