Rohdetherm

Hardening

Hardening is defined by DIN 17014 as follows: Cooling from a temperature above the transition points A3 or A1 at such a speed that the surface or entire workpiece incurs a significant increase in hardness, generally due to the formation of martensite. The heating must be carried out to a temperature above the transition points Ac1 or Ac3 and the cooling from a temperature above the transformation points Ar3 or Ar1. The hardening comprises a heating, correspondingly long holding of the workpiece at the hardening temperature and then quenching. The steel obtains a martensitic microstructure as a result of the hardening. The degree of hardness achieved depends firstly on the carbon content and secondly on the dimensions of the workpiece. Accordingly, in order to achieve the highest degree of hardening for steel, approx. 0.8 % carbon content is required. Heating to the hardening temperature must be implemented evenly and comprehensively. In general, the pre-heated workpiece is placed in the hardening oven, which is already at the hardening temperature.

The carbon required for hardening is bound in carbides prior to hardening and these must be released through holding at the hardening temperature. The ability of the carbides to be released varies depending on their composition, whereby for example, tungsten carbide and vanadium carbide are considered difficult to release.
The hardening temperature of all steels must lie at least 30 °C above the transition point.

Great care and attention must be paid to the holding time at the hardening temperature. Too long a holding time creates large dimensional changes and grain coarsening. Too short a holding time does not allow all of the carbides to be released and this results in a lower hardness after quenching. Nowadays there are various different processes and techniques in use. Hardening is carried out in electrically-heated or gas-heated furnaces. A further option is salt baths.
The chemical composition of the steel must not be changed by the hardening process. It is said that steel must be ‘neutrally’ hardened. This is guaranteed through the use of protective gases or through evacuating the interior of the furnace (vacuum systems). It is important that the carbon potential of the protective gas or salt bath is appropriate for the carbon potential of the workpiece to be hardened so that there is no transfer of carbon either from the workpiece to the environment or the reverse. After the correct heating and sufficient holding at the hardening temperature, workpieces are quenched. Quenching is carried out in water, oil, air or gases (N2, Ar), whereby the most appropriate quenching medium must be selected to suit the type of steel being used.

Quenching in a warm bath is another option. With this method the workpieces are quenched in a salt bath, the temperature of which is just above the martensite point. The workpiece is held at this temperature until it has adopted the temperature itself. Then it can be cooled further in air and only then will the martensite formation occur.