Internal sealing surface material of steel valves (HY-industry technical centre)
Internal parts mainly refer to the sealing surface of the valve closing part, the valve stem, and the bushing (upper sealing seat). In the world, the internal parts are often represented by trim.
Selection principle of internal materials is based on the main material, medium characteristics, structural characteristics, and the role and force of the parts. For conventional general-purpose valves, the internal parts materials have been specified in the standard or several materials have been specified by the designer according to the specific situation. For some valves with special requirements, such as working conditions such as high temperature, high pressure, and corrosion of the medium, it is necessary to select the materials of the internal parts according to the working conditions.
1, Closure sealing surface material
Closing parts are the gate (disc) and the valve seat. The sealing surface of the closing part is one of the main working surfaces of the valve. Whether the material selection is reasonable and its quality status directly affect the function and service life of the valve.
1.1 Working conditions of the sealing surface of the closure
Because the valve has a wide range of uses, the working conditions of the valve sealing surface vary greatly. The pressure can range from vacuum to ultra-high pressure, the temperature can range from -269°C to 816°C, and some working temperatures can reach 1200°C. The working medium ranges from non-corrosive media to strong corrosive media such as various acids and alkalis. Judging from the force of the sealing surface, it is squeezed and sheared. From the perspective of tribology, there are friction wear, corrosion wear, surface fatigue wear, erosion and so on. Therefore, suitable sealing surface materials should be selected according to different working conditions.
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Abrasive wear：Abrasive wear refers to the wear that occurs when a rough hard surface slides on a soft surface. The hard material is pressed into the surface of the softer material, and a tiny groove is drawn on the contact surface. The material dropped off by this groove is pushed away from the surface of the object in the form of debris or loose particles.
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Corrosion and wear：When the metal surface is corroded, a layer of oxide is produced. This layer of oxide usually covers the corroded parts, which can slow down the further corrosion of the metal. However, if sliding occurs, the oxide on the surface will be removed and the exposed metal surface will be further corroded.
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Surface fatigue wear：Repeated cyclic loading and unloading will cause fatigue cracks on the surface or the subsurface, forming debris and pits on the surface, and ultimately lead to the destruction of the surface.
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Erosion：Damage ring of the material is caused by the impact of sharp particles against the object. It is similar to abrasive wear, but the surface is very rough.
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Scratches：Abrasion refers to the damage caused by friction of the material during the relative movement of the sealing surface.
1.2 Requirements for sealing surface materials
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Ideal for sealing surface should be corrosion resistant, erosion resistant, scratch resistant, have sufficient extrusion strength, have sufficient oxidation resistance and thermal fatigue resistance at high temperatures, and the sealing surface has a similar linear expansion coefficient to the body. Good welding performance and processing performance.
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Above-mentioned requirements for sealing surface materials are only an ideal state, and it is impossible to have such a perfect material. Therefore, the selection of materials should be based on the specific situation to resolve the main contradictions.
1.3 Types of sealing surface materials
Commonly used sealing surface materials are divided into two categories: soft materials and hard materials. Soft materials are various rubber, nylon, fluoroplastics, etc. Hard materials are various metals and alloys.
1. Soft sealing material
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Nitrile butadiene rubber (NBR). Nitrile butadiene rubber has excellent oil resistance; heat resistance is better than natural rubber and styrene butadiene rubber; air tightness and water resistance are better. The nitrile rubber can be divided into nitrile -18, nitrile-26 and nitrile -40. Phase eye rubber is suitable for petroleum products, benzene, toluene, water, acid, and alkali media with a temperature of -60 to +120 degrees.
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Fluorine rubber (FKM). Fluorine rubber is resistant to heat, acid and alkali, oil, saturated water and steam, has small compression set and good air tightness. Fluorine rubber is suitable for petroleum products, water, acid and alcohol with a temperature of -30～+220 degrees.
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Polytetrafluoroethylene (PTFE). PTFE is resistant to high temperature, chemical corrosion, low friction coefficient, but low mechanical strength, easy to creep, and low elasticity. It is suitable for corrosive media with a temperature less than 170 degrees.
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Polytetrafluoroethylene (PTFE). PTFE is resistant to high temperature, chemical corrosion, low friction coefficient, but low mechanical strength, easy to creep, and low elasticity. It is suitable for corrosive media with a temperature less than or equal to 170 degrees.
2. Hard sealing material
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Copper alloy. Copper alloy has good corrosion resistance and abrasion resistance in water or steam. It is suitable for media with PN≤1.6MPa and temperature not exceeding 200 degrees. It can be fixed on the body by inserting ring structure or surfacing welding and casting methods. . Commonly used grades are ZCuAl10Fe3 (aluminum bronze), ZCuZn38Mn2Pb2 (cast brass).
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Chrome stainless steel. Chromium stainless steel has good corrosion resistance, and is usually used for water, steam and oil, and the temperature does not exceed 450 degrees. Commonly used grades are 2Cr13 and 1Cr13.
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Stellite alloy. Stellite cemented carbide has corrosion resistance. The comprehensive properties of erosion resistance and scratch resistance are very good. It is suitable for valves of different purposes and various media with a temperature of -268～+650 degrees, especially strong corrosive media. One of the ideal sealing surface materials. Because of the high price of Stellite alloy, surfacing welding is often used.Frequently used are stellite 1, stellite 6, stellite 12, stellite 21, etc. -
Nickel-based alloys. Nickel-based alloys are another important material in the field of corrosion resistance. There are three commonly used sealing surface materials: Monel, Hastelloy B and Hastelloy C. Monel is the main material resistant to hydrofluoric acid corrosion. It is suitable for alkali, salt, food, and air-free acid solvent media with a temperature of -240 to +482 degrees. Hastelloy B and Hastelloy C are the most comprehensive corrosion resistant materials in valve sealing surface materials, suitable for corrosive mineral acids, sulfuric acid, phosphoric acid, wet HCI gas, and strong oxidizing media at a temperature of 371 degrees (hardness 14RC); At the same time, it is suitable for non-chloric acid solution and strong oxidizing medium at a temperature of 538 degrees (23RC).
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Iron-based alloys. Iron-based alloy is a newly developed sealing surface material in my country. Its wear resistance and scratch resistance are better than 2Cr13, and it has certain corrosion resistance, which can replace 2Cr13. Suitable for non-corrosive media with temperature less than or equal to 450 degrees. Commonly used grades are WF311 and WF312 iron-based powder.
