Nitriding

Nitriding is the diffusion of nitrogen into the surface of special alloy steel to give a hard surface and soft core without the need for further treatment. Processing is generally carried out in the temperature range 470ºC to 530ºC in an atmosphere of ammonia, although other processing media can be used, such as salt baths and plasma.

Nitriding is only carried out on special alloy steels containing chromium or aluminium. It is the reaction of the nitrogen with these alloy elements that causes the hardening, so that, unlike carburising and carbonitriding, quenching is not required after processing. The nascent nitrogen is obtained from an atmosphere of ammonia gas, which at 500°C dissociates into its constituent elements, nitrogen and hydrogen. The nitrogen, which is in solution in the iron, diffuses inward and forms aluminium or chromium nitrides producing high hardness in the surface of the nitrided component. A layer of iron nitride and alloy nitrides forms on the surface (the “white layer”). Since this is brittle it is normally removed from bearing surfaces before service. As with carburising the case depth is time and temperature dependent.

Owing to the fact that nitriding is a low-temperature treatment, it is carried out on steels that have already been hardened and tempered. Final tempering must have been carried out at least 50ºC above the nitriding temperature. All steels for nitriding have to contain molybdenum in order to avoid temper brittleness caused by holding the steel for a long time at about 500ºC.

Nitriding produces advantages in addition to a freedom from distortion, which is due to the low treatment temperature and the fact that quenching is not required. The hardening response is due to the dislocation blocking capability of the alloy nitrides dispersed throughout the nitrided layer. Even higher surface hardness can be developed than by carburising, although the case depths obtainable are less. Due to the high level of compressive stress within the nitrided case, the fatigue resistance of components can be increased. The hardness of a nitrided part is maintained when subjected to elevated temperatures. Whereas temperatures of 200°C are sufficient to cause a carburised case to begin to soften, it requires temperatures above that of nitriding or very extended exposure to cause softening of a nitrided case.

Whilst the nitriding process itself is virtually ‘distortionless’, it does cause a small, predictable amount of growth of the nitrided component, and it is necessary to ensure that a component is in a stress free condition prior to nitriding, otherwise distortion can result. Thus, it is preferable to include a stabilisation treatment after the rough machining stage. Since in most component applications core strength is important, the usual planning sequence is as follows:

• 1. Oil harden and temper to produce specified core properties
• 2. Rough machine
• 3. Stabilise at 550/580°C for a time suitable for the section size
• 4. Finish machine
• 5. Nitride
• 6. Polish to remove ‘white layer’.

Selective nitriding can be achieved by the use of electroplated tin, or copper or using tin-based protective paint to blank off areas to be kept soft, thus preventing the diffusion of nitrogen from taking place there.

See also plasma nitriding, gas nitriding, Corr-I-Dur®.