1. What are the requirements for friction pair materials of mechanical seals?

　　The material of mechanical seal is the basis for its existence and development. The friction pair must resist the corrosion of the medium in the high-speed rotation, withstand a certain pressure, and carry out heat exchange. In the case of relative motion and friction to complete the task of sealing, so the material put forward the following requirements:

　　1) It should have good chemical stability in anti-corrosion, and can resist the corrosion, abrasion, dissolution and swelling of the medium;

　　2) In terms of physical and mechanical properties, it is necessary to have high elastic modulus, strength and allowable PV value, low friction coefficient and linear expansion coefficient, excellent wear resistance and self-lubrication and good impermeability;

　　3) In terms of thermal performance, it should have good thermal conductivity, heat resistance, cold resistance and temperature resistance;

　　4) Convenient material source, easy processing and manufacturing, low cost.

　　2. What kinds of materials are used to make friction pairs?

　　At present, there are many materials for making friction pairs. Commonly used non-metallic materials are carbon-graphite, ceramics, polytetrafluoroethylene and plastics; Commonly used metal materials are tungsten carbide, nickel chromium steel, chromium steel, bronze, carbon steel and cast iron; In addition, surfacing welding, sintering, spraying and other surface treatment and composite processes are used to change or improve the surface properties of friction pairs. All kinds of materials have certain characteristics, in the selection of friction materials, should be promoted to avoid weaknesses, according to the specific conditions reasonable selection.

　　3. Why is carbon-graphite commonly used as a friction pair?

　　Carbon-graphite (hereinafter referred to as graphite) is often used as a material in friction pairs, because it has many excellent properties, as follows:

　　1) High thermal conductivity. The thermal conductivity of carbon is 20~40W/(m.K), and the thermal conductivity of graphite is 40~128w/(m.K), which is second only to silver, copper and aluminum, and is the only material with high thermal conductivity in non-metallic materials, which is much higher than that of some metals (such as Stely carbide and Hastelloy, etc.). Therefore, the friction heat generated can be dissipated in time, so as to reduce the temperature of the seal end face, which is necessary to improve its reliability and service life.

　　2) Low coefficient of linear expansion. The linear expansion coefficient of graphite is 2×10ˉ6 ~ 6×10ˉ8/℃, which is about 1/2 ~ 1/4 of the metal. When the temperature rises, the thermal deformation is small, which is advantageous for keeping the parallel end face of the seal. The low coefficient of expansion and high thermal conductivity make it have good thermal stability, good heat resistance, cold resistance and heat shock performance, and no cracks when subjected to temperature changes.

　　3) Good corrosion resistance. The chemical stability of graphite is very good, stable below 400 ° C in the air, in addition to strong oxidizing media such as aqua aqua, chromic acid, concentrated sulfuric acid and halogen elements, it can resist the corrosion of other acids, bases, salts and all organic compounds. It should be noted that the degree of corrosion resistance varies with the type of graphite and the different impregnating agent.

　　4) Excellent self-lubricity. This is because when graphite is in relative friction with metal or other materials, graphite is transferred to the metal surface, forming a relative friction between graphite and graphite. In addition, graphite has a low friction coefficient, the friction coefficient of graphite and metal grinding is 0.04 ~ 0.05, under the condition of full liquid lubrication, the friction coefficient is only 0.008 ~ 0.01.

　　5) The tensile strength of graphite is low, the compressive strength is high, and it is a brittle material. Due to its low hardness, it is commonly used as a soft ring in the friction pair. It can be easily processed. Its prominent disadvantage is that the porosity is large, generally 18% to 22%. In order to compensate for this shortcoming, the graphite used in practice is impregnated to plug the porosity and improve the tightness.

　　4. How many kinds of graphite are there? What are the characteristics of each?

　　Graphite is made of coke powder and graphite powder (or carbon black), with asphalt as a binder, and sintered at high temperatures by molding. Depending on the raw materials used and the sintering temperature and time, graphite with different physical and mechanical properties can be made. The utility model is called high-strength graphite, also known as carbon graphite, which is characterized by hard and brittle quality, low thermal conductivity and difficult processing; The other is graphite fossil ink, also known as electrochemical graphite, which is characterized by soft quality, low strength and good self-lubrication.

　　Due to the graphite manufacturing process, after high temperature sintering, the asphalt volatilized, forming pores, only impregnation can be used. The impregnated graphite has no obvious effect on thermal conductivity, but the strength and hardness are significantly improved.

　　In addition to the above graphite, there is resin graphite, which is made of synthetic resin as a binder, mixed evenly with graphite powder, pressed into shape, and heated to the resin curing temperature. The PV value allowed for this graphite is not high, and the petrochemical industry is less used, and its advantage is that the price is cheap.

　　5. What are graphite impregnators? What are the characteristics of each?

　　The properties of graphite impregnators determine the chemical stability, thermal stability, mechanical strength and application temperature of impregnated graphite. Currently commonly used impregnators can be divided into two categories: synthetic resins and metals. Under normal circumstances, most of the impregnated synthetic resin, and impregnated metal graphite is used less. This is because the synthetic resin has no tendency to melt and will not fail as quickly as metal-impregnated graphite when the permissible temperature limit is exceeded; The metal impregnated with graphite, when melted, often sticks together with the metal it is grinding. The corrosion resistance of synthetic resin impregnated graphite is better than that of metal impregnated graphite. Two problems should be noted when choosing metals as impregnators: the melting point of metals should not be too low, and practice has proved that low melting point metals such as lead, tin and babbitt can not be used as impregnators; Another problem is that the impregnated metal must resist the corrosion of the medium. In the chemical properties are not very clear, or the chemical composition of the large change in the medium, can not be rushed to use metal impregnated graphite. For example, in the seal of the original oil pump and the atmospheric and vacuum tower bottom pump, babbitt alloy and copper (including copper alloy) impregnated graphite can not be used, and antimony impregnated graphite can be selected.

　　At present, various synthetic resin impregnations are widely used at home and abroad. Among them, phenolic resin, furan resin and epoxy resin, etc., according to the properties of different media to choose different impregnation Liu. Phenolic resin has good acid resistance, epoxy resin has good corrosion resistance to alkaline medium, and furan resin has good acid and alkaline resistance, so furan resin is widely used. In addition, there are impregnated polytetrafluoroethylene suspension, corrosion resistance is better, because its process is not stable, so it can not be popularized. When the operating temperature rises, especially after the temperature exceeds 200 ° C, the synthetic resin decomposes and carbonizes, resulting in a decrease in the hardness and strength of the graphite, which intensifies the wear.

　　To overcome these shortcomings, asphalt-impregnated graphite was developed. It is to heat the asphalt to a certain temperature, after pressure impregnation, and then heat up the graphite, so that the asphalt volatilization, decomposition and carbonization, in order to fill the pores in the graphite, so repeated many times, and finally get qualified graphite.

　　6. What metal materials are used to make sealing rings

　　If the soft ring in the friction pair is mostly made of graphite, then the hard ring is mostly made of metal. According to the working condition (pressure, temperature, speed, etc.) and the nature of the medium, different metals are used. Among them are cast iron and sodium carbon, high silicon iron, alloy steel, bronze, corrosion resistant metal and hard alloy. These materials vary greatly in price and the ease of processing and manufacturing, and it is necessary to master their performance and choose suitable materials at the lowest cost.

　　7. What are the characteristics of cast iron and carbon steel? What media is suitable?

　　The earliest mechanical seal friction pair material in China's oil refining industry is cast iron to graphite. Cast iron has the advantages of low price and easy manufacturing. Although the hardness is not high, the wear resistance is no less than that of surfacing cemented carbide, and cracks will not appear in use. The reason is that the thermal conductivity is large, which is 3 to 4 times that of surfacing cemented carbide. The microstructure of cast iron is relatively loose, it can absorb trace liquid, form oil-bearing friction surface, and improve the lubrication condition. Its disadvantage is poor corrosion resistance, so it can only be used in neutral media and low PV value occasions. In order to improve its corrosion resistance, alloy cast iron is used, and appropriate elements (such as aluminum, nickel and chromium, etc.) are added to it, which can significantly extend the service life, and the price is very low. John Crane sealing company uses alloy cast iron as static ring, the use effect is better.

　　The common type of carbon steel is 45or50, after quenching to improve the hardness and wear resistance, can be used for neutral media seal ring.

　　8. What are the characteristics of high silicon iron?

　　High-silicon cast iron is a ferrosilicon alloy containing 10%~17% silicon and 0.5%~1.2% carbon. The silicon content of commonly used high silicon iron is about 14.5%, which is an excellent acid-resistant material. Because there is a protective silicon oxide film on the surface of the material, it has good corrosion resistance to various concentrations of sulfuric acid, nitric acid, organic acid, acid salt and other media. However, it corrodes quickly in hydrofluoric acid, and is not resistant to corrosion of strong alkali, hydrochloric acid and hot ferric chloride solution.

　　The disadvantage is that the quality is brittle, the temperature resistance is poor, and the sudden cold and sudden heat will burst. Therefore, do not beat hard during installation. In addition, the hardness is high (HRc45~50), and the processing is difficult.

　　9. Which alloy steels can be made into sealing rings?

　　Alloy steel after heat treatment, the hardness and wear resistance are greatly improved, processing and manufacturing is relatively easy, the cost is low, and some occasions can be used. Commonly used chromium steel materials are 3Cr13, 4Cr13, 9Cr18, corrosion resistance is better than carbon steel, suitable for weak corrosive media. Also useful 1Crl8Ni9, 1Crl8Ni9Ti, Crl8Ni12Me2Ti sealing ring corrosion resistance is very good, but low hardness, poor wear resistance. There are also sealing rings made of high-speed tool steel and bearing steel, such as W18Cr4V and Gcrg. Shengli Refinery uses W18Cr4V to make moving rings on the high-speed mechanical seals of LPG centrifugal compressors, which run continuously for two years.

　　10. What bronze materials can be used as sealing rings?

　　There are many bronze grades, which can be used as sealing rings :ZQSn6-6-3 and ZQSn1G-1, etc., which has a large elastic modulus, good thermal conductivity, wear resistance, processability and compatibility for hard materials. But soft, poor corrosion resistance, suitable for seawater, oil and other media.

　　11. What material seal ring is used in highly corrosive media?

　　With the development of production, highly corrosive media are gradually increasing, and higher requirements are put forward for materials. For example, hydrofluoric acid, sulfuric acid and hydrochloric acid at various concentrations and temperatures. Production cannot proceed without solving the material problem. Here are two corrosion resistant alloys, nickel aluminum alloy 0Ni65Mo28Fe5V and nickel chromium aluminum alloy 00Cr16Ni-60Mo17W4, which are equivalent to foreign Hastelloy (Has.B) and Hastelloy C (Has.C). The former has good corrosion resistance to hydrochloric acid, sulfuric acid and phosphoric acid, still has high strength and dimensional stability at high temperature, and good cutting performance; Hastelloy C is one of the most corrosion resistant alloys in oxidation (or reduction) highly corrosive media, and is still very corrosion resistant in various concentrations of hydrogen fluoride gas below 600 ° C, and is also very corrosion resistant in high temperature hydrofluoric acid gas, and has good corrosion resistance in various concentrations of nitric acid (except the boiling point), which can be cast, forged and welded, but the processing is more difficult. The biggest disadvantage of the above two corrosion resistant alloys is that they are expensive.

　　12. What is cemented carbide? What are the commonly used cemented carbide?

　　Metal carbides (such as tungsten carbide, etc.) have many excellent properties, making sealing rings is very suitable. However, because the melting point of these carbides is very high, it is not feasible to use ordinary smelting methods. Usually by powder metallurgy method, it is made of carbide into a very fine powder, mixed with a low melting point of the metal, pressing molding, sintering at high temperatures, the lower melting point of the metal melting, the metal carbide bonded into a whole, you can get carbide. Commonly used tungsten carbide carbide with cobalt as a binder; Corrosion-resistant cemented carbide with nickel and chromium as binders; Steel-bound cemented carbide with steel as binder and various hard alloy welding rods for surfacing.

　　13. What are the excellent properties of cemented carbide?

　　1) High hardness. The hardness of tungsten carbide is generally between HRA87 and 94, and the hardness is basically unchanged in a comparable temperature range. Because the wear rate of abrasive wear is inversely proportional to the hardness, the hard alloy material with high hardness is the ideal friction pair material for sealing mechanical seals containing suspended granular media.

　　2) High bending strength. The pit bending strength of cemented carbide is generally above 1400MPa, and the size of the bending strength is related to the size of the WC grain.

　　3) High thermal conductivity and small linear expansion coefficient. The thermal conductivity of WC-Co cemented carbide is 1 to 2 times higher than that of high-speed steel, and the coefficient of linear expansion is much smaller than that of high-speed steel and carbon steel. Therefore, the mechanical seal of the friction secondary ring material made of cemented carbide has strong thermal shock resistance and bearing capacity.

　　4) Have a certain corrosion resistance. Different cemented carbide grades have different corrosion resistance in various media. Therefore, different cemented carbide can be selected according to the specific medium properties.