Mechanical seal material issues
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I’d like to ask: for the mechanical seals (imported) that I usually see, it’s almost never the case that the friction rings are made of cemented carbide; they are generally made of graphite and silicon carbide. Why then do many domestic mechanical seals use cemented carbide? What is the difference in material selection between these two approaches?
Depending on whether the conveying medium is clean or contains solid particles
For imported pumps under the same operating conditions, silicon carbide is used, while those made domestically use alloys. Several pumps are like this – as soon as they are made domestically, alloys are used instead
This is related to your process parameters and mechanical seal flushing scheme! Different flushing schemes may come with different materials for the stationary and rotating rings!
Materials for end-face seal friction pairs: The materials used for such friction pairs include graphite, ceramics, cemented carbide, tungsten carbide alloys, SiC, filled polytetrafluoroethylene, tin bronze, steel-bonded carbide, stainless steel, phenolic plastics, nylon, and more. The properties of commonly used materials are as follows: 1. Tungsten carbide alloy is made up of metal carbides such as tungsten carbide and titanium carbide, which have high hardness and high melting points; it is formed by pressing and sintering using binder powders in a powder metallurgy process. The advantages are high hardness and strength. It is wear-resistant, heat-resistant, corrosion-resistant, and has a low linear expansion coefficient; its disadvantages are brittleness and difficulty in processing. Tungsten carbide is the most widely used material for face seal pairs, and is often used as the stationary ring in medium and low-speed mechanical seals, as well as the rotating ring in high-speed mechanical seals. 2. Graphite: The advantages of graphite include good corrosion resistance and self-lubricating properties, a low coefficient of friction, good resistance to thermal shock, and ease of machining. The disadvantage is low mechanical strength and porosity. These two drawbacks of graphite can be improved through impregnation and carburization. Impregnated graphite can be divided into resin-impregnated and metal-impregnated types. Resin-impregnated graphite has good corrosion resistance, but it is not resistant to high temperatures (with a temperature tolerance of around 170–200°C) ； Impregnated metal graphite has good heat resistance (impregnated with bronze, aluminum, lead, etc., it can withstand temperatures of 400–500°C), but its corrosion resistance is poor. Graphite is the most widely used non-metallic material, used as the rotating ring in medium and low-speed mechanical seals and the stationary ring in high-speed mechanical seals. Good graphite appears dense to the naked eye; it doesn’t shed much powder when touched by fingers, nor does it darken those fingers. 3. Ceramics: The advantages of ceramics are their good corrosion resistance, high hardness, and excellent wear resistance. The disadvantages are their high brittleness and the difficulty in processing them due to their excessive hardness. Alumina ceramics are widely used, as are cermets. Ceramics are commonly used in corrosive media and at medium to low speeds. 4. Hardfacing with cemented carbide: Cemented carbide is applied as a hardfacing layer on the sealing surfaces of carbon steel, chromium steel, and chromium-nickel steel. Its advantages include high hardness, good wear resistance, good temperature resistance (up to 500°C), and good corrosion or cavitation resistance. The disadvantages are the tendency to develop pores, inclusions, and uneven surface hardness.
There are many possible combinations of materials for the friction pairs in mechanical seals, and the choice of materials should be based on the operating conditions and requirements. Generally, it is one soft material paired with one hard material; the most common combination is silicon carbide paired with carbon. Carbon (graphite) is a soft material with excellent self-lubricating properties; it can reduce the friction coefficient while also serving as a seal. If the conveying medium contains solid particles, silicon carbide is generally used against silicon carbide.
Tungsten carbide production is fast, low-cost, and yields good results
I feel that the precision of our local pump and valve manufacturing industry is generally low, with large fluctuations; however, the cemented alloys do not break, offering higher reliability. However, many domestic joint-venture pump manufacturers still use a large amount of graphite in the production of mass-produced pumps instead of silicon carbide: lol