Cryogenic Valve Technology and Standard

Austenitic stainless steels (e.g., 304, 316L) are widely used for cryogenic valves due to their excellent low-temperature toughness. However, they are metastable at room temperature. When the temperature drops below the martensite transformation start point (Ms point, typically between -50°C and -100°C), some of the austenite will irreversibly transform into martensite. This phase transformation presents two major risks: Volume expansion – martensite has a larger specific volume than austenite, leading to localized volumetric expansion; and microstructural stress – non-uniform phase transformation and temperature differences across different valve components generate complex thermal and microstructural stresses. When these stresses exceed the material's yield strength, permanent plastic deformation occurs in the pressure-containing parts (especially the precision-finished seat sealing surfaces), destroying the geometric accuracy required for sealing and resulting in internal leakage.

To eliminate this risk, a cryogenic treatment involving immersion in liquid nitrogen at -196°C must be performed before and after rough machining:

Induces the transformation of the majority of unstable austenite into martensite. Measurements show that at this stage, the average deformation of the valve seat sealing surface can reach 2.25 μm.

The already-deformed sealing surfaces are re-lapped to the design precision.

Prompts the transformation of the remaining trace amounts of unstable austenite. Since the vast majority of phase transformation was completed during the first treatment, the deformation after the second treatment is extremely small, with measured averages of only 0.37 μm, fully meeting high sealing requirements. Metallographic analysis confirms that after the second treatment, the microstructure is virtually stable, fundamentally guaranteeing the dimensional stability of the valve under actual low-temperature operating conditions.

To standardize the technical requirements for cryogenic valves, major industrial countries and regions worldwide have established stringent standards. The three most influential standards are:

Media temperatures from -196°C to -29°C, nominal pressures PN16~PN400 / Class150~Class2500.

The 2019 version of the standard explicitly requires cryogenic treatment for austenitic stainless steel valves used in service conditions below -100°C.

Detailed provisions on the selection principles for low-temperature cast steels (e.g., LC3, LC9) and austenitic stainless steels.

Provides more refined control over leak rates.

Imposes stringent requirements on fugitive emissions from the valve stem and body-bonnet joint (e.g., stem leakage ≤ 100 ppmv).

Gate, globe, ball, butterfly, check valves, etc. The 2019 version added axial flow check valves, top-entry ball valves, and top-entry butterfly valves.

As one of the earliest standards dedicated to cryogenic valves, BS 6364 is widely adopted in the global LNG industry and serves as a de facto international specification.

Mandates an extended bonnet design to ensure the packing box remains at ambient temperature, preventing packing failure and stem freezing due to low temperatures.

Specifies a complete procedure from ambient temperature pressure testing to cryogenic performance testing.

Provides systematic regulations on low-temperature impact toughness of materials, welding procedures, non-destructive examination, etc.

Focuses on the specific requirements for isolating valves (e.g., gate, globe, ball valves) in low-temperature applications.

Designed to be used in conjunction with basic standards such as ISO 5208 (Industrial valves – Pressure testing).

Provides standardized cryogenic test apparatus, procedures, and acceptance criteria.

Emphasizes key design elements such as pressure relief holes, blow-out-proof stems, and fire safety.

The reliability of cryogenic valves is the lifeline of cryogenic engineering. Through the scientific two-stage cryogenic treatment process, the issue of low-temperature phase transformation deformation in austenitic stainless steel can be fundamentally resolved. The three major standards – GB/T 24925, BS 6364, and ISO 28921 – collectively form a technical regulatory system covering the entire chain of materials, design, manufacturing, and testing. In practical engineering, the appropriate standard(s) should be selected and followed based on the project location, owner specifications, and media characteristics to ensure long-term, safe, and leak-free operation of valves in extreme low-temperature environments.