How do cryogenic valves resist low temperatures?
The special materials used in cryogenic valves resist low temperatures in the following ways to ensure safe and reliable operation in extreme environments:
1. Maintain high toughness and prevent brittle fracture
Face-centered cubic crystal structure: Materials such as austenitic stainless steel (304, 316), copper and aluminum have a face-centered cubic structure and are not prone to brittle transformation at low temperatures. This crystal structure allows dislocations to move freely, allowing the material to maintain good toughness and plasticity and avoid low-temperature brittle fracture.
The role of alloying elements: Adding alloying elements such as nickel and chromium can improve the low-temperature toughness of the material. For example, nickel can stabilize the austenite structure and enhance the material's impact resistance at low temperatures.
2. Prevent organizational transformation and performance degradation
Stable austenitic structure: Austenitic stainless steel will not undergo martensitic transformation at low temperatures, maintain organizational stability, and ensure that the mechanical properties and corrosion resistance of the material do not decrease with temperature changes.
Controlling carbon content: Reducing carbon content can reduce the precipitation of carbides at low temperatures and prevent the material from becoming brittle. For example, using low-carbon or ultra-low-carbon stainless steel (such as 316L, 304L) can improve low-temperature stability.
3. Improve low-temperature corrosion resistance
Corrosion-resistant alloy elements: Adding elements such as molybdenum and copper to enhance the corrosion resistance of materials in low-temperature media. For example, 316 stainless steel contains molybdenum and has better resistance to chloride corrosion.
Surface treatment: Form a protective layer on the surface of the material through nickel plating, chrome plating, etc. to prevent low-temperature media from corroding the substrate.
4. Reduce thermal conductivity and reduce cold loss
Select low-thermal conductivity materials: Non-metallic materials such as polytetrafluoroethylene and flexible graphite have low thermal conductivity, which can effectively reduce cold transfer and prevent the valve from cooling too quickly.
Structural optimization: Use a long-necked bonnet design to increase the length of the thermal bridge, reduce the heat transfer rate, and protect the stuffing box and operating parts.
5. Enhanced hardness and wear resistance
Surface hardening treatment: Hardening treatment of key parts such as valve stem and sealing surface, such as hard chrome plating, nitriding, surfacing of hard alloy, etc., to improve surface hardness and wear resistance, and prevent wear and leakage at low temperature.
Select high-hardness materials: such as nickel-based alloys, cobalt-based alloys, etc., which have high hardness and good low-temperature performance, suitable for manufacturing seals and valve cores.
6. Good processing and welding performance
Optimized heat treatment process: Through appropriate heat treatment, such as deep cryogenic treatment, tempering treatment, etc., internal stress is eliminated, the low-temperature toughness and dimensional stability of the material are improved, and deformation and cracking at low temperature are prevented.
Welding process control: Select appropriate welding materials and processes to ensure that the welded joint has sufficient strength and toughness at low temperature and avoid welding defects.
7. Dimensional stability
Low-temperature stability test: Conduct low-temperature impact test and dimensional stability test on the material to ensure that the dimensional change of the material at low temperature is within an acceptable range to prevent the valve from failing due to cold shrinkage.
Summary: The materials of cryogenic valves resist the various challenges brought by low temperatures by optimizing crystal structure, adding alloy elements, surface treatment, reducing thermal conductivity, enhancing hardness and wear resistance, etc., ensuring that the valves maintain good mechanical properties and sealing properties under extreme conditions and ensuring the safe operation of the cryogenic system.