Carbonitriding

Carbonitriding is an austenitic (above A3) case hardening process similar to carburising, with the addition of nitrogen (via NH₃ gas), used to increase wear resistance and surface hardness through the creation of a hardened surface layer.

Benefits

Carbonitriding is applied primarily to produce a hard and wear resistant case. The diffusion of both carbon and nitrogen increases the hardenability of plain carbon and low alloy steels, and creates a harder case than carburising. The carbonitriding process is particularly suited for clean mass production of small components. Due to the lower temperature required for the carbonitriding, compared to carburising, distortion is reduced. Mild quenching speed reduces the risk of quench cracking.

Application & materials

Austenitic carbonitriding is successfully applied to generally mass produced components, and those of smaller dimensions, where great resistance to wear is required and where the case depth requirements ranges from 0.1 to max 0.75 mm. Typical applications include:

·         gears and shafts

·         pistons

·         rollers and bearings

·         levers in hydraulic, pneumatic and mechanical actuated systems.

Primarily to improve wear resistance and fatigue strength of plain carbon steels.

A wide variety of steels can be carbonitrided from plain carbon steels to mild steels (with reduced aluminium content), low alloy steels with max. 0.25% carbon, free cutting steels, and sintered steel.

Process details

(Austenitic) carbonitriding is a thermochemical treatment involving the incorporation of both carbon and nitrogen into the surface of the component, usually simultaneously. The process is carried out at lower temperatures, and generally for shorter times than carburising, and therefore components are less prone to distortion. The diffused nitrogen has a stabilising effect on austenite and lowers the critical quenching speed and, as a consequence, the hardenability of the steel.

Less severe quenching media like oil, instead of water quenching needed for mild steel, can be applied for reducing distortion.

Carbonitriding is usually carried out in a temperature range of 820-900°C in a gaseous atmosphere adding between 0.5 to 0.8% carbon and 0.2-0.4% (< 5%) nitrogen to the surface of plain carbon steel or low alloy steel. After diffusion time the components are directly quenched in oil. The attained case hardened depth (CHD) is usually not greater than about 0.7 mm and depends not only on carbonitriding depths, but also on the hardening temperature, the quench rate, the hardenability of the steel and the dimensions of the component. The heat treatment is completed by low temperature tempering between 150-200°C for the higher case depth range reducing brittleness and depending on tribological circumstances.