The Source of Nitrogen
Nitrogen in steel is primarily absorbed during exposure to atmospheric conditions in molten steel processing. Electric furnace steelmaking, including secondary refining arc heating, accelerates gas dissociation, resulting in higher nitrogen content. Prolonged smelting durations in open hearth furnaces further increase nitrogen levels. Improper control of converter reblowing operations and delayed switching between nitrogen and argon shielding gases will also elevate nitrogen concentrations. Additionally, nitrogen carried by ferroalloys, scrap iron, and slag materials can be introduced into the molten steel during charging.
The Form of Nitrogen
A portion of nitrogen in steel exists as metallic nitrides or interstitial solid solutions. Most alloying elements added to special steels form stable nitrides under specific conditions. These nitride-forming elements include manganese, aluminum, boron, chromium, vanadium, molybdenum, titanium, tungsten, niobium, tantalum, zirconium, silicon, and rare earth metals. Given that many nitride-forming elements can create multiple simple or complex compounds, over 70 distinct nitride phases may potentially form in steel. The remaining nitrogen exists as atomic nitrogen dissolved in the iron matrix. In exceptional cases, nitrogen may form molecular gas bubbles or adsorb onto steel surfaces.
The Effect of Nitrogen
Nitrogen should not be categorically classified as a detrimental gaseous element, as certain special steels intentionally incorporate nitrogen additions. All steel grades contain nitrogen, with specific concentrations determined by production methods, alloy composition and addition techniques, and casting parameters. In specific stainless steel grades, controlled nitrogen increases can reduce chromium requirements, effectively lowering production costs while maintaining performance. However, nitrogen predominantly exists as metallic nitrides in ferrous alloys. For instance, steel products exhibiting strain aging after storage become unsuitable for deep drawing applications (e.g., automotive body panels) due to tear formation during non-uniform plastic deformation. This phenomenon results from coarse grain structures combined with Fe₄N precipitation at grain boundaries.
Another critical example occurs in stainless steels where Cr₂N formation at grain boundaries depletes interfacial chromium content, leading to intergranular corrosion susceptibility. This detrimental effect can be mitigated through titanium additions, which preferentially form stable TiN compounds, preserving chromium content at grain boundaries.
