Cryogenic treatment of stainless steels is just one method that can be used to reduce commonly occurring microstructural defects in stainless steels.
With cryogenic treatment the general strength properties of stainless steels can be improved and with the additional heat treatment, plasticity can also be effectively preserved.
The word, “Cryogenics” is taken from two Greek words – “kryos” which means ‘frost’ or freezing, and “genic” meaning to ‘produce’ or generated. Technologically, it means the study and use of materials (or other requirements) at very low temperatures. Deep Sub-zero treatment of metals and alloys is a deep stress relieving technology. Whenever material is subjected to any manufacturing operation, it is subjected to stresses. The stress manifests itself in the nature of defects in the crystal structure of materials. The most commonly observed defects are in the form of vacancies, dislocations, stacking faults etc. As the level of stress increases, the density of these defects increases, leading to increase in inter atomic spacing. When the distance between the atoms exceeds a certain critical distance, cracks develop and failure takes place. The third law of thermodynamics states that entropy is zero at absolute zero temperature. Deep subzero treatment uses this principle to relieve stresses in the material. The materials are subjected to extremely low temperatures for a prolonged period of time leading to development of equilibrium conditions. This leads to ironing out of the defects in the material and also attainment of the minimum entropy state.
According to the laws of thermodynamics, there exists a limit to the lowest temperature that can be achieved, which is known as absolute zero. Molecules are in their lowest, but finite, energy state at absolute zero. Absolute zero is the zero of 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 often considered to be that below approximately 120 K (-153°C). The common permanent gases referred to earlier change from gas to liquid at atmospheric pressure at the temperatures shown in Table 1, called the normal boiling point (NBP). Such liquids are known as cryogenic liquids or cryogens.
The complete treatment process of the steels consists of hardening that is austenitizing and quenching, cryo-treatment or deep cryogenic treatment (DCT), and Tempering. To achieve better microstructure of the steel to get most desired properties, it is recommended by the most researchers to execute DCT after completion of quenching and before tempering in conventional heat-treatment cycle as shown in Figure 1.
Mechanical characteristics obtained for all tests are listed in Table 3. It can be said that yield strength and ultimate tensile strength of modified 316LN increase with deformation and the rolling temperature has impact to the final mechanical properties. On the other hand, the total elongation significantly decreases with deformation what is caused by exhaustion of plasticity and increase of the deformation resistance of material during experimental rolling. Therefore, after rolling process, the heat treatment should be done.
– With higher deformation and decreasing test temperature, the yield strength and ultimate tensile strength increase, while plasticity is significantly decreases. – The austenitic stainless steel can be strongly reinforced by plastic deformation under cryogenic conditions.
– In order to preserve plasticity of the final material, additional heat treatment is required. But there are are applications where the high strength is required without any need for plastic properties. Such strong materials can be prepared by rolling under cryogenic conditions.
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.
Date Published: Sep-2018