In material conveying pipelines such as polysilicon, coal chemical industry, power plant ash removal, mud and boiler slag discharge, the valve sealing parts are not only subject to sliding friction and wear of the sealing sub-components, but also subject to high-speed impact of high-temperature and high-hardness gas-solid two-phase mixture, and flash evaporation and cavitation caused by high-pressure fluid, which in turn lead to damage and aggravate valve failure.
Overview
In material conveying pipelines such as polysilicon, coal chemical industry, power plant ash removal, mud and boiler slag discharge, the valve sealing parts are not only subject to the sliding friction and wear of the sealing sub-components, but also bear the high-speed impact of high-temperature and high-hardness gas-solid two-phase mixture, and the flash and cavitation caused by high-pressure fluid, which leads to damage and aggravates valve failure. Therefore, under harsh working conditions such as powder conveying, wear resistance becomes an important performance evaluation indicator of valves.
2. Analysis
According to the conditions and mechanisms of friction, wear has many different forms, the common ones are abrasive wear, adhesive wear, fatigue wear and corrosive wear, as well as erosion wear.
Common parameters for characterizing material wear performance include wear amount, wear rate, wear degree, wear resistance and relative wear resistance. The basic requirement for wear-resistant materials is that the surface of the object must have high hardness (the surface hardness should exceed the hardness of the abrasive), and it should also have good oxidation resistance at working temperature. The effective way to control or minimize wear is to improve the hardness and wear resistance of the material. Sintered tungsten carbide has high strength and high surface finish. When used in pairs with other materials, the friction coefficient is smaller than that of steel, which can significantly reduce the friction of the contact surface and effectively reduce the operating torque.
3. Features
The integral sintered tungsten carbide is made by heating a mixture of tungsten and carbon at high temperature. Most tungsten carbides have high hardness, and their microhardness is second only to diamond. The melting point is 2870℃. The boiling point is 6000℃, and the relative density is 15.63 (18℃). It is not easy to decompose at high temperature and has good oxidation resistance. The investigation of on-site use shows that the wear resistance of tungsten carbide under abrasive wear, erosion wear and abrasion is 100 times higher than that of tool steel, stainless steel, iron and brass, and the rigidity is 2 to 3 times higher than that of steel, 4 to 6 times higher than that of cast iron and brass, and the impact resistance is similar to that of hardened tool steel.
4. Application
In coal gasification and polysilicon, which are high temperature, high pressure, highly corrosive and contain solid particles of slurry and powder, the valve disc and valve seat of the commonly used hard-sealed ball valve, Venturi black water control valve, coal powder control valve, disc valve and slide valve are respectively sprayed with tungsten carbide hard alloy to form a sealing pair. Since the thickness of the tungsten carbide coating is <2mm, the hardness is <60HRC, and the bonding force between the coating and the substrate is <1000psi. Spraying is usually carried out under high temperature conditions of 10,000℃, and the operating conditions are harsh. It is difficult to guarantee the service life of the valve for 10,000 cycles, which cannot meet the long-term stable production requirements of systems such as coal chemical slurry and polysilicon powder. On the other hand, the strength of sprayed tungsten carbide is mainly provided by the substrate material. When the linear expansion coefficient of the two materials is large, the use is limited by the temperature and it is difficult to exceed 450℃. The valve disc and valve seat of the new hard-sealed ball valve, Venturi black water control valve (Figure 2), coal powder control valve (Figure 3), disc valve (Figure 4) and slide valve are all made of integrally sintered tungsten carbide hard alloy to form a sealing pair, and the valve performance has been significantly improved.
(1) High hardness. The hardness is >80HRC, which can withstand the high-speed erosion of multi-phase granular media such as water-coal slurry, coal powder, and silicon powder.
(2) High temperature resistance. It can work for a long time at a high temperature of 750℃. The strength, bonding force and thermal expansion are not limited by temperature, which fully meets the high-temperature working conditions such as coal chemical industry.
(3) High pressure resistance. The transverse fracture strength of integrally sintered tungsten carbide reaches 4000MPa, which is more than 10 times the strength of conventional steel. It can work for a long time at a working pressure level of 25MPa.
(4) Corrosion resistance. The chemical properties of integrally sintered tungsten carbide are stable. Even when heated, it is insoluble in water. It does not react with hydrochloric acid and sulfuric acid, and it will not be dissolved even in aqua regia. It is sufficient to meet the special requirements of special industries such as coal chemical industry for corrosion resistance.
(5) Wear resistance. The high hardness and high stability of integrally sintered tungsten carbide ensure the excellent wear resistance of the sealing auxiliary material, meeting the wear requirements of special media such as coal powder and organic silicon (silicon powder particle hardness is 62HRC).
(6) Erosion resistance. Conventional valves use sprayed tungsten carbide coating as the sealing surface. The valve internal parts are subjected to severe erosion. Usually, under the working condition of 250℃, they will become honeycomb within 1 month and completely lose their function. The Venturi black water control valve and coal powder control valve using integrally sintered tungsten carbide as the control part have a service life of 12 months at a temperature of 450℃ (other working conditions are the same). The number of switching times of frequently opened disc valves and slide valves is >300,000 times, which fully meets the long-term operation requirements of 8000h in coal chemical industry.
(7) Flexible high-temperature action performance. The ball and valve seat are both integrally sintered tungsten carbide hard alloy, and their linear expansion coefficients are 1/3 to 1/2 of conventional steel, which effectively avoids the high-temperature jamming phenomenon of conventional valves and ensures their excellent performance under high temperature difference conditions.
(8) Low friction. The use of sintered tungsten carbide anti-friction gaskets not only extends the high-temperature service life of the gasket, but also greatly reduces the friction between the two and the valve operating torque due to its low friction coefficient, which is usually only 1/3 to 1/2 of the conventional matching material.
5. Conclusion
Integrally sintered tungsten carbide hard alloy has high strength, high hardness, high melting point, high stability, low friction coefficient, wear resistance, erosion resistance and cavitation resistance, and corrosion resistance. It is used to manufacture wear-resistant valve seals for harsh working conditions, improves the applicability of valves, expands the scope of use of valves, extends the working life of valves, ensures various performance indicators of valves, and meets the development needs of the chemical industry.
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