Nitridering

Nitridering is a term that is sometimes used to refer to nitriding, a thermochemical surface treatment in which nitrogen diffuses into the surface of a metal substrate to form nitride phases. The goal is to increase surface hardness and wear resistance, often with beneficial effects on fatigue performance. The word nitridering is not standardized in the literature; most sources describe the process simply as nitriding.

The main methods of nitriding are gas nitriding, plasma (ion) nitriding, and molten salt bath nitriding. Gas nitriding uses ammonia- or nitrogen-based atmospheres at about 500 to 550 degrees Celsius, forming a nitride layer on steels and cast irons over hours to days. Plasma nitriding operates at lower temperatures and uses active nitrogen ions, allowing finer control of case depth and reduced distortion. Molten salt bath nitriding employs cyanide- or cyanide-nitrate salts at similar temperatures, offering high diffusion rates but presenting greater safety and environmental concerns.

Materials commonly treated are iron- and nickel-based alloys, especially steel alloys with alloying elements such as chromium, molybdenum, or vanadium that promote nitride formation. The nitride layer typically contains iron nitrides such as gamma'-Fe4N and epsilon-Fe3N. Resulting surface hardness often reaches well beyond the core hardness, with case depths typically from about 0.1 to 0.5 millimeters, and depths up to about 1 millimeter or more in specialized cases.

Benefits include increased wear and fatigue resistance, lower friction, and sometimes improved corrosion resistance in nitrogen-rich environments. Limitations include potential distortion if not controlled, incompatibility with certain subsequent heat treatments, and varying suitability for different alloys. Nitriding is widely used in automotive components, gears, valves, and tooling.