Nitrided

Nitrided describes a material that has undergone nitriding, a diffusion-based surface hardening process in which nitrogen is introduced into the surface of a metal, typically iron- or steel-based alloys. The aim is to form a hard nitride-containing surface layer and a nitrogen-diffused case beneath it, improving wear resistance, fatigue life, and sometimes corrosion resistance, while preserving a resilient core.

Common methods include gas nitriding, salt bath nitriding, and plasma (ion) nitriding. Gas nitriding uses ammonia or nitrogen-containing gas at typically 500–550°C and is widely used for ferrous alloys because it can produce deep, uniform cases. Salt bath nitriding uses high-temperature baths containing nitriding agents but is less common today due to environmental and safety concerns. Plasma nitriding (ion nitriding) employs nitrogen-containing plasma at lower temperatures, offering precise control over depth and uniformity and often shorter cycles.

Materials typically treated are low- to medium-carbon steels and certain alloy steels; some stainless steels and specialty alloys can be nitrided with appropriate processes. The resulting microstructure features a nitrided surface layer consisting of hard nitride phases such as epsilon-Fe2-3N and gamma’-Fe4N, atop a diffusion zone where nitrogen has penetrated the substrate.

Benefits include increased surface hardness, improved wear and abrasion resistance, enhanced fatigue strength, and better load-bearing capability in sliding contacts. Post-treatment steps vary, but many nitrided parts are left as-is or subjected to low-temperature tempering to relieve stresses.

Limitations include material compatibility requirements, potential brittleness of the outer layer under certain conditions, and cost considerations. Nitrided parts are common in gears, camshafts, crankshafts, valve components, and bearing surfaces where long wear life is important.