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A cryogenic treatment is the process of treating workpieces to cryogenic temperatures (i.e. below −190 °C (−310 °F)) in order to remove residual stresses and improve wear resistance on steels. In addition to seeking enhanced stress relief and stabilization, or wear resistance, cryogenic treatment is also sought for its ability to improve corrosion resistance by precipitating micro-fine eta carbides, which can be measured before and after in a part using a quantimet.
The process has a wide range of applications from industrial tooling to the 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.
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Cryogenic hardening is a cryogenic treatment process where the material is cooled to very low temperatures. By using liquid nitrogen, the temperature can go as low as −190 °C. It can have a profound effect on the mechanical properties of certain materials, such as steels or tungsten carbide.
Applications of cryogenic hardening
- Aerospace & Defense: communication, optical housings, weapons platforms, guidance systems, landing systems.
- Automotive: brake rotors, transmissions, clutches, brake parts, rods, crank shafts, camshafts axles, bearings, ring and pinion, heads, valve trains, differentials, springs, nuts, bolts, washers.
- Cutting tools: cutters, knives, blades, drill bits, end mills, turning or milling inserts. There are two main types of cryogenic treatments of cutting tools: Cryogenic treatments of cutting inserts can be classified as follows: Deep Cryogenic Treatments (DCT) or Shallow Cryogenic Treatments (SCT). A different minimum tool cooling temperature is used in the two mentioned treatments: -196 °C for DCT and -80 °C for SCT.
- Forming tools: roll form dies, progressive dies, stamping dies.
- Mechanical industry: pumps, motors, nuts, bolts, washers.
- Medical: tooling, scalpels.
- Motorsports and Fleet Vehicles: See Automotive for brake rotors and other automotive components.
- Musical: Vacuum tubes, brass instruments, guitar strings and fret wire, piano wire, amplifiers, magnetic pickups, cables, connectors.
- Sports: Firearms, knives, fishing equipment, auto racing, tennis rackets, golf clubs, mountain climbing gear, archery, skiing, aircraft parts, high pressure lines, bicycles, motor cycles.
Cryogenic machining is a machining process where the traditional flood lubro-cooling liquid (an emulsion of oil into water) is replaced by a jet of either liquid nitrogen (LN2) or pre-compressed carbon dioxide (CO2). Cryogenic machining is useful in rough machining operations, in order to increase the tool life. It can also be useful to preserve the integrity and quality of the machined surfaces in finish machining operations. Cryogenic machining tests have been performed by researchers since several decades, but the actual commercial applications are still limited to very few companies. Both cryogenic machining by turning and milling are possible.
Cryogenic rolling or 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.
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. Where as 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.
- ASM Handbook, Volume 4A, Steel Heat Treating Fundamentals and Processes. ASM International. 2013. pp. 382–386. ISBN 978-1-62708-011-8.
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