A heat treating process that increases the surface hardness to a part by immersing it in a carbon and nitrogen-rich atmosphere at elevated temperatures. This results in the diffusion of carbon and nitrogen into the surface of the part. The depth of diffusion depends on the temperature and the amount of time that the part is held at that temperature. The amount of carbon and nitrogen entering the part can be controlled by adjusting the amount of carbon and nitrogen (called potential) in the furnace atmosphere. Carbonitriding produces a shallow high hardness surface layer (also called case hardness) and is commonly performed on parts that are thin or have relatively small cross sections and require enhanced wear resistance in service, such as self-tapping screws.
An improperly adjusted carbonitriding atmosphere can result in alterations to a material’s microstructure that can actually decrease its surface hardness. Improper carbonitriding can also produce sub-surface voids or holes in parts. This can significantly reduce fatigue strength.