A wide range of austenitic stainless steel, but in the manufacture of cryogenic valves, widely used is the Cr-Ni austenitic stainless steel, the American standard 300 series stainless steel. Among them, F304, F304L, F316 and F316L are ultra-low temperature valve used in several more grades. All four brands of steel belong to the metastable stainless steel, martensitic transformation (ie phase change) occurs at very low temperatures, if you want to get a stable austenitic structure, optional F310 austenitic stainless steel. A variety of stainless steel can be used at ultra-low temperature -254 ℃ or lower. In some foreign PTFE valve standards or norms, the use of austenitic stainless steel temperature range has a clear rule. For example, the Japanese High-Pressure Gas Act stipulates that SUS304L and SUS316L (equivalent to fluorine-containing valves on F304L and F316L) allow the use of -269 ° C while SUS304 and SUS316 (equivalent to F304 and F316) only allow the use of -254 ° C, probably because SUS304 and SUS316 in the welding process, the carbon steel valve heat-affected zone prone to grain boundary carbides, can lead to low temperature deterioration caused by the impact toughness decline. Properties As the temperature decreases, the strength of austenitic stainless steels increases significantly (Table 2). From an intensity point of view, this means that valves that have adequate strength at ambient temperatures must also be safe at very low temperatures. The increase in strength, the impact on toughness is small, only a slight decline. This is the main reason that austenitic stainless steel can become the material of ultra-low temperature valve. Austenitic stainless steel has a face-centered cubic structure with no brittle transition at low temperatures. Unless there is a second phase or in an environment that causes stress corrosion cracking, no brittle fracture will occur and the toughness will not suddenly decrease with decreasing temperature. The main reason is that when the temperature decreases, the yield strength of face-centered cubic metal does not change significantly, and it is not easy to produce deformation twins, the dislocation is easy to move, the local stress is easy to relax, the crack is not easy to spread, generally there is no transition from ductility to brittleness (Temperature cast iron valve). Therefore, at ultra-low temperatures, gray cast iron valves are still able to withstand high impact toughness (Table 3) and well beyond 27 J (this value is for low temperature steel as specified in the Pressure Equipment Directive of the European Union 97/23 / EC Must be the minimum average impact energy), can fully meet the requirements of ultra-low temperature valve. Casting in the manufacture of cryogenic valves, often used austenitic stainless steel castings as the main body of the valve material. Casting easy to shape, forging method is difficult to shape the parts, the casting method is very easy to obtain. At present, the international common is the United States ASTMA351CF stainless steel castings. As the casting may have defects such as alloy segregation and dendritic microstructure, combined with a class CF stainless steel castings contain a certain amount of ferrite, so its low temperature toughness is bound to be affected. Under normal circumstances, the CF type stainless steel castings low temperature impact value is lower than the same type of deformation alloy. And the United States ASMEB1613 "Corrosion-resistant valve art pipeline" under the CF-type stainless steel casting temperature range of application, the Japanese JIS8243 "structure of the pressure vessel" and high-pressure gas management will be austenitic stainless steel valve stainless steel castings The minimum service temperature is limited to -196 ° C (Table 4). Cr-Mn-N stainless steel Cr-Mn-N pneumatic valve stainless steel is a nodular austenitic stainless steel, such as 03Cr13Ni5NMn19 (for -253 ℃), 07Cr21Mn7Ni5N (can be used for -269 ℃) and so on. Unlike Cr-Ni austenitic stainless steels, Cr-Mn-N stainless steels have a ductile-brittle transition at low temperatures. This is due to the fact that steel contains Mn, which forms a large number of faults at low temperatures, hindering the intersection of dislocations. Cryogenic valve materials are usually rarely used Cr-Mn-N stainless steel. Process treatment Solution treatment Solution treatment is the basic heat treatment of cast steel valve austenitic stainless steel, is an important means to prevent intergranular corrosion. As the ultra-low temperature valve used in austenitic stainless steel, solution treatment is designed to fully dissolve carbides to improve its resistance to brittleness, which can be safely used at lower temperatures. For Ti, Nb stabilized austenitic stainless steel, solution treatment is mainly to get more uniform composition and organization, to ensure its good toughness and plasticity. For castings, solution treatment allows for improved segregation during solidification, resulting in a homogenous microstructure. In addition, the austenitic stainless steel welding, the heat-affected zone due to carbide precipitation and ferrite, will significantly reduce the material's low temperature toughness. Therefore, after soldering should be carried out solution treatment to restore its low temperature toughness. Cryogenic treatment Most Cr-Ni austenitic stainless steels are metastable at room temperature and undergo martensitic transformation due to lattice distortion in the ultra-low temperature range. The temperature at which martensite begins to change is the martensitic transformation point (also known as the phase transition point), indicated by the symbol Ms. Ms point of the steel valve temperature depends mainly on the amount of alloying elements dissolved in austenite. When austenitic stainless steel working temperature is equal to or lower than the martensitic transformation point Ms, martensitic transformation occurs. Due to the larger specific volume of martensite than austenite, the resulting volumetric expansion and structural stress can change the dimensions of the part, eventually leading to valve leakage. In order to prevent the material during the use of martensitic transformation, the need for its soft-sealed valves cryogenic treatment. Cryogenic treatment is the austenitic stainless steel material immersed in the coolant cooling, cold, so that a martensitic transformation process. Cryogenic treatment allows the material to undergo a martensitic transformation in advance to ensure tissue stability during use. Cryogenic treatment is generally carried out before the parts are finished. Cryogenic treatment temperature should be based on the Ms point of the material. Different materials, Ms different points. Even the same grade of material, due to the different batches (or furnace number), Ms points are also different, but very different. Some in the ultra-low temperature range near the upper limit to produce martensitic transformation.
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