Cryogenic treatment
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A cryogenic treatment is the process of reducing the temperature of components over an extended period of time to extreme cold levels, usually slightly below −190 °C (−310.0 °F), which is why it is called a cryogenic process. Liquid nitrogen (LN2) is a common fluid for the process being relatively inexpensive and making up more than 70% of our atmosphere.
As the LN2 boils off from liquid to gas at around −195 °C (−319.0 °F), the components in its proximity are also cooled. The process is controlled by microprocessors so that thermal shock is not generated at the same time resulting in damage to components. Before these microprocessors were created, people would dip parts in liquid nitrogen and virtually turn them to brittle instantaneously.
As the material cools its molecular structure is drawn together through contraction and stress and dislocation brought about by production methods is removed or reduced. Both Einstein and Bose of Germany realized why cryogenic treatment was able to remove residual stresses. Cryogenic treatment removes heat from an object which then allows the object to enter its most relaxed state or a condition with the least amount of kinetic energy. After heat treatment, steels still have a certain percentage of retained austenite which can be transformed into martensite via cryogenic treatment. Other effects are the production of martensite and the precipitation of Eta type carbides. All metals including copper and aluminum, not just steel benefit from the residual stress relief that cryogenic treatment promotes.[1]
The process has a wide range of applications from industrial tooling to improvement of musical signal transmission. Some of the benefits of cryogenic treatment include longer part life, less failure due to cracking, improved thermal properties, better electrical properties including less electrical resistance, reduced coefficient of friction, less creep and walk, improved flatness, and easier machining.
It has been found and proved that cryogenic treatment improves wear resistance of many alloy steels to a great extent.
Cryorolling
Cryorolling is one of the potential techniques to produce nanostructured bulk materials from its bulk counterpart at cryogenic temperatures. It can be defined as rolling that is carried out at cryogenic temperatures. Nanostructured materials are produced chiefly by severe plastic deformation processes. The majority of these methods require large plastic deformations (strains much larger than unity). In case of cryorolling, the deformation in the strain hardened metals is preserved as a result of the suppression of the dynamic recovery. Hence large strains can be maintained and after subsequent annealing, ultra-fine-grained structure can be produced.
Advantages
Comparison of cryorolling and rolling at room temperature:
- In Cryorolling, the strain hardening is retained up to the extent to which rolling is carried out. This implies that there will be no dislocation annihilation and dynamic recovery. Whereas in rolling at room temperature, dynamic recovery is inevitable and softening takes place.
- The flow stress of the material differs for the sample which is subjected to cryorolling. A cryorolled sample has a higher flow stress compared to a sample subjected to rolling at room temperature.
- Cross slip and climb of dislocations are effectively suppressed during cryorolling leading to high dislocation density which is not the case for room temperature rolling.
- The corrosion resistance of the cryorolled sample comparatively decreases due to the high residual stress involved.
- The number of electron scattering centres increases for the cryorolled sample and hence the electrical conductivity decreases significantly.
- The cryorolled sample shows a high dissolution rate.
- Ultra-fine-grained structures can be produced from cryorolled samples after subsequent annealing.