|Annealing, Hardening, Tempering - Course: Working techniques of heat treatment of steel. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 27 p.)|
In practice, heat treatment is a systematic sequence of various processes of "heating" and "cooling".
In this way, special properties of unalloyed and alloy steels can be changed.
Such properties are:
Hardness - Strength - Toughness - Elasticity.
Figure 1 - Principle of heat treatment - 1 heating, 2 cooling
The appropriate heat treatment process is selected either to increase or to decrease the properties.
Therefore, temperatures to be set during heating, the period of heating and the cooling speed are of special importance. Cooling can be effected either quickly or slowly or in individual stages.
Properties of steels are changed because the crystal structure of steels having a latticed configuration is changed in its internal structure due to temperature factors.
According to the selection of the process, a structural change of the entire workpiece or of certain parts on the surface only can be effected.
Tools must be hard and robust so that they are suitable for working other materials. In general, they must, however, only be hard at points by which working is effected:
- cutting edge is hard and wear-resistant
- tang is tough and elastic
If the chisel would be hard throughout, it could break when the hammer is striked onto it!
Figure 2 - Cut through a hardened chisel - 1 cutting edge (hard), 2 twig (tough)
Measuring and testing means must not be worn out even when they are used frequently. They must especially be wear-resistant on their surfaces.
Moving machine parts (ball bearing, shafts, spur wheels) must not be worn out or only to a little extent. They must be hard and wear-resistant, but also tough at the same time.
Alloy steels containing apart from iron also other alloying elements have in general especially distinct properties.
Therefore, an exact heat treatment can only be carried out when the conditions given by the steel manufacturer are strictly adhered to. Precise temperature data for heat treatment processes are therefore supplied with by the steel manufacturer.