Case hardening and carbonitriding
The aim of case hardening is a soft and tough core combined with a hard surface of the material. Case hardening is understood as a process combination of carburizing, hardening and tempering of a low-alloy steel workpiece.
In this process, the surface of components is carburized by a carbon-emitting medium (gas) and then quenched. This significantly improves the mechanical properties of the component surface layer (e.g., wear resistance or fatigue strength). The subsequent quenching may be either directly from the carburizing temperature, or after lowering to a material-specific hardening temperature. However, these are only two variants of temperature-time sequences in case hardening.
Carburizing is usually carried out between 880°C and 980°C. After the carburized components have been hardened, tempering is predominantly required in order to reduce the stresses resulting from the hardening and to achieve the required service life.
Carbonitriding is a special type of case hardening. This is usually used at a low to medium hardening depth. Carbon and ammonia are diffused into the surface layers. The critical quench rate is thereby reduced, which in turn improves the surface hardenability of the steel. Providing the possibility to use steels, such as unalloyed or low-alloyed steels as well as construction steels, for hardening.
Compared to casehardening, which is mainly used for case-hardening steels, carbonitriding can achieve a significant increase in surface hardness even in unalloyed materials. Apart from this an improved friction resistance and better dry-running properties are provided.