Characteristics of the most hot forged steel low t

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Features of forged steel low-temperature stop valve

forged steel low-temperature stop valve is applicable to the management system of low-temperature liquid storage and transportation equipment. It has the characteristics of flexible switching and reliable sealing. It can also be used in the management system of other low-temperature and cryogenic media. The normal temperature stop valve refers to the service temperature of the medium between -40~+500 degrees, and the material of the valve body is also different. Low temperature stop valve mainly refers to the stop valve in the low-temperature liquid medium system with the medium service temperature of -196~-150 ℃

low temperature valves, especially ultra-low temperature valves, have extremely low operating temperature. In the design of such valves, in addition to the design principles of general valves, there are some special requirements

I. design requirements for low temperature valves: according to the service conditions, the low temperature P-point stress is the proportional limit σ The design of P valve has the following requirements: 1) the valve shall have the ability to work for a long time under low temperature medium and ambient temperature. 2) The valve shall not be a significant heat source for the cryogenic system. This is because in addition to reducing the thermal efficiency, excessive inflow of heat will also cause rapid evaporation of internal fluid and abnormal pressure rise, resulting in danger. 3) Low temperature medium shall not have harmful effect on handwheel operation and packing sealing performance. 4) Valve assemblies in direct contact with low-temperature media shall have explosion-proof and fire-proof structures. 5) Valve assemblies working at low temperatures cannot be lubricated, so structural measures need to be taken to prevent friction parts from scratching

II. Material selection of low-temperature valve

1. Main body material of low-temperature valve

(1) factors to be considered in the selection of main body material

from the perspective of metallography, in addition to austenitic steel, copper, aluminum, etc. with face centered cubic lattice, general steel will appear low-temperature brittleness at low temperature, thus reducing the strength and service life of the valve. When selecting the main material, the material suitable for the electronic universal testing machine with lower than 30t and more advantageous temperature shall be selected first. Aluminum will not appear low-temperature brittleness at low temperature. However, due to the low hardness of aluminum and aluminum alloy, and the poor wear and scratch resistance of aluminum sealing surface, there are certain restrictions on its use in low-temperature valves, and it is only used in low-pressure and small-diameter valves

materials working at low temperature should ensure their low temperature performance, mainly to ensure their cold impact strength. Valve internals must be properly selected to have sufficient cold impact strength to prevent fracture. The impact strength of C and Cr alloy steels will soon lose when the temperature is lower than -20 ℃, so the service temperature is limited to -30 ℃ and -50 ℃ respectively. The nickel steel with 3.5% Ni content can be used to -100 ℃ of 6 electrically controlled experimental machines, and the nickel steel with 9% Ni content can be used to -192 ℃. Austenitic stainless steel, nickel, Monel, Hastelloy, titanium, aluminum alloy and bronze can be used at lower temperatures (-273 ℃)

in addition, the material selection of low-temperature valves should also consider the following factors:

1) minimum service temperature of valves; 2) The mechanical properties required by metal materials to maintain working conditions at low temperature, especially impact toughness, relative elongation and structural stability; 3) Good friction resistance at low temperature and without oil lubrication; 4) Good corrosion resistance; 5) Welding performance of materials shall also be considered when welding connection is adopted

(2) selection of materials for valve body, bonnet, valve seat and disc (gate). The selection principles of materials for these main parts are as follows: ferritic steel shall be selected when the temperature is higher than -100 ℃; Austenitic steel shall be selected when the temperature is lower than -100 ℃; Low pressure and small diameter valves can be made of copper, aluminum and other materials. Appropriate materials shall be selected according to the minimum service temperature during design

(3) the valve rod and fastener shall be made of

when the temperature is higher than -100 ℃, the valve rod and bolt shall be made of Ni, cr-m and other alloy steels with the joint efforts of various enterprises. After appropriate heat treatment, the tensile strength shall be improved and the thread bite shall be prevented. When the temperature is lower than -100 ℃, it shall be made of austenitic stainless acid resistant steel. However, the hardness of acid resistant steel is low, which will cause mutual abrasion between the valve rod and the packing, resulting in leakage at the packing. Therefore, the valve rod surface must be plated with hard chromium (plating thickness 0..06mm), or nitrided and nickel phosphorus plated to improve the surface hardness. In order to prevent the nut and bolt from seizing, the nut is generally made of Mo steel or Ni steel, and the thread surface is coated with molybdenum disulfide

2. Selection of gasket and packing materials for low temperature valve: with the decrease of temperature, fluoroplastic shrinks greatly, which will reduce the sealing performance and easily cause leakage. Asbestos fillers cannot avoid permeable leakage. Rubber has foaming property to liquefied natural gas and cannot be used at low temperature. In the design of low-temperature valves, on the one hand, the structural design ensures that the packing works at a temperature close to the ambient temperature. For example, the long neck bonnet structure is adopted to keep the stuffing box away from the ambient temperature

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