Figure 1: Typical Cryogenic Treatment Cooling Curve. Image: Republished with permission of ASM International, from ASM Handbook, Vol 4A, F. Diekman, “Cold & Cryogenic Treatment of Steel,” 2013; permission conveyed through Copyright Clearance Center, Inc.
Cryogenic treatment is the process of cooling materials to cryogenic temperatures temporarily to improve their material properties at room temperature. This is distinct from cooling materials down to cryogenic temperatures to take advantage of phenomena such as superconductivity that only occur at cryogenic temperatures. Cryogenic treatment, some- times also referred to as deep cryogenic treatment, is best thought of as an adjunct to other material processing steps such as heat treatment, quenching and cold work.
Depending on both the process applied and the material used, a number of material properties may be improved, including hardness, wear resistance (thus increasing lifetime), fatigue life and electrical conductivity. As an example, Barron (1982) described an increase of wear resistance of tool steel treated down to 77 K and ascribed the cause to a more complete transition from the austenite phase to the harder martensite phase.
While steel was one of the first materials to undergo cryogenic treatment—and the use of this technique to increase the lifetime of machine tools is one of its major applications—cryogenic treatment has been applied to a wide range of materials including Aluminum, brass, titanium, nickel alloys, some plastics and even carbon nanotubes.
Cryogenic treatment generally occurs at roughly 77 K (liquid nitrogen temperatures). Figure 1 shows a typical time versus temperature curve for cryogenic treatment. Some processes use dry ice temperatures (189 K) Which, while above the nominal 120 K limit of cryogenics, are also some times referred to as cryogenic treatment.
Cryogenic treatment is a very active area of research that has produced many reproducible and industrially useful results. However, cryogenic treatment is not a panacea and not all claims can be scientifically verified. Care must be taken in interpreting results in this area. Towards that end, the Cryogenic Society of America has established the Cryogenic Treatment Database (https://www.cryo- genictreatmentdatabase.org).
This database, updated quarterly, con- tains articles and scientific papers that have been vetted by an impartial committee of experts. Other good sources of information on cryogenic treatment include the proceedings of the International Cryogenic Materials Conference and the journal Cryogenics.
A related topic is cryogenic machining. Here the material is cooled to cryogenic temperatures during the machining pro- cess. This is done to optimize the machining or to optimize the final machined piece, for example in terms of its surface finish, porosity or hardness. Again, as in the case of cryogenic treatment, the resulting material is typically used at room temperature.
A detailed introduction to cryogenic treatment is given in “Cold & Cryogenic Treatment of Steel” by F. Diekman, ASM Handbook, Vol. 4A (2013). Other examples of cryogenic treatment include: ”Internal Friction Measurements of Phase Transformations During the Process of Deep Cryogenic Treatment of a Tool Steel,” Shaohong Li, et al., Cryogenics 57 (2013); “Cryogenic Treatment of Metals to Improve Wear Resistance,” R.F. Barron, Cryogenics, August (1982); “Effect of Cryogenic Treatment on the Plastic Property of Ti-6Al-4V Titanium Alloy,” K.X. Gu, et al.; Advances in Cryogenic Engineering (AIP Conf. Proc. 1574, 42 (2014)); and “Improvement of Carbon Nanotubes Using Cryogenic Treatment,” Dae-Weon Kim, et al. Japanese Journal of Applied Physics, Vol. 46, No. 45 (2007).
Applications of cryogenic machining are given in “Cryogenic Machining of Porous Tungsten for Enhanced Surface Integrity ,” J. Schoop, et al., Journal of Materials Processing Technology, 229 (2016); “Cryogenic Machining of Biomedical Implant Materials for Improved Performance, Life and Sustainability,” I.S. Jawahir, et al. Procedia CIRP (2016); and “Enhanced Surface Integrity of AZ31B Mg Alloy by Cryogenic Machining Towards Improved Functional Performance of Machined Components,” Z. Pu, et al., International Journal of Machine T ools and Manufacture, 56 (2012).