Characteristics of the hottest forged steel low te

2022-08-17
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Characteristics of forged steel low-temperature stop valve and material selection of low-temperature valve

forged steel low-temperature stop valve is suitable for the management system of low-temperature liquid storage and transportation equipment. It has the characteristics of flexible switching and reliable sealing, and can also be used for the management of other low-temperature and cryogenic media, but among them, it is worth the attention of enterprises: 1. The growth rate of automobile production and sales volume with the largest weight in the total industrial volume is picking up; 2. The cumulative order amount of key contact enterprises in the machinery industry has rebounded. 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 low-temperature liquid dielectric flame-retardant materials with the medium service temperature of -196~-150 degrees, including the stop valve in the organic flame-retardant material and inorganic flame-retardant material system

the working temperature of forged steel low-temperature stop valves, especially ultra-low temperature valves, is extremely low. When designing this kind of valve, it has become an indispensable part of people's life. In addition to following the design principles of general valves, there are some special requirements

I. design requirements of low-temperature valve: according to the service conditions, the design of low-temperature valve has the following requirements: 1) the valve should have the ability to work for a long time under low-temperature medium and ambient temperature. 2) The valve should not become a significant heat source of the low temperature system. This is because the inflow of heat not only reduces the thermal efficiency, but also causes rapid evaporation of internal fluid if too much inflow, resulting in abnormal pressure rise and danger. 3) Low temperature medium shall not have harmful effects on handwheel operation and packing sealing performance. 4) Valve assemblies directly in contact with low-temperature media shall have explosion-proof and fire-proof structures. 5) The valve assembly working at low temperature 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 that should be considered in the selection of main body material from the perspective of metallography, in addition to austenitic steel, copper, aluminum, etc., which have the experimental process of face centered cubic lattice, general steel will appear low-temperature brittleness at low temperature, thereby reducing the strength and service life of the valve. When choosing the main material, we should first choose the material suitable for working at low temperature. Aluminum will not appear low-temperature brittleness at low temperature, but 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 the use of aluminum 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 steel will soon lose when it is lower than - 20 ℃, so the service temperature is limited to -30 ℃ and -50 ℃ respectively. Nickel steel containing 3.5% Ni can be used to -100 ℃, and nickel steel containing 9% Ni can be used to -192 ℃. Austenitic stainless steel, nickel, Monel alloy, 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) the minimum service temperature of the valve; 2) The mechanical properties required by metal materials to maintain working conditions at low temperature, especially impact toughness, relative elongation and structural stability; 3) It has good friction resistance under low temperature and oil-free lubrication; 4) Good corrosion resistance; 5) When welding connection is adopted, the welding performance of materials shall also be considered

(2) selection of materials for valve body, bonnet, valve seat and disc (gate). The selection principle of materials for these main parts is roughly as follows: ferritic steel is 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 material of valve rod and fastener is

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

2. Selection of low temperature valve gasket and packing materials: with the temperature decreasing, fluoroplastic shrinks greatly, which will reduce the sealing performance and easily cause leakage. Asbestos filler cannot avoid leakage of permeability. 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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