Nitriding

Nitriding is a surface hardening process in which nitrogen atoms diffuse into the surface layer of steel. Unlike carburizing, nitriding does not heat the steel into the austenitic region and requires no post process quenching, making it particularly advantageous for precision components with low distortion tolerances.

Nitriding is typically carried out at relatively low temperatures ranging from 500°C to 580°C, generally through the dissociation of ammonia gas (NH3) at the component surface, allowing nascent (atomic) nitrogen to diffuse into the material. Because this temperature range remains within the ferritic region, phase transformations and their associated volume changes do not occur in the base material.

Consequently, distortion in nitrided parts is significantly lower compared to carburized parts.The increase in hardness results from the precipitation of hard nitride compounds formed by nitrogen combining with iron and alloying elements (particularly chromium, aluminum, and molybdenum) at and near the surface. Therefore, nitriding yields maximum efficiency when applied to specialized alloy "nitriding steels" containing aluminum, chromium, or vanadium.

A nitrided surface typically exhibits two distinct zones:

Compound layer (white layer): A thin, outermost surface layer consisting of hard yet brittle iron nitrides.

Diffusion zone: The region beneath the compound layer where dissolved nitrogen and alloy nitrides are dispersed within the matrix, thereby enhancing fatigue strength.