Quenching

Quenching is the rapid cooling of steel after heat treatment by immersion in water, or oil.

Water is used for plain carbon steels and very low alloy steels. Where the fastest possible quench is required, salt can be added to the water which is then called brine. Oil is used for higher-alloy steel to cool more gently and minimise distortion. It is possible to quench very high alloy steels using air or another suitable gas, such as nitrogen or even argon.

The ease with which a steel can be hardened is known as its hardenability. The higher the hardenability, the easier it is to harden and the slower the quenching rate can be. It is the quantity and type of alloy in the steel that determines its hardenability.

With most steels, quenching causes a great increase in hardness. In general, the higher the carbon content, the higher the hardness that can be achieved. Typically, the hardness of a fully-hardened steel varies from 40Rc for 0.1% carbon to 60Rc for 0.8% carbon.