The word cryogenics is taken from two Greek words “Cryo” – meaning frost or freeze and “genics” – meaning produce or generate. In physics, cryogenics is the study of production of low temperatures and to study behavior of materials at very low temperatures. Some of the advantages of cryogenic treatment includes:
- Not only the surface, the whole structure changes
- Grinding and other surface finishing processes does not effect
- Increases wear resistance
- Increases performance and durability
- Increases tensile strength and toughness
- Release internal stresses
Conventional cryogenic treatment consists of a slow cool down of -3oC per minute from ambient to -196oC, a soak from 24 to 72 hours in liquid nitrogen. Slow cool down cycles helps in controlling the temperature more accurately and avoiding thermal shock to the material. At this temperature, the conversion of austenite to martensite occurs slowly, but almost completely. The parts are then warmed up to ambient temperature. The next step is temper treatment (149oC to 538oC) for a minimum of one hour, of the cryogenically treated parts in order to stabilize the martensite. This kind of conventional cryogenic treatment may leave some small pockets of austenite to get transformed into martensite .
Deep cryogenic treatment proves to be a solution for this problem. In deep cryogenic processing parts are cooled to more than 300°C below zero compared to about -120°C used in conventional cryogenic processing. Deep cryogenic processing involves cooling parts at a controlled rate which is governed by a microprocessor .
Cryogenic cooling systems commonly used are: Gradual immersion, direct nebulization and heat exchanger. Accessories include containers, pressure vessels, cold traps, purifiers and piping. Transfer lines are used for safe transportation of supercooled liquids, such as liquid helium, nitrogen, oxygen and argon .
Wear resistance via cryogenic treatment
Several researchers used different kinds of tests with different kinds of steels or other materials in order to show increase in wear resistance via conventional or deep cryogenic treatment and this makes it harder to compile all results together. Some of the most popular theories behind the increase in wear resistance via cryogenic treatment are depicted below :
- Cryogenic process promotes complete transformation of retained austenite into martensite, and this can be attributed to the enhanced wear resistance of the steels.
- Cryogenic process enhances the formation of fine carbides in martensite matrix with uniform distribution which is responsible to increase wear resistance.
- Tempering of low-temperature conditioned martensitic structure, result in the precipitation of a finer distribution of carbides in the microstructure with consequent increase in toughness as well as in wear resistance.
Apart from the above theories, there is also a great influence of process parameters:
- Longer cryogenic treatment times will result in even finer microstructure and higher surface hardness, which has no effect on abrasive wear resistance but it considerably improves galling properties .
- Increasing the austenizing temperature will lead to reduced volume fraction of carbides and consequently to increased friction and wear .
- For holding times of up to 24h a nearly constant wear rate was seen. The wear rate reaches a minimum for a longer holding time of 36h and increases again with further holding .
- Tempering of steels at around 200oC prior to cryogenic treatment helps in increasing hardness of steel. There is a possibility of creation of nuclei sites during the 200oC tempering, where new segregations of carbon and alloying elements could cluster during the cryogenic treatment producing an increase in the hardness .
- Cryogenic processing of metals, techcommentary, EPRI Center for material fabrication, 1251 Dublin Road, Columbus, OH 43215
- Free, Miles. “Martensite- Five Facts.” Speaking of Precision Blog. N.p., n.d. Web. 16 Nov. 2016. https://pmpaspeakingofprecision.com/tag/why-martensite-is-harder-than-austenite/
- Gerrow, Tom. “The Science of Cryogenic Processing.” Metal Science Services, Web. 16 Nov. 2016. http://www.metalscience.com/techinfo_ASM.php
- “Cryogenics.” Cryogenics | Certified Heat Treating Inc. Web. 16 Nov. 2016. http://www.heattreating.com/value-added-services/cryogenics/
- Patil, Nagraj, ShriShail Kakkeri, Dr., and Sangamesh. “Effect of Cryogenic Treated and Untreated Tool on Its Tool Life-Review.” International Journal of Science and Research (IJSR) 3.8 (2014): 141-145
- “More about Cryogenic Equipment.” ThomasNet. Web. 16 Nov. 2016. http://www.thomasnet.com/about/cryogenic-equipment-21111604.html
- Singh, Swarndeep, Simranpreet Singh, and Jagdev Singh. “Improvement in Tool Life of M2hss Tools by Cryogenic Treatment.” Journal of Metallurgical Engineering (2011): 47-61
- Podgornik, B., F. Majdic, V. Leskovsek, and J. Vizintin. “Improving Tribological Properties of Tool Steels through Combination of Deep-cryogenic Treatment and Plasma Nitriding.” Wear 288 (2012): 88-93.
- Oppenkowski, A., S. Weber, and W. Theisen. “Evaluation of Factors Influencing Deep Cryogenic Treatment That Affect the Properties of Tool Steels.” Journal of Materials Processing Technology 210.14 (2010): 1949-955.
- Preciado, M., P.m. Bravo, and J.m. Alegre. “Effect of Low Temperature Tempering Prior Cryogenic Treatment on Carburized Steels.” Journal of Materials Processing Technology 176.1-3 (2006): 41-44.
- Technical papers used – open access and abstract only; Image used is “Labelled for reuse with modification/ for free and use them commercially”.