Cryogenic treatment
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A cryogenic treatment is the process of treating workpieces to cryogenic temperatures (i.e. below −190 °C (−310 °F)) to remove residual stresses and improve wear resistance on steels.
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.
Principle
Liquid nitrogen (LN2) is a commonly used to cool the workpiece, because it boils at −195 °C (−319 °F), therefore the workpiece is cooled to near this temperature. 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 turn them brittle instantaneously.
As the material cools its molecular structure contracts, which relieves residual stresses. This occurs because the low temperatures cause the material to have a low 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 carbides. All metals including copper and aluminum, not just steel benefit from the residual stress relief that cryogenic treatment promotes.[1]
Processes
Cryogenic deburring
Cryogenic hardening
Cryogenic rolling
Cryogenic rolling, also known as 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.
Audiophiles
Many British/European and American/canadian companies who supply accessories to their customers currently rave about the benefits of DCT on both equipment and all types of cables and also on Compact Discs. In the 1980's some audiophiles experimented with their CDs by freezing them rapidly in the freezer compartment of their domestic fridge followed by a slow de-thawing-out over 24 hours in the chiller compartment claim to have experienced a significant improvement in the sound quality from their CDs treated in this way. In the UK an audio accessory dealer offers a full DCT service for CD owners. The suspicion is that the transparent side of the CD has its refractive index improved as well as microscopic gas bubbles removed in the transparent plastic layer.
References
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