In some mechanical seals, metal materials are often used as sealing surface materials due to the good processing properties of metals. In general, alloys are used to make seals, such as nickel alloy (nickel alloy cast iron), cemented carbide, etc.; for some special steels that are not resistant to corrosion, such as cast iron, die steel and bearing steel, they are often used for low load, oil liquid. In Daya Bay Nuclear Power Plant, there are some examples of using metal as mechanical sealing surface. The moving ring of the mechanical seal of the CRF circulating water pump is made of seawater resistant cast iron: the static ring of the mechanical seal of the SRI cooling water pump is made of stainless steel. At present, the main mechanical seal materials are mainly cemented carbide (YG6), SiC, aluminum oxide, etc., and there is a harder material boron carbide in marine mechanical seals. Due to its higher hardness, it is more difficult to process. So use less.
Alumina has high hardness and good wear resistance, but has low thermal shock resistance. Alumina cermet is made by adding a small amount of metal elements to alumina. 88%~92% of its composition is AL203, and the rest is Fe, Cr, Ni and other metal elements. Adding metal elements can improve the thermal conductivity and reduce the brittleness of alumina cermet, but its corrosion resistance decreases. This ceramic is often used in mechanical seals for automotive cooling water pumps in large batches. Since the use of industrial ceramics in mechanical ceramic seals, it has continued to be replaced by high-performance silicon carbide. Only alumina with the best corrosion resistance and a purity of 99.5% has a certain amount of application in petrochemical industry.
Cemented tungsten carbide is formed by sintering hard carbide particles with a ductile metal binder. Typically used for heavy duty (high PV) applications. Although it is more expensive, it is more and more used in mechanical seals ring because of its toughness, rigidity and good wear resistance. 6% drill-based tungsten carbide is the most widely used; nickel-based tungsten carbide is used more in petrochemicals because of its better corrosion resistance, but its comprehensive performance is worse than that of drill-based tungsten carbide, but its corrosion resistance is better than that of drill-based tungsten carbide. Tungsten carbide is better.
Compared with silicon carbide, silicon nitride is somewhat lacking in hardness and corrosion resistance, and more importantly, the material is brittle and is easily damaged during processing and installation, so it is less used in mechanical seals. The hardness of silicon carbide is close to that of diamond and has outstanding wear resistance. Silicon carbide is one of the most widely used materials in mechanical seals. In addition to melting strong alkali and strong oxidizing gas at high temperature, silicon carbide can also be used stably in full concentration sulfuric acid and caustic alkali with a close boiling point, especially maintaining corrosion resistance and wear resistance in a wide temperature range.