Scope of application and technical principles
A deep cryogenic treatment makes sense only for materials containing residual austenite at room temperature. Although primarily applied on high-alloy ledeburitic tool steels, it is also used for eutectoid tool steels. In the case of non-alloy and low-alloy steels, residual austenite forms only when the carbon content is at least 0.5%.
A deep cryogenic treatment is usually applied after hardening. However, because of the high risk of cracking during the cryogenic treatment, this now increasingly takes place after the first tempering treatment. The objective of this process is to eliminate dispersed η-carbide particles. However, this effect has not been proven conclusively.
It depends on the material whether the residual austenite content should be stabilised by repeated tempering or by deep cryogenic treatment.
Steels Subjected to deep cryogenic treatment are also hardened, and they achieve dimensional stability. This is achieved by cooling to a temperature between -90 °C and -196 °C. At these temperatures, the residual austenite in the material is converted to martensite.
Unless the residual austenite content is reduced, it may cause changes in the microstructure and volume of the component during subsequent use. The reason for this is the soft residual austenite, which becomes converted to the harder martensite over a number of weeks. The deep cryogenic treatment of steel prevents this slow gradual change in dimensional stability. This is especially important for precision components and high-precision tools.
INFO: Cryogenic methods
The development of various methods has made it possible to achieve ever lower temperatures during deep cryogenic treatments. Air is cooled down as low as -60 °C in deep-freeze chests or cabinets. Temperatures can be lowered far below -60 °C by means of mixtures of alcohol, dry ice, and liquefied gas. Finally, the use of liquid nitrogen and liquid helium allows cryogenic treatment down to -196 °C and -269 °C respectively.
What happens during deep cryogenic treatment?
In deep cryogenic treatment, the cooling process after hardening is extended, in order to accelerate the rate of conversion from austenite to martensite. For this purpose, the component is generally cooled to -90 °C. To achieve an even higher conversion rate, the material can also be cooled over an extended period to as low as –196 °C. This step is followed by at least one tempering cycle.
This procedure transforms the previously heterogeneous microstructure into a homogeneous lattice structure. This reduces residual stresses in the microstructure. Finally, hardness and wear resistance are increased because of the higher martensite content.
Cryogenic treatment is primarily intended to prevent slow gradual changes in dimensional stability and offers the following advantages:
- Consolidation of dimensional stability
- Reduction of residual stresses
- Reduced wear thanks to increased wear resistance
- Suitable for automation, and reproducible
- Ideal for precision tools
Basic requirements for deep cryogenic treatment are a carbon content of at least 0.5 % and a sufficient content of alloying elements with a martensite finish temperature (Mf) below 30 °C. All steels that meet these requirements are suitable for cryogenic treatment. Examples are:
- Ledeburitic chromium steels (e.g. 1.2080, 1.2379, 1.2436)
- High-speed steels
- Eutectoid tool steels (e.g. 1.2842)
Our deep cryogenic equipment has the following dimensions:
1,150 x 750 x 600 mm/500 kg
Refer to our interactive location overview to learn which Härtha locations offer deep cryogenic treatment of steel.
Customer specifications for deep cryogenic treatment
Would you like to subject your components to hardening and deep cryogenic treatment at our company? We will gladly provide you with a proposal. Please tell us the material designation, dimensions, weight, and quantity of workpieces to be treated.