H-O-T uses the environmentally friendly method of vacuum technology to harden medium to high-alloyed
steels. Vacuum hardening is a thermal process that achieves excellent results, especially with workpieces
that are prone to distortion. With precisely controllable parameters and a great deal of practical knowledge
accumulated over 50 years, we ensure high-quality results in mass production.


Areas of application

Automotive industry | Medical technology | Aerospace industry
Electrical industry | Textile industry | Mechanical engineering | Tool making


Material groups

Medium to high-alloyed steels

Vacuum hardening heat treatments

  • Stress-relief annealing

    Stress-relief annealing is necessary when existing residual stresses have a detrimental influence on distortion behavior during hardening. Corrections of the resulting changes in shape and size are taken into consideration by adding a certain machining allowance. The temperature must be below the Ac1 transition temperature but should nevertheless be as close as possible to this temperature so that it is not necessary to maintain the temperature after heating. Heating and cooling must be carried out in such a way that no additional or new residual stresses can develop. In the case of cold-formed tools, the normalizing process is preferred, whereas in the case of stress-relief annealing, coarse grain formation might occur as a result of re-crystallization.

    Quenching and tempering

    Hardening with subsequent tempering, mostly above 550°C, to reach a desired combination of mechanical properties, in particular to increase the toughness compared to the hardened condition.


    Hardening serves to achieve a high hardness in the component, preferably by martensite accumulation. It consists of the following two stages: austenitizing and cooling at an adequate speed.

    Precipitation hardening

    Process to increase stability by accumulation caused by precipitation. It consists of the solution treatment and precipitation heat treatment sub-processes.


    A single or repeated heating of a hardened component to a pre-determined temperature (<ac1), maintenance of the given temperature and subsequent adequate cooling.

    Soft annealing

    In the soft annealing process, heat treatment takes place to achieve the lowest possible hardness with high malleability at the same time. By heating and maintaining the temperature in the range of the Ac1 transition temperature for several hours, hardening structure constituents and strain hardening are eliminated and the cementite fins of the pearlite take on a spheroid form (this is also referred to as annealing to achieve spheroidal carbides). In most cases the soft annealed condition is the most suitable form for machining and cold forming as well as hardening. Exceptions are steels with lower carbon contents that tend to “smear” when being turned or milled.


    Consists of heat treatment to austenitize the steel, together with subsequent cooling in calm air.

    Structural heat treatment

    Like stress-relief annealing, the purpose of structural heat treatment is to disperse stresses caused by machining the material; the process is normally carried out after rough machining. The treatment consists of hardening followed by soft annealing. As a result of the change in structure, reduced changes in shape and dimensions can be anticipated during subsequent heat treatment.

    Steam tempering

    A dark blue-grey-black oxide layer (magnetite) is deposited on the surface of the tools during steam tempering. As well as changing the optical appearance, steam tempering improves corrosion-resistance and minimizes the coefficient of friction.

    Sub-zero cooling

    Treatment after hardening to convert a large amount of the residual austenite to martensite. The treatment consists of a cooling process and the maintenance of a given temperature below room temperature.

H-O-T hardening shop –
Special treatment processes upon customer request

  • Partial tempering by induction
  • Special batches for complex shapes
  • Sub-zero cooling processes
  • Special heat treatment for workpieces that are subsequently coated, eroded, photo-etched or are subject to further heat treatment such as nitriding
  • Oxidation of tools in our vacuum furnaces