All steels contain some nitrogen, which can enter the steel as an impurity or as an intentional alloying addition. It is generally undesirable as an impurity, causing embrittlement issues and affecting strain aging. However, nitrogen produces a marked (intersititial solid solution) strengthening when diffused into the surface of the steel, similar to the strengthening observed during case hardening (i.e. nitriding). Combined with aluminum, it produces a fine grain size.

In austenitic steels, the addition of nitrogen can simultaneously improve fatigue life, strength, work-hardening rate, wear and localized corrosion resistance. The solubility of nitrogen in ferrite is reported[2] to be at least two and five times greater than that of carbon at room temperature and 1100°F (590°C). Therefore, nitrogen as an interstitial atom is superior to carbon. Nitrogen can also improve the corrosion and creep properties as well as the wear resistance of iron-based metals. The powerful strengthening effect of interstitial nitrogen has made high-nitrogen steels a new class of engineering materials.

Nitrogen has a particularly strong affinity for boron, forming boron nitride and effectively destroying the hardenability effectiveness of boron additions. Protective additions (such as titanium) are added to boron steels to scavenge the nitrogen.

Although detrimental to ductility, notch impact strength and formability, the usefulness of nitrogen (in concentrations of about 0.010–0.015% N) as an inexpensive strengthening agent should not be overlooked. Strain aging can further improve strength levels. Steels containing columbium and/or vanadium (e.g., HSLA steels) will benefit from nitrogen additions through a combination of grain refinement and precipitation hardening.

Recently, an area of intense research interest is in martensitic stainless steels where high nitrogen content produces improved resistance to localized corrosion (e.g., pitting, crevice and intergranular corrosion). Alloy additions of both carbon and nitrogen in excess of approximately 0.4 wt% have shown promise.[3] The nitrogen and carbon reportedly remained as interstitials without the formation of nitride or carbide precipitates even in carbon and nitrogen concentrations as high as 2 wt%. Nitrogen and carbon alloying increased hardness, strength, toughness and wear resistance. Precipitate formers such as niobium and solid solution additives such as silicon and molybdenum enhanced yield strength to a much smaller extent than interstitial alloying. Also, additions of silicon and molybdenum significantly improved oxidation protection at all temperatures and are necessary to prevent spalling at temperatures above 980°F (525°C).