Under low temperature conditions, the strength and hardness of metal materials increase, but the plasticity and toughness decrease, showing different degrees of low temperature brittleness, which seriously affects the performance and safety of valves. In order to prevent low-stress brittle fracture of materials at low temperatures, when designing low-temperature valves, ferritic stainless steel materials are generally used when the temperature is higher than -100°C, and when the temperature is lower than -100°C, the valve body, valve cover, valve stem, Sealed valve seats are mostly made of austenitic stainless steel, copper and copper alloys, aluminum and aluminum alloys with face-centered cubic lattices. However, due to the low hardness of aluminum and aluminum alloy, the wear resistance and scratch resistance of the sealing surface are poor, so it is rarely used in low temperature valves.

1. Generally, austenitic stainless steel materials are mostly used, such as 0Cr18Ni9, 00Cr17Ni12Mo2 (SS304, SS316L), etc. These materials do not have a low-temperature cold and brittle critical temperature, and can still maintain high toughness under low temperature conditions.

2. Austenitic stainless steel also has some shortcomings as the metal sealing material of cryogenic valves. Because most of these materials are in a metastable state at room temperature, when the temperature drops below the transformation point (MS), the austenite in the material will transform into martensite. The martensite density of the body-centered cubic lattice is lower than that of the austenite of the face-centered cubic lattice, and because some carbon atoms are regularly arranged to occupy the position of the body-centered cubic lattice, the lattice grows along the C-axis direction, thus The change of volume causes the increase of internal stress, which causes warping and deformation of the sealing surface that meets the sealing requirements after grinding, resulting in sealing failure.

3. In addition to the deformation and failure of the sealing surface caused by the low-temperature phase change, the temperature change stress will also be generated due to the temperature difference between parts or the difference in physical properties between different materials, which will cause uneven shrinkage. When the stress is lower than the elastic limit of the material, reversible elastic distortion will occur on the sealing surface. When the temperature stress of a certain part exceeds the yield limit of the material, the part will undergo irreversible distortion and deformation, which will also cause the failure of the sealing surface and affect the sealing effect.

4. In view of the impact of low temperature on the metal sealing pair, corresponding measures must be taken to minimize the deformation of the metal sealing surface or the deformation of the sealing surface has the least impact on the sealing performance. First of all, in terms of materials, try to choose materials with high metallographic stability (such as 316L but the cost is high). Secondly, parts made of austenitic materials such as valve body, bonnet, valve stem, and seals must be treated at low temperature, so that the martensitic transformation and deformation of the material can be fully carried out before finishing. The temperature of the low temperature treatment should be lower than the material phase transition temperature (MS) and lower than the actual working temperature of the valve, and the treatment time should be 2 to 4 hours. If necessary, multiple low temperature treatments or appropriate aging treatments can be carried out. In addition to the above measures, structural design should also be considered to reduce the impact of sealing surface deformation on sealing performance. For example, elastic sealing structures can be considered in the design of gate valves, ball valves and butterfly valves to partially compensate for low temperature deformation. For the stop valve, the tapered surface sealing structure should be adopted, so that the low temperature deformation has little influence on the sealing surface.