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1. What are the requirements for the materials of the friction pairs in mechanical seals? The materials used in mechanical seals form the basis on which they can function and develop. During high-speed rotation, the friction pair must resist corrosion from the medium, withstand certain pressures, and facilitate heat exchange. The task of sealing must be accomplished in the presence of relative motion and friction; therefore, the following requirements are imposed on the materials: 1) Good chemical stability to resist corrosion, abrasion, dissolution, and swelling by the surrounding media; 2) High elastic modulus, strength, and allowable PV value, low friction coefficient and linear expansion coefficient, excellent wear resistance and self-lubricating properties, as well as good impermeability; 3) Good thermal conductivity, heat resistance, cold resistance, and tolerance to sudden temperature changes; 4) The materials should be readily available, easy to process and manufacture, and cost-effective. 2. What are the categories of materials used to manufacture friction pairs? There are many materials available for manufacturing friction pairs at present. Commonly used non-metallic materials include carbon-graphite, ceramics, polytetrafluoroethylene, and plastics; common metal materials include cemented carbide, nickel-chromium steel, chromium steel, bronze, carbon steel, and cast iron. In addition, surface treatment methods such as surfacing, sintering, and spraying, as well as composite processes, are used to modify or improve the surface properties of friction pairs. All materials possess certain properties; when selecting materials for friction pairs, it is necessary to make use of their strengths while avoiding their weaknesses, and to choose them appropriately based on specific conditions. 3. Why is carbon-graphite commonly used for friction pairs? Carbon-graphite (hereafter referred to as graphite) is a material frequently chosen for friction pairs due to its many excellent properties, such as: 1) High thermal conductivity. The thermal conductivity of carbon is 20–40 W/(m·K), while that of graphite is 40–128 W/(m·K). It ranks just after silver, copper, and aluminum, and it is the only non-metallic material with high thermal conductivity; its thermal conductivity is much higher than that of certain metals such as stellite and Hastelloy. Thus, the generated frictional heat can be dissipated in a timely manner, thereby reducing the temperature of the sealing surface; this is essential for improving both its reliability and service life. 2) Lower linear expansion coefficient. The linear expansion coefficient of graphite is 2×10ˉ6 to 6×10ˉ8/°C, which is about 1/2 to 1/4 that of metals. As the temperature rises, its thermal deformation is minimal, which is advantageous for maintaining the parallelism of the sealed end faces. Its low coefficient of expansion combined with high thermal conductivity gives it good thermal stability, as well as excellent resistance to heat, cold, and thermal shock; it does not develop cracks when exposed to sudden temperature changes. 3) Good corrosion resistance. Graphite has excellent chemical stability; it remains stable at temperatures below 400°C in air. Apart from strongly oxidizing media such as aqua regia, chromic acid, concentrated sulfuric acid, and halogen elements, it can resist corrosion by other acids, bases, salts, and all organic compounds. It should be noted that its corrosion resistance varies depending on the type of graphite and the impregnant used. 4) Excellent self-lubricating properties. This is because when graphite rubs against a metal or other material, graphite transfers to the surface of the metal, resulting in friction between graphite particles. In addition, graphite has a low coefficient of friction; the friction coefficient between graphite and metal is 0.04–0.05, while under fully liquid lubrication conditions it is only 0.008–0.01. 5) Graphite has a low tensile strength and a high compressive strength; it is a brittle material. Due to its low hardness, it is commonly used as the soft ring in friction pairs. It can be easily processed in various ways. Its prominent drawback is its high porosity, generally ranging from 18% to 22%. To overcome this drawback, graphite used in practical applications is impregnated to fill the pores and improve sealing performance. 4. How many types of graphite are there? What are their characteristics? Graphite is produced by mixing coke powder and graphite powder (or carbon black), using asphalt as a binder, pressing it into shape, and then sintering it at high temperatures. Depending on the raw materials used, as well as the sintering temperature and time, graphite with different physical and mechanical properties can be produced. One type is known as high-strength graphite, also called carbon graphite; it is characterized by being hard and brittle, having a low thermal conductivity, and being difficult to process. The other type is graphitized graphite, also known as electrochemical graphite; it is soft, has low strength, but exhibits good self-lubricating properties. During the production of graphite, high-temperature sintering causes the pitch to evaporate, resulting in the formation of pores; therefore, it must be impregnated before it can be used. Impregnated graphite has no significant effect on thermal conductivity, but its strength and hardness are significantly improved. In addition to the graphite mentioned above, there is also resin graphite, which is made by using synthetic resin as a binder, mixing it evenly with graphite powder, pressing it into shape, and then heating it to the temperature at which the resin solidifies. This type of graphite allows for a low PV value to be used, and it is less commonly employed in the petrochemical industry; its advantage is its low cost. 5. What are the graphite impregnants? What are their respective characteristics? The properties of graphite impregnants determine the chemical stability, thermal stability, mechanical strength, and operating temperature of the impregnated graphite. The commonly used impregnants at present can be divided into two main categories: synthetic resins and metals. Under normal circumstances, it is mostly impregnated with synthetic resin, while graphite impregnated with metal is used less frequently. This is because synthetic resins do not have a tendency to melt, and when the operating temperature exceeds the allowable limit, they do not fail quickly like metal-impregnated graphite; when the metal in impregnated graphite melts, it often sticks to the metal with which it is in contact. Graphite impregnated with synthetic resin also has superior corrosion resistance compared to graphite impregnated with metals. When choosing a metal as an impregnant, two issues need to be considered: the melting point of the metal must not be too low; experience has shown that metals with low melting points such as lead, tin, and babbitt cannot be used as impregnants. Another issue is that the metal used for impregnation must be resistant to corrosion by the medium. In media whose chemical properties are not well understood or in which the chemical composition varies significantly, graphite with embedded metal should not be used rashly. For example, in the sealing of crude oil pumps and bottom pumps in atmospheric and vacuum distillation towers, babbitt and graphite impregnated with copper (including copper alloys) cannot be used; instead, graphite impregnated with antimony can be selected. Currently, various synthetic resin impregnants are widely used at home and abroad. ***Resins such as phenolic resin, furan resin, and epoxy resin require different impregnating agents depending on the properties of the medium. Phenolic resin has good acid resistance, epoxy resin is resistant to corrosion in alkaline media, and furan resin has good resistance to both acids and alkalis; therefore, furan resin is widely used. In addition, there are those that use impregnated polytetrafluoroethylene suspensions, which offer better corrosion resistance; however, their manufacturing process is not very stable, so they have not been widely adopted. As the operating temperature rises, especially when it exceeds 200°C, the synthetic resin decomposes and carbonizes, resulting in a decrease in the hardness and strength of graphite and thereby exacerbating wear. To overcome the aforementioned drawbacks, impregnated graphite has emerged. It involves heating the asphalt to a certain temperature and impregnating it under pressure; thereafter, the graphite is heated to cause the asphalt to volatilize, decompose, and carbonize, thereby filling the pores in the graphite. This process is repeated multiple times until high-quality graphite is obtained. 6. What metal materials are used to manufacture sealing rings? While the soft rings in friction pairs are usually made of graphite, metal is generally used to make the hard rings. Depending on the operating conditions (pressure, temperature, speed, etc.) and the properties of the medium, different metals are used. ***Cast iron, sodium carbonate, high-silicon iron, alloy steel, bronze, corrosion-resistant metals, and cemented carbide, etc. These materials vary greatly in terms of price and the difficulty of processing them; it is necessary to understand their properties in order to select the appropriate material at the lowest cost. 7. What are the characteristics of cast iron and carbon steel? What media are they suitable for? The earliest mechanical seal friction pair materials used in China’s metallurgical industry were cast iron against graphite. Cast iron has the advantages of low cost and easy processing. Although its hardness is not high, its wear resistance is comparable to that of cemented carbide, and cracks or similar issues do not occur during use. The reason is its high thermal conductivity, which is 3 to 4 times that of cemented carbide. The cast iron structure is relatively porous, allowing it to absorb trace amounts of liquid and form an oil-containing friction surface, thereby improving the lubrication conditions. Its drawback is poor corrosion resistance, so it can only be used in neutral media and in applications with a low PV value. To improve its corrosion resistance, alloy cast iron is used; adding appropriate amounts of elements (such as aluminum, nickel, and chromium) to it can significantly extend its service life while keeping the cost very low. John Crane Sealing Company has achieved good results by using alloy cast iron for the stationary ring. Commonly used carbon steels include grades 45 and 50; after quenching, their hardness and wear resistance are increased, making them suitable for use as sealing rings in neutral media. 8. What are the characteristics of high-silicon ferroalloy? High-silicon cast iron is a ferroalloy containing 10%~17% silicon and 0.5%~1.2% carbon. Common high-silicon ferroalloys contain about 14.5% silicon and are excellent acid-resistant materials. Due to a highly protective silicon oxide film on its surface, it exhibits excellent corrosion resistance to acids of various concentrations, such as sulfuric acid, nitric acid, organic acids, and acidic salts. However, it corrodes rapidly in hydrofluoric acid, and is also not resistant to the corrosion caused by strong alkalis, hydrochloric acid, and hot ferric chloride solutions. Its drawback is that it is brittle, has poor tolerance to rapid temperature changes, and can crack when exposed to sudden cooling or heating. Therefore, do not strike hard during installation. Furthermore, it has a high hardness (HRc45~50), making it difficult to machine. 9. Which alloy steels can be used to make sealing rings? After heat treatment, alloy steels exhibit increased hardness and wear resistance; they are easy to process and manufacture at a lower cost, making them suitable for use in certain applications. Common chromium steel materials include 3Cr13, 4Cr13, and 9Cr18; they have better corrosion resistance than carbon steel and are suitable for use in mildly corrosive environments. Sealing rings made from 1Crl8Ni9, 1Crl8Ni9Ti, and Crl8Ni12Me2Ti also have good corrosion resistance, but they have low hardness and poor wear resistance. There are also sealing rings made from high-speed tool steels and bearing steels, such as W18Cr4V and Gcrg. Victory Refinery used W18Cr4V to manufacture the rotating ring for the high-speed mechanical seal of its LPG centrifugal compressors, which operated continuously for two years. 10. Which bronze materials can be used for sealing rings? There are many bronze grades, and those that can be used for sealing rings include ZQSn6-6-3 and ZQSn1G-1. These metals have high elastic moduli, good thermal conductivity, wear resistance, machinability, as well as the ability to form bonds with hard materials. However, it is soft in texture and has poor corrosion resistance, making it suitable for use in seawater, oil, and other similar media. 11. What materials should be used for sealing rings in highly corrosive media? With the development of production, there is an increasing amount of highly corrosive media, which imposes higher requirements on the materials used. Such as hydrofluoric acid, sulfuric acid and hydrochloric acid at various concentrations and temperatures, etc. Production cannot proceed without resolving the material issues. Here, two corrosion-resistant alloys are introduced: the nickel-aluminum alloy 0Ni65Mo28Fe5V and the nickel-chromium-aluminum alloy 00Cr16Ni-60Mo17W4, which correspond respectively to the foreign Hastelloy Has.B and Hastelloy C (Has.C). The former exhibits excellent corrosion resistance to hydrochloric acid, sulfuric acid, and phosphoric acid; it maintains high strength and dimensional stability at high temperatures, as well as good machinability. On the other hand, Hastelloy C is one of the most corrosion-resistant alloys in strongly corrosive oxidizing (or reducing) media. It remains highly resistant to hydrogen fluoride gas at various concentrations below 600°C, as well as to hydrogen fluoride gas at high temperatures. It also shows good corrosion resistance in nitric acid at various concentrations (except at its boiling point). It can be cast, forged, and welded, though it is relatively difficult to machine. The biggest drawback of these two corrosion-resistant alloys is their high cost. 12. What is cemented carbide? What are the commonly used types of cemented carbide? Metal carbides (such as tungsten carbide) possess many excellent properties, making them very suitable for manufacturing sealing rings. However, due to the very high melting points of these carbides, ordinary smelting methods are not feasible. It is usually produced by powder metallurgy: the carbides are turned into very fine powders, mixed with a metal with a lower melting point, compressed into a shape, and then sintered at high temperatures. The metal with the lower melting point melts, bonding the metal carbides together to form cemented carbide. Commonly used cemented carbides include tungsten carbide cemented carbides with cobalt as the binder; corrosion-resistant cemented carbides with nickel and chromium as binders; steel-bonded cemented carbides with steel as the binder, as well as various cemented carbide electrodes for surfacing applications. 13. What are the excellent properties of cemented carbide? 1) High hardness. The hardness of tungsten carbide hard alloys generally ranges from HRA 87 to 94; within a fairly wide temperature range, the hardness remains essentially unchanged. Since the wear rate due to abrasive wear is inversely proportional to hardness, high-hardness cemented carbide materials are ideal friction pair materials for mechanical seals that operate in media containing suspended particles. 2) High bending strength. The flexural strength of cemented carbides is generally above 1400 MPa, and the value of this strength is related to the size of the WC grains. 3) High thermal conductivity and low linear expansion coefficient. The thermal conductivity of WC-Co hard alloys is 1–2 times higher than that of high-speed steel, while their coefficient of linear expansion is much lower than that of high-speed steel and carbon steel. Therefore, mechanical seals that use cemented carbide as the material for the friction pair rings possess strong resistance to thermal shock and high load-bearing capacity. 4) It has a certain degree of corrosion resistance. Different cemented carbide grades exhibit varying corrosion resistance in various media. Therefore, different cemented carbides can be selected based on the specific properties of the medium.