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

• Applicants: Professor Javad Mola, Ph.D.; Professor Dr.-Ing. Thomas Niendorf; Professorin Dr.-Ing. Olena Volkova
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 433662460

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.

• DFG Programme: Research Grants
• International Connection: Denmark
• Cooperation Partner: Professor Dr. Marcel A. J. Somers