Heat Treatment

For the purpose of hardening steel is heated to a temperature where austenite is formed (about 760 to 870°C). Then it is cooled quickly to room temperature (quenching); however quenching generates high tensions inside the steel which have to be removed by tempering, or annealing.

The size, form and distribution of cementite in the metal can be controlled by the means of tempering. This distribution is a main factor determining the physical properties of steel. There are many variations of this method.

Metallurgists discovered that the conversion of austenite to martensite takes place during the last phase of cooling. Due to the lower density of martensite this conversion leads to a change in volume. To avoid fracturing three main methods have been developed.

Slow cooling removes the steel from the cooling bath when it reaches the temperature at which martensite starts forming. The last phase of cooling takes place in air.

Martempering removes the steel at the same temperature and then it is put into another bath with the same temperature until the temperature inside the steel is equal at all points. Afterwards the steel is removed from the cooling bath and cooled to room temperature in air. The start temperature for the forming of martensite is usually at about 288°C.

Austempering tempers the steel in a bath of metal or salt which is kept at a constant temperature at which the wished structural changes take place. The metal is kept inside this bath until the conversion is completed.

There are other methods of quenching and tempering. Such as case hardening; this infuses carbon or nitrogen into the outer layer of the steel for hardening (carburizing, nitriding). These compounds react with the steel and form a layer of harder alloy in the upper surface. For carburization the raw steel is heated in coke, methane or carbon monoxide.

For the process of nitriding the steel has to contain a certain amount of nitride forming elements such as chromium or titanium. The steel is heated in ammonia gas. This method is often used in the automobile and machine industry as well as in tool factories.