Hardening up to 1,300°C
1. The process
Hardening is a heat treatment that consists of austenitizing and cooling, performed under conditions that allow an increase in hardness to be brought about by a more or less complete transformation of the austenite generally into martensite.
Austenitization is the stage of the process in which the workpiece is brought to austenitizing temperature and the steel matrix becomes austenitic through complete phase transformation and carbide dissolution. Austenitizing is followed by cooling. To allow the entire workpiece to take on a martensitic microstructure, the speed of the temperature drop must be greater than the critical cooling speed of the respective steel.
Cooling can take place in a variety of media; these differ characteristically in terms of their cooling effect in the various temperature ranges.
After hardening, the microstructure of so-called hypereutectoid steels is usually composed of martensite plus residual austenite plus carbide. Great importance is attached to the proportions of these phases, for instance in the heat treatment of tool steels, because characteristics such as resistance to wear and dimensional accuracy are affected by the structural state after hardening. Following hardening, tempering is necessary to relax the martensitic microstructure and to set the required strength. At low tempering temperatures (generally up to 250°C), the martensitic microstructure can be impacted on without any great compromise to hardness. Only at higher temperatures (dependent on the material, but as a rule at T>250°C), is structural relaxation accompanied by an appreciable loss in hardness. The component’s hardness properties can be adjusted by way of the tempering temperature. At considerably higher tempering temperatures (from around 400°C), the process is referred to as steel hardening and tempering. It is then possible to adjust both hardness and mechanical properties of the steel.
2. Suitable materials
In principle, any steel can be hardened to varying degrees. However, hardenability depends chiefly on the steel’s chemical composition. Hardenability is the ability of a steel to assume an increased hardness to a greater or lesser depth in the zone close to the surface. The term hardenability refers to both the extent and the distribution of the increase in the workpiece’s hardness. As a general rule, carbon is responsible for hardness. If there is no carbon in the steel (min. 0.22%C), it will hardly be possible to achieve an increase in hardness. Besides carbon, the quantity, type and number of alloy elements in the steel have an effect on hardness, toughness, through-hardenability, resistance to wear, and other mechanical values.
3. Benefits of this heat treatment
Hardening is used to give components and tools sufficient hardness and strength to withstand mechanical stress, such as static or dynamic deformation through tension, pressure, bending or wear.
4. Customer information required for heat treatment
The following information is required in all cases:
- Material designation
- Required hardness
- Processing state of the workpiece upon submission (e.g. preliminary hardening and tempering)