Favoured for components that are subjected to heavy loading, nitriding imparts a high surface hardness which promotes high resistance to wear, scuffing, galling and seizure. Fatigue strength is increased mainly by the development of surface compressive stresses. The wide range of possible temperatures and case depths, which allow adjustment of different properties of the treated parts, give gas nitriding a broad field of applications.
Typical applications include gears, crankshafts, camshafts, cam followers, valve parts, springs, extrusion screws, die-cast tooling, forging dies, aluminium-extrusion dies, injectors and plastic-moulds.
Nitriding is most effective when applied to the range of steels containing nitride-forming elements such as chromium, molybdenum, vanadium and aluminium. The process is also applicable to tool steels such as hot-work, cold-work and mould steels. A low temperature application is nitriding of spring steels to prolong the fatigue life of springs for automotive use. In general, all ferrous materials can be gas nitrided up to 5% chromium. For higher contents of alloying elements and for gas nitriding of stainless steel, plasma nitriding might be considered. Gas nitriding of sintered steels with low density is not recommended.
For optimum results, the material should be in a hardened and tempered condition prior to gas nitriding.
Gas nitriding is a low temperature (typically 520°C/970°F), low distortion "thermochemical" heat treatment process carried out to enhance the surface properties of finished or near finished ferrous components. If a carbon spending gas is added, the process is called gas nitrocarburising. The layer usually consists of two zones – the compound layer (white layer) which can be a cubic or hexagonal nitride and the diffusion layer below with dissolved nitrogen and hard nitride precipitations. The compound layer on the surface of the parts is responsible for the major benefit of high resistance to wear, scuffing, galling and seizure. The diffusion layer contributes improved fatigue strength and works as a support for the hard compound layer. By controlling and adjusting the process atmosphere, the constitution of the layer can be influenced from thin compound layers for fatigue strength improvement to thick nitrogen and carbon rich compound layers in case of gaseous nitrocarburising and post oxidation if good wear and corrosion resistance is desired. In this case Corr-I-Dur® might be considered as the preferred choice.