Cryogenic valve related knowledge - cryogenic treatment of metal materials

According to the requirements of article 5.10 of the national standard GB/T24925-2019 low temperature valve technical conditions. For low temperature valves below -100℃, such as austenitic stainless steel, valve body, valve cover, closing parts, valve seat, valve stem and other parts should be cryogenic treatment not less than 2 times before finishing.

What is cryogenic treatment?
The parts are immersed in liquid nitrogen for cooling, and when the temperature of the parts is stable at -196℃, the parts are kept warm for 2h~4h according to the thickness of the parts, and then the process is taken out of the box and restored to room temperature naturally. (This concept comes from GB/T24925-2019)

The role of cryogenic treatment
Low temperature valves usually use austenitic stainless steel, although austenite is relatively stable, but when the temperature is lower than the Ms point, martensitic transformation will also occur, because the specific volume of martensitic than austenite, so martensitic transformation will cause changes in macro size; The material manufacturing process, including heat treatment, machining will produce residual stress, and the release of residual stress will lead to the size of the parts is unstable, especially in the low temperature environment prone to deformation.
Cryogenic treatment will reduce the key parts of the valve to a certain low temperature, and insulation, so as to promote the unstable austenite in the stainless steel organization into martensite in advance, because the process is irreversible, so as to effectively ensure the organizational stability of the material.
The cryogenic treatment process uses the interaction between the stress generated by the lattice shrinkage during the cryogenic process and the original residual stress of the material to promote the micro-plastic deformation of the material, so that the residual stress is released in advance, effectively reducing the residual stress of the material and improving the dimensional stability.