Mechanical properties of metals at ambient temperatures may be improved using various heat-treatment procedures, which have been established for many years. The concept of improving steel tensile and mechanical characteristics by exposure to low temperatures is quite old. For many years, certain industries have used subzero cold treatment to improve the serviceability of components or equipment. The effectiveness of cryogenic treatments makes them a highly preferred choice among material engineers.
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Basic Knowledge Regarding Cryogenic Treatment
Cryogenic treatment of materials involves the material being subjected to extremely low temperatures, resulting in a significant improvement in material performance. The procedure differs from cold treatment since it uses significantly lower temperatures, has separate time/temperature profiles, and may be used for materials other than steel. Cryogenic treatment significantly enhances wear resistance compared to cold treatment with hardened steels.
The scientific community defines cryogenic temperatures as those below -150 °C (238 °F, or 123 K). Cryogenic Treatment (CT) involves exposing materials to temperatures ranging from -190 °C to -230 °C for an extended length of time. CT has the effect of infiltrating the inside of materials rather than being restricted to the surface, making it a useful method for improving mechanical characteristics.
The development of microprocessor-based temperature controls in the 1960s and 1970s, as well as pioneering research by Randall Barron of Louisiana Tech University, facilitated successful cryogenic therapy.
The Difference in Cold Treatment vs Cryogenic Treatment Process
Cold treatment is commonly used for high-precision parts and components because it accelerates the transition of austenite to martensite. Depending on the material and quenching settings, the optimal temperature range for cold treatment is often -60 to -80°C as per the article published in the Journal of Materials Processing Technology. Many industries employ this type of treatment to increase their surface hardness and thermal durability. One notable distinction from cold treatment is that cryogenic processing necessitates a gradual decrease in temperature to fully realize its benefits. The temperature decline typically occurs at a rate ranging from 0.25 to 0.5 °C/min (32.5 to 32.9 °F/min).
The article highlights that deep cryogenic treatment performed in the range of −125 to −196°C enhances certain mechanical properties of tool steels beyond what is achieved through standard cold treatment.
The complete transformation from austenite to martensite, coupled with the formation of microscopically dispersed carbides in the tempered martensitic structure, is the primary factor contributing to this improvement. The most significant enhancement in properties occurs when the deep cryogenic treatment is conducted between quenching and tempering.
However, even treating the steel tools after the typical heat treatment cycle, i.e., the finished tools, yields a noteworthy improvement. This latter approach is more versatile, allowing the treatment to be applied for various practical applications.
The ASM Handbook, Volume 4A provides essential information regarding the processes for the treatment of steel. A standard cryogenic treatment involves a slow descent from ambient temperature to around –193 °C (–315 °F), where it is maintained for an appropriate duration. Based on the type of steel tool and the application, the specimen under treatment is held between 4 hours to 48 hours. It is followed by bringing the specimen back to room temperature at 2.5 °C/min (36.5 °F/min).
Effect on the Mechanical Properties of Steel
Cryogenic treatments have been known to augment the mechanical properties of steel. This was confirmed by researchers who performed an experimental study on AISI 4340 steel published in the Journal of Materials Processing Technology. The impact of cryogenic treatment before any subsequent tempering process revealed a slight rise in the average hardness from 54.5 HRC when the treatment was performed immediately after quenching. The cryogenically treated samples exhibited slightly higher hardness levels across the tempering temperature range compared to conventional heat treatment.
Regarding the impact energy absorption, the toughness after Deep Cryogenic Treatment (DCT) was measured at 7.7 J, while for regular quenching, it was 10.8 J. The slight decrease in toughness after cryogenic treatment could be attributed to an increase in the amount of martensite.
The fatigue limit of the steel demonstrated improvement after undergoing cryogenic treatment and subsequent tempering. This enhancement was linked to the increased hardness and strength of the material resulting from the treatment process. Hence, the cryogenic treatment led to a significant improvement in the mechanical properties of steel tools.
How Does Steel Type and Composition Affect the Performance of Deep Cryogenic Treatment?
In recent years, the promising technique of deep cryogenic treatment has taken a new step in improving the properties of various materials, especially steels. A study published in the Journal of Material Research and Technology focuses on the influence of a selected heat treatment process involving deep cryogenic treatment on the microstructure and microstructural evolution of four different steel grades (bearing steel 100Cr6, cold work tool steel X210Cr12, hot work tool steel X38CrMoV5-3, and stainless steel X17CrNi16-2).
The study was performed under different heat treatment conditions, emphasizing the effectiveness of DCT with various austenitizing and tempering temperatures. The evolution of the microstructure was investigated sequentially using various analytical techniques.
The transformation of retained austenite into other steel phases was more pronounced after higher austenitization and lower tempering temperatures. The effectiveness of DCT on matrix changes was mainly associated with the homogenization of the matrix.
In general, the application of DCT increased the precipitation of carbides after tempering. These carbides were also more homogeneously distributed, smaller, and had a more spherical shape. The DCT process induced the preferential formation of secondary transiting carbides M7C3 and M2.4C, directly contributing to the increased formation of M23C6 and M7C3 carbides. The dissolution of primary carbides is slightly increased after DCT, linked to the enhanced nucleation of secondary carbides.
Cryogenic treatments lead to a significant improvement in the mechanical and corrosion properties of steel, improving the lifetime of steel tools in extremely harsh environments and reducing financial losses.
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References and Further Reading
Dossett, J. & Totten, G., (2013). Cold and Cryogenic Treatment of Steel. [Online]
Available at: https://ctpcryogenics.com/wp-content/uploads/2017/12/asmcryodef.pdf
Molinari, A. et. al. (2001). Effect of deep cryogenic treatment on the mechanical properties of tool steels. Journal of materials processing technology, 118(1-3), 350-355. Available at: https://doi.org/10.1016/S0924-0136(01)00973-6
Saeed, Z., (2005). Effect of cryogenic treatment on the mechanical properties of steel and aluminum alloys. [Online]
Available at: https://spectrum.library.concordia.ca/id/eprint/8600/1/MR10278.pdf
Zhirafar, S., Rezaeian, A., & Pugh, M. (2007). Effect of cryogenic treatment on the mechanical properties of 4340 steel. Journal of Materials Processing Technology, 186(1-3), 298-303. Available at: https://doi.org/10.1016/j.jmatprotec.2006.12.046
Jovičević-Klug, P. et. al. (2021). Impact of steel type, composition and heat treatment parameters on effectiveness of deep cryogenic treatment. Journal of Materials Research and Technology, 14, 1007-1020. Available at: https://doi.org/10.1016/j.jmrt.2021.07.02
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