Specifications for cryogenic treatment of low-temperature valves

First, it is necessary to understand the current state of cryogenic treatment. Secondly, it is specified in the national standard for technical requirements of low-temperature valves, GB/T 24529

In the national standard GB/T 24925, 5.10.1 b) states that at the operating temperature, the microstructure of the material must be stable in order to prevent volume changes caused by phase transformations in the material. For low-temperature valves used at temperatures below 101·C, components such as the valve body, valve cover, valve disc, valve seat, and valve stem must undergo cryogenic treatment before precision machining ； So it’s understandable that -101 degrees isn’t extremely cold, right?

In low-temperature environments, austenitic stainless steel tends to undergo a transformation in which some of the austenitic structure within the material changes to martensitic structure; this is known as martensitic transformation. The formation of martensitic structure directly affects the stability of austenitic stainless steel at low temperatures. As a result of this transformation, the volume of the material increases, leading to plastic deformation and thus damaging the original design structure, which can cause internal or external leakage in the valve. We refer to the temperature at which the martensitic transformation begins upon cooling to a certain temperature as the martensitic critical transformation temperature Ms, which can be predicted using empirical formulas. If the Ms point of the material is significantly lower than the minimum operating temperature of the valve, we consider the material to be stable at low temperatures. If, on the other hand, the Ms point is close to or higher than the valve’s minimum operating temperature, it is necessary to subject the material to deep cooling treatments at least twice prior to precision machining, in order to prompt the transformation of residual austenite into martensite and to ensure that this transformation and deformation occur fully before proceeding with machining, thus ensuring the material’s stability at low temperatures.

BS6364 is for cryogenic testing, applied to valves that have been assembled. Cryogenic treatment is generally used to treat components before valve production.