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Hardening & Tempering

Hardening

Hardening is the heat treatment of steel, whereby heating to the Austenising temperature, followed by very rapid cooling results in the formation of martensite microstructure (hardened structure).

The austenising temperature and speed of cooling are dependant on the steel quality and can be calculated for every steel from their appropriate TTT-diagram.  The aim of hardening is to obtain a uniform microstructure with almost all the carbon in solution.  The TTT-diagram charts show the maximum possible hardenability of a steel.  The term „hardenability“ includes the extent as well as the distribution of the increased hardness within the component.

As a rule, hardening is generally followed by a further heat-treatment process: tempering.

Suitable Materials

All steels can be hardened to some degree

Advantages of Hardening

Increased resistance to drawing, compression, bending and abrasion.

Tempering

Tempering is a thermal process that, like hardening, changes the mechanical properties of the whole component from the surface to the core.

Tempering is carried out after quenching and complete cooling of the component and follows on immediately after hardening. 

Depending on the steel composition, the hardened component has a high degree of hardness but, at the same time, is also very brittle and cannot be used or worked in this condition.  A general rule of thumb is that the higher the hardness the lower the toughness. 

Tempering is used to reduce the hardness of a hardened component to enable the required toughness to be achieved.  The resulting loss in hardness must be tolerated. Hardness and toughness are integrally linked to each other and it is not possible to influence either property without having an effect on the other.

Tempering can be carried out in protective-gas equipment, vacuum equipment, salt-baths or in equipment under air (e.g. induction equipment). The process can be repeated as often as necessary.  The choice of atmosphere affects the surface characteristics of the component.

Suitable Materials

All qualities of steel.

Advantages of Tempering

  • Improvement in toughness
  • Reduction in brittleness
  • Reduction in stresses within the component resulting from hardening
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