Most of Cr Ni austenitic stainless steels are in metastable state at room temperature, but martensitic transformation occurs in ultra-low temperature range due to lattice distortion. The temperature at which martensite begins to transform is the martensite transformation point (also known as transformation point), which is represented by the symbol Ms. The temperature of MS point mainly depends on the amount of alloying elements dissolved in austenite.
Martensitic transformation occurs when the working temperature of austenitic stainless steel is equal to or lower than its martensitic transformation point Ms. Because the specific volume of martensite is larger than that of austenite, the volume expansion and structural stress caused by this will change the size of parts, and eventually lead to valve leakage. In order to prevent martensitic transformation in the process of use, cryogenic treatment is necessary.
Cryogenic treatment is a process to cool and keep the austenitic stainless steel material in industrial refrigerator to make it undergo martensitic transformation. Cryogenic treatment can make the material undergo martensitic transformation in advance to ensure the structural stability in use. Cryogenic treatment is usually carried out before the finishing of parts. The temperature of cryogenic treatment should be based on the MS point of the material. Different materials have different MS points. Even for the same brand of materials, due to the different batch (or heat number), the MS points are different, and the difference is very big. Some of them can produce martensite transformation near the upper limit of ultra-low temperature range. The transformation amount of martensite increases with the decrease of temperature. In order to ensure the structural stability of workpiece in service, the temperature of medium used in cryogenic treatment should be equal to or lower than the working temperature of valve. The cooling medium of cryogenic treatment is industrial refrigerator. It can be determined according to the service temperature of the valve. When the parts in the industrial refrigerator reach the medium temperature (bubbles on the surface of the medium completely disappear), the cold holding time can be calculated. According to practical experience, the purpose of treatment can be achieved by holding cold for 1-2 hours. If the time is too long, there is no obvious effect on the transformation of martensite. After cold insulation, the parts can be taken out and cooled to room temperature in air.
After one cryogenic treatment, the martensitic transformation of austenitic stainless steel is basically completed, which can meet the requirements of service in general. The times of cryogenic treatment can be increased for ultra-low temperature check valves with strict sealing performance or sealing by medium pressure