Material selection for low-temperature valves
Under low temperature conditions, the tensile strength and hardness of the material increase, plasticity and toughness decrease, and exhibit varying degrees of low-temperature brittleness, seriously affecting the normal operation of the valve. Therefore, in the selection of materials for low-temperature valve components, the following points must be noted:
(1) The minimum operating temperature of the valve;
(2) The mechanical properties necessary to ensure that the parts do not lose their working ability at low temperatures, especially low-temperature impact toughness, relative elongation, and organizational stability;
(3) Ensure that the parts meet the compatibility conditions for low-temperature medium explosion-proof performance,
(4) Ensure the necessary thermal physical properties of the parts, including thermal conductivity, cold shrinkage performance, etc;
(5) Ensure that the parts have necessary wear resistance under low temperature and oil-free lubrication conditions;
(6) Ensure that the parts have the necessary corrosion resistance;
(7) When using welding, the welding performance of the material should also be considered.
1. Material selection for low-temperature valve body, valve cover, valve seat, opening and closing parts, etc
Ferritic stainless steel can be used when the temperature is above -100 ℃, austenitic stainless steel can be used when the temperature is below -100 ℃, and copper alloy or aluminum alloy can be used for low pressure and small diameter valves.
2. Selection of Low Temperature Valve Stem Materials
Made of austenitic stainless and acid resistant steel, appropriate heat treatment is required to improve tensile strength, and hard chromium plating (coating thickness 0.04-0.06mm) or nitriding treatment must be carried out to improve surface hardness.
3. Selection of Fastener Materials for Low Temperature Valves
When the temperature is higher than -100 ℃, the bolt material is made of alloy steel such as Ni and Cr Mo, which needs to undergo appropriate heat treatment to prevent thread bite; When the temperature is low -100 ℃, austenitic stainless steel can be used as the bolt material. The nut material is generally Mo steel or Ni steel, and the thread surface is coated with disulfide key.
4. Selection of gasket materials for low-temperature valves
When the operating temperature is above -196 ℃ and the maximum operating pressure at low temperatures is 3MPa, asbestos rubber sheets made of long fiber self asbestos can be used; When the operating temperature is above -196 ℃ and the maximum operating pressure at low temperatures is 5MPa, stainless steel asbestos wound gaskets, stainless steel PTFE wound gaskets, or stainless steel expanded graphite wound gaskets can be used.
It should be emphasized that all low-temperature material components must undergo cryogenic treatment before precision machining to reduce the shrinkage deformation of low-temperature valves under low-temperature conditions.
