Carbonitriding

Carbonitriding is a surface hardening process in which carbon and nitrogen are introduced into the outer layer of steel to form a hard, wear-resistant case while preserving a tougher core. The treatment is usually performed in a furnace with a gas atmosphere containing a carbon source (such as methane, propane, or natural gas) and a nitrogen source (typically ammonia or nitrogen).

The process operates at elevated temperatures, commonly in the range of 800 to 950°C. After heating, the part is held at temperature to allow diffusion of carbon and nitrogen into the surface for a period of time that depends on desired case depth and alloy content. The part is then quenched and often tempered to achieve a balance of hardness and toughness. In carbonitriding, nitrogen forms diffusion-generated nitrides while carbon promotes carbide formation, producing a surface layer that combines nitrides and carbides.

Materials typically treated include low to medium carbon steels and certain alloy steels chosen for wear resistance. Typical results include a surface hardness around 50–60 HRC with a case depth ranging from fractions of a millimeter up to about a millimeter, depending on time and material. The core remains relatively ductile to preserve toughness.

Advantages of carbonitriding include improved wear resistance and fatigue strength, good surface hardness, and potentially lower distortion than some high-temperature carburizing processes. Limitations involve the need for controlled atmospheres and specialized equipment, environmental and safety considerations (especially with ammonia or cyanide salts in some variants), and sensitivity to alloy content and geometry.

Variants include gas carbonitriding, salt bath carbonitriding, and vacuum carbonitriding. Common applications are gears, crankshafts, camshafts, bearing surfaces, and other components requiring a hard, wear-resistant surface.