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In cryogenic processing, materials are cooled to extremely low temperatures (-196 ºC) to achieve the desired metallurgical and microstructural properties. Reaching these temperatures is made possible by feeding the system with computer-controlled liquid nitrogen (N2) and using the most suitable insulation materials. Cryogenic processing is divided into "deep" and "shallow" types. In shallow cryogenic processing, the temperature of the materials is lowered to around -150 ºC. Although there is a significant improvement in the mechanical properties of the materials, the desired properties are not fully achieved. In deep cryogenic processing, the materials are cooled to -196 ºC, and the effects of this process are significantly increased compared to shallow cryogenic processing.
Application of Cryogenic Processing
Cryogenic processing can be applied to a wide range of materials, including:
- Steels,
- Cast irons,
- Non-ferrous metals,
- Alloys,
- Carbides,
- Plastics,
- Ceramics.
The process involves cooling at rates of 1-2 ºC/min and takes between 36 and 72 hours, varying according to the type and weight of the material.
There are three cycles in cryogenic processing:
- Controlled Cooling
- Soaking
- Controlled Heating
Due to the computer-controlled system and prevention of liquid nitrogen-part contact, there is no change in the dimensions of the part, and the risk of cracking is eliminated. This special process is not a surface treatment; it creates the same effect throughout every point of the material.
Effects of Cryogenic Processing
Transformation of Residual Austenite to Martensite
In traditional heat treatment processes, to harden steels, austenite phase is first obtained and then this phase is rapidly cooled to obtain the martensite phase. However, not all austenite phase can be transformed into martensite in traditional heat treatment. At this point, cryogenic processing comes into play and transforms almost all residual austenite phase into martensite.
Change in Carbide Forms
In traditional hardening methods, carbides enter the crystal structure, causing stresses in the crystal structure. During cryogenic processing, carbide particles are freed from the crystal lattice and distributed within the material. Thus, residual stresses decrease significantly, and high hardness carbide structures are formed, leading to a significant increase in wear resistance.
Refinement of Grain Structure
Atoms forming the alloy prefer to stay where they are most stable in the structure. Through cryogenic processing, these atoms achieve an optimum arrangement, leading to a refinement of the grain structure. As a result, molecular bonds strengthen, increasing the material’s wear resistance.
Relief of Internal Stresses
Due to the rearrangement of atoms and the resulting refinement of the grain structure, atomic vacancies (defects) in the crystal structure are eliminated, increasing the atomic density per unit volume. This significantly reduces internal stresses in the material.
As a result of these microstructural changes, materials exhibit increased strength, toughness, and hardness, with a significant increase in wear resistance. In particular, the increase in wear resistance can reach up to 800%. As a result, the lifespan of the material is significantly extended.
Increases in wear resistance after shallow and deep cryogenic processing in some commonly used metallic materials in industry.
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Significant Benefits of Cryogenic Processing
- Extends the service life of materials
- Permanent and applied only once
- Increases wear resistance
- Increases tensile strength and toughness
- Eliminates residual stresses
- Reduces brittleness
- Affects not only the surface but the entire material
Cryogenic processing is a continuation of traditional heat treatment and allows for the achievement of high mechanical properties. Cryogenic processing, widely applied in Europe and America with the presence of large companies, has become quite common abroad. However, in our country, despite having such superior features, cryogenic processing applications are almost never used. Our company aims to create awareness in this regard through its well-equipped team and academic experiences.
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