Carburising heat treatment is used for hardening the outer surface of a metal.

Carburising is a heat treatment process that is used to increase the surface hardness of low carbon steels. It involves introducing carbon into the surface of the steel by heating it in a carbon-rich environment. The carbon diffuses into the surface of the steel, forming a high-carbon layer known as a carburized case. This carburized case is then hardened by quenching, resulting in a hardened outer surface with a tough core.

The process of carburising involves several steps:

1. Heating: The steel component is heated in a furnace to a temperature between 850°C and 950°C. The temperature and time at which the steel is heated depend on the desired depth of the carburized case.

2. Carburizing: Once the steel is at the desired temperature, it is introduced into a carbon-rich atmosphere. This can be achieved by using a carbon-containing gas, such as methane or propane, or by placing the steel in a container with a carbon-rich solid, such as charcoal or carbon powder. The carbon atoms diffuse into the surface of the steel, increasing the carbon concentration and forming iron carbides.

3. Cooling: After the carburizing process, the steel is rapidly cooled to room temperature to harden the carburized case. This can be done by quenching the steel in a suitable medium, such as oil or water. The rapid cooling transforms the iron carbides into a hardened structure, increasing the surface hardness of the steel.

The purpose of carburising heat treatment is to:

- Hardening outer surface: The main purpose of carburizing heat treatment is to increase the surface hardness of low carbon steels. The carburized case formed during the process provides a hard outer surface, which improves wear resistance and extends the service life of the component.

In conclusion, carburising heat treatment is used for hardening the outer surface of a metal. It involves introducing carbon into the surface of the steel, forming a carburized case, and then hardening the case by quenching. This process increases the surface hardness of the steel, improving its wear resistance and durability.