Nitrogen Regulation and Its Chemical State in FeCr17Mn12Mo3.5N Powders Based on High-Pressure Nitriding

The demand for high-nitrogen austenitic stainless steel (HNASS) powders has become increasingly urgent due to the rapid development of advanced manufacturing processes. However, it still remains a challenge to accurately control the nitrogen content and its chemical state. In this work, an innovative process combining high-pressure metallurgy and solid-state powder nitriding is proposed to prepare ultra-high nitrogen austenitic stainless steel powders. The prepared powders not only exhibit fine austenite grains, but also achieve a high nitrogen content of up to 5.63 wt.% at 1000 °C, 2.5 h, and 2.5 MPa. The results demonstrate that the phase composition of the powders, as well as the size and distribution of nitrides can be effectively regulated by carefully controlling the key processing parameters including temperature, time and pressure. Nitrogen is predominantly uniformly distributed as solid solution, with minor nanoscale nitride precipitates. XPS analysis of the powder surface indicates that the peak area ratios of N1s (N 1s core-level) in the form of solid solution and nitrides are ~82.95% and ~17.05%, respectively. And the peak area ratios of N1s at different depths of the powder do not show significant changes. Furthermore, the high-pressure nitriding mechanism reveals that the synergy between a high-pressure nitrogen atmosphere and solid-state nitriding enhances nitrogen diffusion flux and increases nitride nucleation density, enabling precise control of nitrogen content and precipitate size. Moreover, the high-pressure nitriding process can effectively keep nitrogen in a solid solution, prevent the precipitation of coarse nitrides, and consequently improve the quality of the powders. This research provides in-depth guidance and insights into the design and preparation of HNASS powders.