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AS-C420 double cartridge mechanical seal is customized dual seal made as per drawing
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How to choose and use metal parts for mechanical seals (part 3)
（1）What are the characteristics of steel bonded carbide，a commonly-used mechanical seal materials？
Steel bonded carbide is a commonly-used material in producing mechanical seals. What are their characteristics？
Steel-bonded cemented carbide is a cemented carbide with steel as the binder phase and titanium carbide as the hard phase. It has high wear resistance and rigidity (high modulus of elasticity), as well as high strength, hardness and certain toughness. Due to the small specific gravity of TiC, it is lighter in weight; at the same time, the TiC particles are round, the friction coefficient is greatly reduced, and it has better self-lubricating properties. The alloy also has good thermal shock resistance and can be used in situations where the temperature changes drastically. Its outstanding feature is that it has good processing technology and can be heat treated. After heat treatment, the dimensional change is small, and the deformation is within the range of dimensional error. In addition, it has good weldability and malleability. The corrosion resistance of steel-bonded carbide varies with the type of steel base. They have good corrosion resistance in seawater and alkali solution, and the corrosion resistance and mechanical properties in other media are shown in the following two tables:
Item | Base material | HCl 30% | HNO388% | H2SO4 50% | NaCl 10% | NaCl 50% | CH3COOH 10% | Temperature℃ | |
CT35 | Chromium Molybdenum Medium Alloy Steel | 14.5 | 2.0 | 5.0 | 0.28 | 0.04 | 3.21 | 13—21 | |
R5 | High carbon high chromium molybdenum | 11.6 | 1.7 | 4.7 | 0.16 | 0.04 | 0.94 | 13—21 | |
R8 | High chromium stainless steel | 12.4 | 0.6 | 3.8 | 0.10 | 0.03 | 0.12 | 13—21 | |
Parameter values of corrosion resistance of steel-bonded cemented carbide mg/Cm2 ·24h
Item | Density g/cm² | Hardness HRC | Anti-bending strength hMPa | Impact Toughness N·m/㎡ | Modulus of Elasticity MPa | Linear expansion coefficient × 0.00001(1/℃) | |||
Annealing | Quenching | Temperature ℃ | a | ||||||
CT35 | 6.50 | 39—46 | 68—72 | 1800 | 6 | 3060 | 2980 | —200 —700 | 8.43 11.83 |
R5 | 6.40 | 46 | 70—73 | 1300 | 3 | 3210 | 3130 | —200 —700 | 9.16 11.13 |
R8 | 6.25 | 45 | 62—66 | 1100 | 1.5 | —200 —700 | 7.58 10.60 | ||
Parameters characterizing the mechanical properties of steel-bonded carbide
（2）What are the characteristics of cobalt-based cemented carbide commonly used in mechanical seal materials
Cobalt-based tungsten carbide is the most widely used cemented carbide for mechanical seals. It is made of extremely hard tungsten carbide (WC) plus cobalt (Co) or other iron group metals as a binder, and is pressed and sintered by powder metallurgy. Its biggest feature is high hardness, high thermal conductivity and low linear expansion coefficient. Hard phases such as WC in cemented carbide have strong corrosion resistance and are very stable in corrosive media such as acid and alkali. However, the corrosion resistance of the bonding metal is poor, and the sealing medium of mechanical seals often corrodes the bonding metal first, so that the hard phase WC and the like are isolated and lost, and are quickly worn away in relative friction.So WC-Co cemented carbide is not suitable for corrosive occasions. However, it has strong corrosion resistance in media such as sulfuric acid at ordinary temperatures and caustic soda below the boiling point. WC-Co material has low impact toughness and high brittleness, and the machining cost is expensive.
The physical and mechanical properties of commonly used cobalt-based cemented carbides in making mechanical seals are shown in the table below.
Types of tungsten carbide | TC grade | Density g/cm² | Strength MPa | Hardness HRA | Linear expansion coefficient 0~500℃（1/℃） |
Tungsten cobalt | YG6 | 14.5~15.0 | 1400 | >89.5 | 4.5 |
YG8 | 14.4~14.8 | 1470 | >89 | 4.5 | |
Tungsten Cobalt Titanium | YT5 | 12.5~13.2 | 1274 | >89.5 | 6.06 |
YT14 | 11.2~12.4 | 1176 | >89.5 | 6.21 |
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