Nitrogen is a common element in steel, originating from high-temperature absorption from the charge or molten steel. Nitrogen stabilizes the austenite structure and significantly improves the strength and toughness of steel after solution treatment. Nitrogen can improve the macrostructure of high-chromium and high-chromium-nickel steels, making them dense and strong. Therefore, replacing nickel with nitrogen in stainless steel and heat-resistant steels has significant economic value and development prospects. Nitrogen has a strong affinity for alloying elements such as titanium and aluminum in steel, combining with them to form highly stable nitrides. These disperse within grain boundaries and act as precipitation strengthening, inhibiting creep deformation at high temperatures and improving creep and long-term strength. Surface penetration methods such as nitriding and carbonitriding can form nitrides and cyanides on the steel surface, thereby increasing the hardness, strength, wear resistance, and corrosion resistance of the surface layer. In addition to ferritic stainless steel, almost all types of stainless steel, especially austenitic and duplex stainless steel, have been widely alloyed with nitrogen in the past decade, bringing austenitic and α+γ duplex stainless steel into the modern stainless steel era. Stainless steel includes controlled nitrogen ([N]≤0.10% or [N]＜0.12%), medium nitrogen ([N]≤0.40% or [W]＜0.50%) and high nitrogen ([N]＞0.4% or [N]≥0.50%) stainless steel, super austenitic stainless steel and super martensitic stainless steel containing low nitrogen. Although nitrogen cannot prevent the precipitation of σ phase in duplex stainless steel, it can inhibit the formation of σ phase. The addition of nitrogen to duplex stainless steel led to the emergence of second- and third-generation duplex stainless steels (super duplex and economical duplex), establishing a family of duplex stainless steels alongside the three major categories of stainless steel: martensitic, ferritic, and austenitic. Because nitrogen has extremely low solubility in ferritic stainless steel and high solubility in austenite, its beneficial effects in duplex stainless steel are primarily reflected in improving the properties of the austenitic structure.
Nitrogen in duplex stainless steel delays high-temperature welding of the austenite (γ) phase and promotes the precipitation of secondary austenite (γ2) in the heat-affected zone after welding, thus facilitating the production, processing, and welding of duplex stainless steel. Nitrogen is the only gaseous alloying element used in stainless steel. It is readily available, unlimited in quantity, inexpensive, and easy to incorporate during production. Its beneficial effects are significant, with few side effects, making it a highly promising alloying element for the development of modern stainless steel.
To emphasize the beneficial effects of nitrogen on corrosion resistance in stainless steel, sufficient amounts of chromium and molybdenum are essential. It is generally believed that nitrogen promotes the enrichment of chromium in the passive film, improving the passivation ability of the steel. Nitrogen can form NH₃ and NH₄+, raising the pH of the solution in the micro-region. Chromium-rich nitrides form at the interface between the metal and the passive film, further enhancing the stability of the passive film. Research has shown that nitrogen can also combine with molybdenum in duplex stainless steel to form Ni₂Mo₂N, further stabilizing the passive film and thus enhancing the corrosion resistance of the steel.
Nitrogen significantly enhances the room-temperature and high-temperature strength of austenitic and duplex stainless steels through solid solution strengthening. Adding 0.1% nitrogen to chromium-nickel austenitic stainless steel can increase the strength by 60-100 MPa, but at an appropriate nitrogen content, it does not significantly reduce the steel’s ductility or toughness. Large amounts of nitrogen can impart very high strength to high-nitrogen austenitic stainless steel without compromising fracture toughness.
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