Hardening

Hardening processes are used to impart specific mechanical properties to a component in order to render it fit for use. Hardening occurs when a steel component is heated to the austenitic range and cooled rapidly by quenching in a suitable medium such as water, oil or an inert gas. The choice of quenchant is determined by the composition of the steel and the geometry and application of the component being treated.

Steel must be in its austenite phase before it can be hardened. The temperature from which a steel can be hardened (called the hardening temperature) depends on its composition and can be determined from the equilibrium diagram. The rapid cooling during quenching causes the structure of the steel to change to martensite, which is very hard. Slow cooling would cause the austenite to transform to the much-softer ferrite.

The main points to be considered when selecting a hardening treatment are the application for which the component has been designed, its geometry and the steel composition which has been selected to provide the required mechanical properties. These will, to a large extent, dictate the hardening treatments which are suitable and the choices available. All stages of the manufacture of the component can affect the efficiency of the hardening treatment and the overall manufacturing economics can be greatly influenced by the choice of heat treatment. All methods of manufacture, every steel composition and each hardening treatment has its advantages and disadvantages. Care is required if the optimum choice is to be made and advice from heat treatment specialists, such as Bodycote, should be sought at an early stage of component design.

Various designs of heat treatment furnaces are available, including gas fired or electric continuous furnaces or sealed quench furnaces with integral oil quenching chambers, electrically heated vacuum furnaces with inert gas cooling facilities and gas or electrically heated pit furnaces. Other heat treatment equipment including fluid beds, salt baths, flame hardening and induction heat treatment sets offer a wide selection for the economic heat treatment of components of various sizes and in quantities from one-offs to mass production volumes.

The heating and cooling regimes required for hardening must be closely controlled if optimum results are to be obtained. There is a risk of component distortion occurring, due to a combination of factors, including the relief of stresses induced by the prior manufacturing history, the production of stresses due to the volume changes accompanying the crystallographic changes during hardening and temperature gradients set up by variations in the cross section of the treated component.