Case Hardening

Carburising and carbonitriding are “thermochemical” treatments, usually conducted at temperatures in the range 800-940°C in the first stage of “case-hardening”.

These processes change the chemical composition of the surface of a low-carbon steel component so that subsequent fast cooling, by “quenching” produces a hard “case” combined with a softer/tougher “core”.

Quenching is normally followed by a low-temperature tempering / stress relieving treatment.

In carburising, controlled levels of carbon are introduced at the surface and allowed to diffuse to a controlled depth. In carbonitriding, nitrogen is also imparted, along with the carbon, to improve case hardenability. The heat treater employs a variety of processing media to achieve these objectives, including controlled gaseous atmospheres and molten salt (“cyaniding”).

What Are The Benefits
Carburising and carbonitriding case-hardening treatments offer a means of enhancing the strength and wear properties of parts made from relatively inexpensive easily worked materials. Generally applied to near-finished components, the processes impart a high-hardness wear resistant surface which, with sufficient depth, can also improve fatigue strength. Applications range from simple mild steel pressings to heavy-duty alloy-steel transmission components.

What sort Of Steels can be Treated?
Low-carbon / non-alloy (mild) steels can be case hardened by carburising or carbonitriding, but do not develop significant core strength. Thus they are normally treated for increased wear resistance only Low-carbon alloy case-hardening steels, intrinsically higher-strength materials, can be carburised to yield a high surface hardness whilst developing significant strength and toughness in the core. They are not normally carbonitrided.