The Selection of Materials Used in Mechanical Seals

The sealing performance and service life of mechanical seal are related to the material of each part, especially the material of the end-face seal (friction pair), auxiliary sealing material and spring material.

1. The material for friction pairs of end face seals

The friction materials include graphite, ceramic, surfacing hard alloy, tungsten carbide alloy, SiC, fill PTFE, tin bronze, steel bonded hard alloy, stainless steel, phenolic plastic, nylon and so on. The properties of commonly used materials are as follows:

(1) The graphite

The advantages of graphite are corrosion resistance and self - lubricity, low friction coefficient, good thermal shock resistance and easy to process. The disadvantage is that the mechanical strength is low and there is porosity. These two shortcomings of graphite can be improved by impregnation and carburizing. The impregnated graphite can be divided into two kinds: immersion resin and impregnated metal. The resin impregnated graphite has good corrosion resistance, but not resistant to high temperature (heat resistant about 170~200 ℃); The impregnated metal graphite has good heat resistant (immersion bronze, aluminum, lead, such as resistance to high temperature can reach 400 ~ 500 ℃), but poor corrosion resistance. The graphite is the most widely used non-metallic material, it is used for low-speed mechanical seal ring and high - speed mechanical seal ring. Good graphite can compact with the naked eye, and with little finger touch, little black fingers.

(2) The cermet

The advantages of ceramics are corrosion resistance, high hardness and good wear resistance. The disadvantage is that it is brittle and hard to process. The Alumina, ceramic and metal ceramic are used more. The ceramic is used for corrosive medium, medium and low speed.

(3) The surfacing hard alloy

The hard alloy is surfacing on the sealing face of carbon steel, chromium steel and chromium nickel steel, it has the advantage of high hardness, good wear resistance, good temperature resistance (below 500 ℃), good corrosion resistance and cavitation. The disadvantage is that the pores, slag and surface hardness are not uniform.

(4) The tungsten carbide alloy

The metal carbide with high hardness and high melting point, such as tungsten carbide and titanium carbide, is used as the method of powder metallurgy with adhesive powder to suppress the sintering process. The advantages are high hardness, high strength, wear resistance, high temperature resistance, corrosion resistance and low linear expansion coefficient. The disadvantage is that it is brittle and difficult to process. Tungsten carbide is the most widely used sealing side material, and it is used as a static ring of low speed mechanical seal, high-speed mechanical seal ring.

2. The materials for auxiliary sealing ring

The requirements for auxiliary sealing materials are elastic, the friction coefficient is small, wear-resisting, heat-resistant and low-temperature property, anti-dielectric corrosion, dissolution and aging. In addition, the residual deformation is required after compression and long-term use. The commonly used auxiliary sealing rings are rubber and polytetrafluoroethylene, as well as soft polyvinyl chloride.

(1) The rubber

The rubber has good elasticity, cushioning, vibration absorption, heat resistance and corrosion resistance. The sealing effect of rubber sealing ring is good and is most widely used. The commonly used rubber has silicone rubber, rubber, chloroprene rubber and fluorine rubber.

(2) The polytetrafluoroethylene

The advantages of polytetrafluoroethylene are chemical stability, oil resistance, solvent resistance, excellent moisture resistance, low friction coefficient, and it is suitable for all kinds of corrosive media. The drawback is that the elasticity is worse than the rubber, and is prone to permanent deformation.

3. The elastomeric material

(1) The spring material

The requirements of spring materials are: good elasticity and corrosion resistance. The commonly used spring materials are stainless steel spring steel (1Cr18Ni9Ti, etc.), chromium steel (3Cr13, 4Cr13, etc.), carbon spring steel (60Si2Mn, etc.) and phosphorus bronze.

(2) The corrugated pipe material

The requirements of bellows material are good welding property, and greater elasticity and certain corrosion resistance. The commonly used bellows material has iron base, copper base and nickel base alloy and titanium etc. It is generally used in the iron base of nickel chromium austenitic strip, especially in 1Cr18Ni125MnMo2Ti. The high nickel elastic alloy is considered to be the ideal material for making corrugated tubes, and the materials containing aluminum will encounter difficulties with the general welding technology. The most widely used bellows materials is AM350 (approximately Cr16Ni45MnMo3N), it belongs to solid solution, and it has the performance of low strength, low hardness and high elongation.