cryogenics
what is the deep cryogenic treatment?
Depending on the material used, a mostly undesirable unstable phase remains in the steel after heat treatment – the retained austenite. The targeted transformation of this phase into a martensitic structure can be achieved by combining conventional heat treatment techniques with deep cryogenic treatment. Deep cryogenic treatment is often the technically and economically simplest and most sensible method for achieving the desired material properties and preventing structural changes during use.
Deep cryogenic treatment is a process in which workpieces are treated at a temperature below room temperature, often in the range between -20 °C to -80 °C. The treatment can be a single or multiple additional steps in the hardening process to achieve optimal results in terms of hardness and structure of the material, or it can be part of an ageing process.
deep cryogenic treatment process
In the case of high carbon steels (> approx. 0.6%) or steels after case-hardening, the quenching temperature of the medium used is not sufficient to achieve a monophase hardening structure (martensite). This means that the austenite structure present during the hardening process at high temperatures cannot be completely transformed and a certain amount of austenite remains, the so-called retained austenite. This retained austenite is significantly softer than the hardened structure, and also has a smaller atomic lattice volume. This could lead to problems when using the components. For this reason, this retained austenite usually has to be subsequently converted into a hardened structure (martensite). The transformation point, depending on the respective carbon content, is in the temperature minus range (< 0 °C).
Liquid nitrogen is used for deep cryogenic treatment, which is converted into gaseous nitrogen in a chamber. The process involves slow cooling to treatment temperature, a certain holding time and slow heating to room temperature.
Each freezing process must be followed by a new tempering process.
advantages of the deep cryogenic treatment
The advantage of deep cryogenic treatment is that the retained austenite transformation can be carried out economically and relatively quickly without having to expose the components to a higher temperature, which is often not possible for technical reasons.
- Achievement of a higher surface hardness
- Reduction of residual structural stresses close to the surface
- Reduction of the risk of cracking
fields of application
Particularly important for components that must not undergo any changes in volume or length due to structural changes during use, e.g. measuring equipment, tools, automotive and aerospace components. They also require maximum surface hardness.
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frequently asked questions – deep cryogenic treatment
What is the deep cryogenic treatment?
The deep cryogenic treatment of steel is a process step in heat treatment in which the workpiece is not heated but deep-frozen to achieve the necessary structural change. In most cases, the residual austenite is converted into a martensitic hardening structure.
In which areas is the cryogenic hardening of steel used?
Mostly for steels with a higher carbon content or for steels that have been case-hardened. Deep cryogenic treatment always takes place after conventional heat treatment.
How long does the cryogenic hardening of steel take?
The entire process (cooling, holding, heating) can take several hours, depending on the component geometry and special customer specifications.
Can all steel parts or components be cryogenic hardened or are there restrictions?
All components can actually be deep-frozen. You should only avoid thermal shock situations. This often happens when components are placed in liquid nitrogen at -196 °C for deep-freezing. Cracking is then possible with complicated component geometries.
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