【Haichuan Chemical Valve Knowledge】What is the purpose of cryogenic treatment for low-temperature valves?

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In cryogenic applications such as liquefied natural gas, air separation units, low-temperature chemical processes, and the storage and transportation of liquid oxygen or liquid nitrogen, cryogenic valves serve as key components for fluid control. Their performance in temperature ranges from -40°C to -196°C directly determines the intrinsic safety and operational continuity of the entire process. Cryogenic treatment is not a supplementary or optional manufacturing process; rather, it is a core component of low-temperature valve engineering. It systematically addresses the key technical challenges in ultra-low temperature environments by considering various factors such as the material’s microstructure, dimensional stability, sealing integrity, and fatigue life. The technical essence of cryogenic treatment lies in inducing a controlled martensitic phase transformation in the austenitic stainless steels commonly used for low-temperature valves (such as 304, 316L, F304, etc.). These materials exhibit excellent toughness and workability at room temperature, but their austenitic structure is thermodynamically metastable. When the operating temperature drops below the martensitic transformation start point (Ms point) of the material, the residual austenite in the structure undergoes an irreversible martensitic transformation spontaneously, accompanied by a volume expansion of about 2%-3%. If this phase change occurs suddenly during operation, it will cause local bulging of the valve seat sealing surface, bending deformation of the valve stem, and loss of the clearance between the mating parts, ultimately leading to internal or external leakage. Deep cryogenic treatment involves placing the components in a liquid nitrogen environment at -196°C prior to finishing, and by controlling the temperature according to a specific curve of gradual cooling, holding at that temperature, and then gradual warming, it induces the residual austenite to undergo martensitic transformation in a timely, uniform, and controllable manner. This process also releases the casting stresses and machining stresses within the material, thereby eliminating the risk of uncontrollable deformation during operation from the very beginning. From the perspective of the specific mechanism of action, cryogenic treatment first ensures the dimensional stability of the parts. By performing two cryogenic treatments after rough machining but before finish machining, it is possible to keep the dimensional change rate of the key components of the valve within 0.01% in an environment with alternating hot and cold conditions. This means that at -196°C, the fit between the valve seat and the valve stem can remain at its designed value, thereby completely preventing the formation of leakage paths caused by low-temperature deformation. Secondly, cryogenic treatment can enhance the overall mechanical properties of the material. The martensitic transformation induced by this process, along with the subsequent grain refinement and reduction of lattice defects, enables the release of 60%-80% of the residual stress within the material. Meanwhile, the higher hardness of martensite itself and its finer lath structure allow the material to maintain good low-temperature impact toughness while also seeing improvements in hardness and tensile strength. Practical applications have shown that valve components subjected to standardized cryogenic treatment can see their resistance to brittle fracture and fatigue life increased by 2–3 times. Third, cryogenic treatment significantly improved sealing reliability. For the sealing pair to maintain its fit accuracy over long periods in extremely cold environments, it is essential to meet A-class zero-leakage standards such as ISO 15848 or TA-Luft. Cryogenic treatment eliminates the uneven expansion and contraction of the sealing surfaces at low temperatures, while also reducing the likelihood of icing and sticking of these surfaces. This approach is particularly suitable for meeting the stringent sealing requirements associated with highly hazardous materials such as LNG and liquid hydrogen. Furthermore, cryogenic treatment increases the density of the material, reduces microsegregation and microscopic pores, making it more difficult for corrosive agents to penetrate along the grain boundaries. Meanwhile, the dispersed carbides that form within the martensitic matrix can significantly reduce the wear rate of the sealing surfaces, thereby extending the service life of the valves in low-temperature media containing trace amounts of corrosive impurities such as H₂S and CO₂.