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Tungsten carbide

  • Author:Michelle
  • Source:Www.xy-global.com
  • Release on:2018-07-20
  Tungsten carbide (Tungsten Carbide)Is a compound composed of tungsten and carbon. The molecular formula is WC and the molecular weight is 195.85.It is a black hexagonal crystal with a metallic luster and a hardness similar to that of diamond. It is a good conductor for electricity and heat. Tungsten carbide is insoluble in water, hydrochloric acid and sulfuric acid, and is easily soluble in a mixed acid of nitric acid-hydrofluoric acid. Pure tungsten carbide is brittle, and if a small amount of metal such as titanium or cobalt is incorporated, brittleness can be reduced. Tungsten carbide used as a steel cutting tool is often added with titanium carbide, tantalum carbide or a mixture thereof to improve the antiknock ability. The chemical properties of tungsten carbide are stable. Tungsten carbide powder is used in cemented carbide production materials.

Active oxidation occurs at temperatures above 500 ° C in the air, and the antioxidant capacity is weak.
Strong acid resistance.
Chemical reaction formula: W + C = WC Note: The reaction is carried out at 1150 °C.
It does not react with chlorine below 400 ° C; it reacts violently with fluorine at room temperature; it is oxidized to tungsten oxide when heated in air.

Using tungsten and carbon as raw materials, the tungsten powder with an average particle diameter of 3 to 5 μm and the carbon black of the same amount are dry-blended with a ball mill, thoroughly mixed, and then placed into a graphite disk by press molding, and then placed in a graphite resistance furnace. Or in an induction furnace to heat to 1400 ~ 1700 ° C, preferably controlled at 1550 ~ 1650 ° C. In the hydrogen stream, W2C is initially formed and continues to react at high temperatures to form WC. Or first, the tungsten hexacarbonyl is thermally decomposed in a CO atmosphere at 650 to 1000 ° C to obtain a tungsten powder, and then reacted with carbon monoxide at 1150 ° C to obtain WC, and a temperature higher than the temperature can generate W2C.
Chemical reaction formula:
2W + C = W2C
W + C = WC
The tungsten trioxide WO3 is hydrogenated and reduced to obtain a tungsten powder (average particle size of 3 to 5 μm). Further, a mixture of tungsten powder and carbon black in an equimolar ratio (dry mixing with a ball mill for about 10 hours) was pressure-molded under a pressure of about 1 t/cm 2 . The pressed molding block is placed in a graphite disk or crucible, and heated in a hydrogen flow (using a pure hydrogen having a dew point of -35 ° C) to a temperature of 1400 to 1700 ° C (preferably 1550 to 1650) using a graphite resistance furnace or an induction furnace. °C), so that carburizing produces WC. The reaction proceeds from around the tungsten particles because W2C is formed in the initial stage of the reaction, and unreacted W and intermediate product W2C remain in addition to WC due to incomplete reaction (mainly low reaction temperature). Therefore, it is necessary to heat to the above high temperature. The maximum temperature should be determined based on the particle size of the raw material tungsten. When the coarse particles having an average particle size of about 150 μm are used, the reaction is carried out at a high temperature of 1550 to 1650 °C.  
Chemical reaction formula:
WO3 + 3H2 → W + 3H2O
2WO3 + 3C → 2W + 3CO2
2W + C = W2C
W + C = WC
Cemented carbide requirements for WC grain size of tungsten carbide, according to different applications of cemented carbide, using different sizes of tungsten carbide; carbide cutting tools, such as cutting machine blade V-CUT knife, etc., finishing alloy using ultra-fine Fine-grained tungsten carbide; coarse-grained alloy using medium-grain tungsten carbide; gravity-cutting and heavy-cutting alloys using medium-thick grain tungsten carbide as raw material; mining tool rock hardness high impact load using coarse-grained tungsten carbide; rock impact small impact load Small, using medium-grain tungsten carbide as raw material wear-resistant parts; when emphasizing its wear resistance and surface roughness, the ultra-fine sub-fine medium-grain tungsten carbide is used as the raw material; the impact-resistant tool is made of medium-thick granular tungsten carbide raw material. the Lord. [5]
The theoretical carbon content of tungsten carbide is 6.128% (atomic 50%). When the carbon content of tungsten carbide is larger than the theoretical carbon content, free carbon (WC+C) appears in tungsten carbide, and the carbonization around it exists in the presence of free carbon. The tungsten grain grows, resulting in uneven grain size of the cemented carbide; tungsten carbide generally requires high carbon (≥6.07%) free carbon (≤0.05%), and the total carbon is determined by the production process and application range of the cemented carbide.
Under normal circumstances, the total carbon of tungsten carbide used for vacuum sintering in paraffin process is mainly determined by the chemical oxygen content in the briquettes before sintering. The oxygen content of a portion of oxygen is increased by 0.75 parts, ie WC total carbon = 6.13% + oxygen content % × 0.75 (assuming sintering The furnace has a neutral atmosphere. In fact, most of the vacuum furnaces used in the carburizing atmosphere have a total carbon carbide content of less than the calculated value. The total carbon content of tungsten carbide in China is roughly divided into three types. The total carbon content of tungsten carbide for vacuum sintering is about 6.18±0.03% (free carbon will increase). The total carbon content of tungsten carbide for hydrogenation in paraffin process is 6.13±0.03. % Rubber Process Hydrogen Sintering Tungsten Carbide Total Carbon = 5.90 ± 0.03% The above processes are sometimes crossed. Therefore, it is determined according to the specific conditions of the total tungsten carbide.
The WC total carbon used in alloys with different ranges of use, different cobalt contents and different grain sizes can be adjusted slightly. For low-cobalt alloys, tungsten carbide with a high total carbon can be used, and for high-cobalt alloys, tungsten carbide with a low total carbon can be used. In short, the specific use requirements of cemented carbides have different requirements on the particle size of tungsten carbide.It is widely used as a high-speed cutting tool, kiln structural material, jet engine parts, cermet materials, and resistance heating elements. 

It is used to manufacture cutting tools, wear parts, smelting crucibles of metals such as copper, cobalt and bismuth, and wear-resistant semiconductor films. 
Used as super hard tool material and wear resistant material. It forms a solid solution with many carbides. WC-TiC-Co carbide tools have been widely used. It can also be used as a modified additive for NbC-C and TaC-C ternary system carbides, which can reduce the sintering temperature while maintaining excellent performance and can be used as aerospace materials. 
The tungsten carbide (WC) powder was synthesized by using tungstic anhydride (WO3) and graphite in a reducing atmosphere at a high temperature of 1400 to 1600 °C. The dense ceramic product can be obtained by hot press sintering or hot isostatic pressing.
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