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Pressureless Sintering Process of Silicon Carbide Sealing Ring

With the rapid development of modern science and technology, in petrochemical, chemical fiber, chemical fertilizer, atomic energy, aerospace and machinery manufacturing and other industrial fields, higher requirements are put forward for mechanical seals. Since the early 1980s, companies in Japan, the United States, Germany and the United Kingdom have successively launched different types of silicon carbide sealing rings. Silicon carbide is currently an excellent material, among which the performance of pressureless sintered SiC and hot-pressed SiC is the best.

Pressureless sintered silicon carbide:

The hardness of pressureless sintered silicon carbide is second only to diamond, boron nitride and boron carbide. The flexural strength is higher than that of alumina ceramics and double that of reaction sintered silicon nitride. Since the specific gravity of silicon carbide is light, only 1/5 of cemented carbide, its specific strength is high. In addition, both pressureless sintered SiC and hot-pressed SiC have no free silicon, so their high-temperature strength and high-temperature oxidation resistance are excellent, and they have excellent high-temperature mechanical properties like silicon nitride ceramics. The thermal conductivity of pressureless sintered silicon carbide is the highest among ceramic materials, 10 times greater than alumina, and similar to cemented carbide. Their coefficient of expansion is small, similar to cemented carbide and carbon graphite. This reflects that silicon carbide is excellent in thermal shock resistance.

Pressureless sintered SiC and hot-pressed SiC have less impurity content, and are resistant to almost all acid and alkali corrosion at room temperature, and are currently the most chemically stable materials. The reaction sintered silicon carbide contains 8 to 15% free silicon and other impurities, so its corrosion resistance will be reduced.

Application of silicon carbide sealing ring

Silicon carbide has a series of excellent physical and chemical properties and sealing properties, and is suitable for some high-parameter mechanical seal applications. Applied to various harsh working conditions:

1.High PV value: Silicon carbide has a light specific gravity, and as a moving ring, the centrifugal force generated during high-speed operation is small, and the vibration and deflection caused are also reduced, so that the pressure on the sealing end face is relatively stable, and the friction track is relatively fixed. In addition, silicon carbide has excellent thermal properties and can avoid thermal cracks. Due to the large elastic modulus of silicon carbide, it is not easy to produce plastic deformation under high medium pressure, and its resistance to mechanical deformation is strong. As a sealing ring with high PV value, it is better to choose hot-pressed SiC.

2.High temperature and low temperature: The excellent high-temperature mechanical and thermal properties of pressureless sintered SiC and hot-pressed SiC make them suitable for use as end face sealing materials in high-temperature or low-temperature fluids. They can be applied to media such as superheated water, hot oil, liquefied petroleum gas, liquefied ethylene, liquid ammonia and liquid nitrogen.

3.Strong corrosive media: Since pressureless sintered SiC and hot-pressed SiC have a single-phase structure and excellent corrosion resistance, they are suitable for various highly corrosive acids and alkalis. However, reaction sintered SiC contains 8-15% free silicon and other impurities, so the corrosion resistance is not as good as pressureless sintered SiC and hot-pressed SiC.

4.Medium containing solid particles: Many fluids in petrochemical industry contain solid particles. These particles not only cause severe abrasive wear on the sealing end face, but also cause local heating of the sealing end face, causing thermal cracking. Silicon carbide not only has high hardness and wear resistance, but also has excellent thermal shock resistance. The friction pair using hot-pressed SIC-pressureless sintering combination or silicon carbide-WC cemented carbide combination is particularly suitable for the above-mentioned working conditions.

Reasonable selection of materials: Since silicon carbide is a brittle material, its impact toughness is poor. Therefore, when designing the sealing structure, it is necessary to maximize the strengths and avoid weaknesses as much as possible, and give full play to the characteristics of silicon carbide materials. The shape of the sealing ring should be simple, and it is not suitable to slot and drill holes to avoid stress concentration. Appropriate chamfering should be done on the sealing end face to avoid edge chipping. For the silicon carbide sealing ring assembled by heat sleeve, the corresponding ring seat material should be selected according to the temperature, pressure and medium. Such as: titanium alloy, 4J-42 (CAP-42) alloy, stainless steel, etc., so as to obtain a good overall use effect.

In addition, it is necessary to improve the technical level of installation and maintenance, and strengthen operation and maintenance.

Pressureless sintered silicon carbide sealing ring process:

The method of vacuum sintering the formed silicon carbide biscuit in a vacuum sintering furnace is called pressureless sintering. The specific process is as follows: silicon carbide raw material——ball milling——adding binder——mixing——press molding—— Pressureless sintering – grinding – assembly – grinding.

Among them, pressureless sintering is a key link in the production of silicon carbide ceramics. The processing effect of this link will directly determine the quality of the finished product. Therefore, the choice of silicon carbide vacuum sintering furnace is very important.

The RVS-S series pressureless silicon carbide sintering furnace produced by SIMUWU can well meet the needs of silicon carbide pressureless sintering. It can be heated up to 2400 degrees Celsius and has good temperature uniformity and vacuum.

SIMUWU has more than ten years of sales and manufacturing experience in the field of vacuum furnaces, which can effectively meet the different process needs of customers. The mature engineer team can help manufacturers complete a series of needs from installation to after-sales.