Cryogenic Treatment of Stainless Steels

Cryogenic treatment of stainless steels is one of the methods that can be used to reduce microstructural defects that commonly occur in stainless steels. Through cryogenic treatment, the general strength properties of stainless steels can be improved, and plasticity can also be effectively preserved with additional heat treatment. The word "cryogenic" comes from two Greek words - "kryos" meaning "ice" or "to freeze," or "genic" meaning "to produce" or "to create." Technologically, it means the examination and use of materials (or other requirements) at very low temperatures. Deep sub-zero treatment of metals and alloys is a deep stress relief technology. When material is subjected to any manufacturing process, it experiences stresses. Stress manifests itself in the nature of defects in the crystal structure of materials. The most commonly observed defects are voids, dislocations, stacking faults, etc. As the stress level increases, the density of these defects increases and causes an increase in the inter-atomic gap. When the distance between atoms exceeds a certain critical distance, cracks form and fracture occurs. The third law of thermodynamics states that entropy is zero at absolute zero temperature. Deep sub-zero treatment uses this principle to reduce stresses in the material. Materials are exposed to extremely low temperatures over an extended period, which leads to the development of equilibrium conditions. This leads to the elimination of defects in the material and also to reaching a minimum entropy state. According to the laws of thermodynamics, the lowest temperature that can be achieved has a limit known as absolute zero. Molecules are at their lowest, but finite energy at absolute zero. Absolute zero is zero on the absolute or thermodynamic temperature scale. It is equal to –273.15°C or –459.67°F. In terms of the Kelvin scale, the cryogenic region is generally considered to be below approximately 120 K (–153°C). At the temperatures shown in Table 1, at atmospheric pressure, the common permanent gases previously referred to as transitioning from gas to liquid are called normal boiling point (NBP). Such liquids are known as cryogenic liquids or cryogens. As mentioned above, temperatures below 273K are generally considered low temperatures. Operating conditions below the freezing point should be included in the construction, transportation, and energy sectors of pipelines. Steels designed under cryogenic conditions are designed for the transport of flowing gases. Finally, it is important to mention the superconductivity of materials secured under temperature conditions below 273K. In materials deformed at temperatures below 273K, dynamic recovery processes are completely prevented and cause accumulation of dislocations. With increasing dislocation density, their movement slows and concentration intensifies. This results in shear and mechanical twinning, which causes the plastic deformation mechanism, the main mechanism of conventional plastic deformation being dislocation glide. With further increase in stress, it is possible to assume that grain size is refined to the nanometer scale. Grain size is related to the mechanical properties of materials; this means that by shaping under cryogenic conditions, it is possible to achieve an increase in the mechanical properties of selected metallic materials. The entire hardening process of steels consists of austenitizing, water quenching, cryo-treatment or deep cryogenic treatment (DCT), and tempering. In order to obtain the most desired properties of steel, to achieve a better microstructure, most researchers recommend that DCT be performed after quenching is completed and, as shown in Figure 1, before tempering in the conventional heat treatment cycle. The objective of A. Fedorikova's work was to define the effect of plastic deformation under cryogenic conditions on mechanical properties of nitrogen-strengthened austenitic stainless steel (SS) 316LN. The steel was shaped by hot forging and then subjected to heat treatment at 1050°C for 60 minutes followed by rapid cooling to room temperature. For this study, samples were cut from the center of the sheet. The chemical composition of the test material is listed in Table 2. To compare results, two types of experimental rolling were performed: at room temperature and under cryogenic conditions. Samples were rolled with approximately 10% reduction per pass, with total deformations of 10%, 30%, and 50%. Cryogenic rolling conditions were secured by immersing samples in liquid nitrogen for 30 minutes before rolling and 10 minutes after each pass. Sample dimensions for experimental rolling: h0 = 11mm, b0 = 41mm, l0 = 80mm. Static tensile testing was performed on a universal test machine (ATLAS, load capacity 650kN) at three temperatures ranging from 4.2K to 293K. Cryogenic temperatures were secured with a cryostat system integrated into the Atlas test machine filled with liquid helium (4.2K) and liquid nitrogen (77K). The mechanical properties obtained for all tests are listed in Table 3. It can be said that the yield strength and ultimate tensile strength of modified 316LN increase with deformation, and there is an effect of rolling temperature on final mechanical properties. On the other hand, total elongation decreases significantly with deformation due to the exhaustion of plasticity during experimental rolling and increased deformation resistance of the material. Therefore, heat treatment should be performed after the rolling process. Based on the results obtained by the experiment performed, the following conclusions can be formulated: - With higher deformation and decreasing test temperature, yield strength and ultimate tensile strength increase while plasticity decreases significantly. - Austenitic stainless steel can be strongly strengthened by plastic deformation under cryogenic conditions. - Additional heat treatment is required to preserve the plasticity of the final material. However, there are also applications where high strength is required without the need for plastic properties. Such strong materials can be prepared by rolling under cryogenic conditions. References • 1. P. I. Patil, R. G. Tated: Comparison of Effects of Cryogenic Treatment on Different Types of Steels: A Review, International Conference in Computational Intelligence (ICCIA) 2012 Proceedings published in International Journal of Computer Applications (IJCA), p.10-29; • 2. A. Fedoriková: Effect of plastic deformation on mechanical properties of austenitic stainless steel under cryogenic condition, METALLURGY JUNIOR 2018, Proceedings, 10. - 11. May 2018, Herľany, Slovak Republic, Faculty of Materials, Metallurgy and Recycling Technical University of Košice, p.39-42, Accessed July 2018   Dr. Zoran Slović \ Chief Engineer Key to Metals AG Total Materia