Overview: high-nitrogen alloying of stainless steels

High-strength austenitic stainless steels can be produced by replacing carbon with nitrogen. Nitrogen has greater solid-solubility than carbon, is a strong austenite stabilizer, potent interstitial solid-solution strengthener, and improves pitting corrosion resistance. Although the solubility of nitrogen in liquid iron is very low, 0.045 wt.% at 1600 °C and atmospheric pressure, nitrogen levels above 1 wt.% can be obtained through alloying and specialized high-pressure melting techniques. An austenitic stainless steel should be considered “high-nitrogen” if it contains more nitrogen than can be retained in the material by processing at atmospheric pressure; for most alloys, this limit is approximately 0.4 wt.%. This article describes melting and processing technologies applicable to high-nitrogen steels and the effects of interstitial nitrogen on a variety of material properties. Thermal stability, nitride precipitation kinetics, and the effects of nitride formation on mechanical properties and corrosion resistance are discussed.