Material selection for mechanical seal friction pair

News | 2018-09-27 16:12


Determination of mechanical seal materials

At present, there are many types of materials used as sealing friction pairs. Commonly used non-metal materials are: graphite, polytetrafluoroethylene, phenolic plastic, ceramics, etc. Commonly used metal materials are: cast iron, carbon steel, chrome steel, chrome-nickel steel, bronze and hard alloy. In addition, there are also friction pair materials made by surfacing, sintering, spraying and other surface treatments and composite processes to change or improve the surface properties of metal materials or friction pairs. All kinds of materials have certain characteristics. When choosing the material of friction pair, you should make use of advantages and avoid disadvantages, and use them reasonably according to specific working conditions.
Material of mechanical seal friction pair:
Graphite is one of the materials with a large amount and a wide range of use in mechanical friction pair materials. Because it has many unique excellent properties.
(1) Higher thermal conductivity. Second only to silver, copper, and aluminum are the only non-metallic materials with high thermal conductivity than some metals (such as Stellite cemented carbide and Hastelloy). Therefore, the generated frictional heat can be dissipated in time, thereby reducing the temperature of the sealing end face, which is very necessary to improve its reliability and service life.
(2) Low linear expansion coefficient. When the temperature is increased, its thermal deformation is small, which is very beneficial to maintain the parallelism of the sealing end face.
(3) Good corrosion resistance. Its chemical stability is very good, it is stable below 400 ℃ in air, in addition to strong oxidizing media such as chromic acid, concentrated sulfuric acid and halogen elements, it can withstand the corrosion of other acids, alkalis, salts and all organic chemicals.
(4) Excellent self-lubricity. Because graphite and metal materials are relatively rubbed, graphite is transferred to the metal surface layer by layer, forming the relative friction between metal and graphite, in addition, it also has a low coefficient of friction.
(5) It has a low tensile strength and belongs to a brittle material. Due to its low hardness, it is often used as a soft ring in a friction pair, which can be conveniently used for various mechanical processing. However, graphite has the characteristics of large porosity and low mechanical strength. To make up for this shortcoming, sintered graphite can be impregnated or carburized multiple times. Choosing the right impregnating agent is very important. At present, the commonly used impregnating resins are phenolic, epoxy, and furan resins, and the commonly used impregnating metals are bronze, pasteurized alloy, lead, aluminum, and the like. When the use temperature is less than or equal to 170 ° C, the impregnated resin is selected; otherwise, the impregnated metal is selected.
Impregnated resin has good corrosion resistance. Phenolic resin has good acid resistance, epoxy resin has good alkali resistance, and furan resin is acid and alkali resistant. Therefore, impregnated furan resin graphite is most commonly used. The 32JF static ring is a furan resin material.
Cemented carbide is another widely used friction pair material in mechanical seals. The cemented carbide here is mainly tungsten carbide (WC), and WC-Co is commonly used. WC-Co is made from powdered metallurgy by high-hardness refractory tungsten carbide and Co as a binder. WC-Co has extremely high hardness and strength, good abrasion resistance and particle erosion resistance, high thermal conductivity, low linear expansion coefficient, and certain corrosion resistance, and can be resistant to sulfuric acid and hydrofluoric acid at ordinary temperatures. As well as caustic soda at the boiling point, it is not resistant to hydrochloric acid and nitric acid. However, WC-Co materials have low impact toughness and high brittleness, are difficult to machine, and are relatively expensive.