Chemical reaction formula:
2W+C=W2C
W+C=WC
Tungsten powder (average particle size 3 ~ 5 microns) was prepared by hydrogenation and reduction of WO3. Then, the mixture of tungsten powder and carbon black with equal molar ratio (ball milling dry mixing is about 1t/cm2) is pressed and molded. Put the pressed compact into a graphite plate or crucible, heat it to 1, 400 ~ 1, 700℃ (preferably 1, 550 ~ 1, 650℃) in hydrogen flow with a graphite resistance furnace or an induction furnace, and carburize it to produce WC. The reaction starts around the tungsten particles, because W2C is formed in the initial stage of the reaction. Due to the incomplete reaction (mainly the low reaction temperature), in addition to WC, unreacted W and intermediate W2C remain. So it must be heated to the above high temperature. The maximum temperature should be determined according to the particle size of raw tungsten. If the average particle size is about 150μm, the reaction is carried out at a high temperature of 1550 ~ 1650℃.
Chemical reaction formula:
WO3 + 3H2 → W + 3H2O
2WO3 + 3C → 2W + 3CO2
2W+C=W2C
W+C=WC
According to the requirements of cemented carbide on the particle size of tungsten carbide WC, according to the different uses of cemented carbide, tungsten carbide with different particle sizes is used; Carbide tools, such as the V-shaped cutter of the foot cutter, are finished with ultrafine sub-fine tungsten carbide; Medium particle tungsten carbide is used as rough machining alloy; Heavy cutting alloy takes medium and coarse tungsten carbide as raw material; Coarse grained tungsten carbide is used for mining tools with high hardness and high impact load; The impact of rock is small, and the impact load is small. The wear-resistant parts are made of medium-sized tungsten carbide. When its wear resistance, pressure resistance and surface smoothness are emphasized, ultrafine sub-fine medium particle tungsten carbide is used as raw material; Impact tools are mainly made of medium-coarse grained tungsten carbide.
The theoretical carbon content of tungsten carbide is 6. 128% (atomic 50%). When the carbon content of tungsten carbide is greater than the theoretical carbon content, free carbon (WC+C) appears in tungsten carbide, which makes the surrounding tungsten carbide grains grow up during sintering, resulting in uneven cemented carbide grains. Tungsten carbide generally requires high bound carbon (≥6.07%) and free carbon (≤0.05%), and the total carbon depends on the production process and application scope of cemented carbide.
In general, the total carbon of tungsten carbide for vacuum sintering in paraffin wax process is mainly determined by increasing the content of bound oxygen in the briquette by 0.75 parts carbon before sintering, that is, the total carbon of WC =6. 13%+ oxygen content %×0.75 (assuming that the sintering furnace is in a neutral atmosphere, the total carbon of most tungsten carbide for vacuum furnace is actually less than the calculated value). The total carbon content of tungsten carbide in China can be roughly divided into three types. The total carbon content of tungsten carbide for vacuum sintering by paraffin wax process is about 6.18 0.03% (free carbon will increase). The total carbon content of tungsten carbide sintered by hydrogen in paraffin method is 6.13 0.03%. The total carbon content of hydrogen sintered tungsten carbide for rubber processing is 5.90 0.03%. The above processes are sometimes carried out alternately, so the determination of total carbon in tungsten carbide should be based on the specific situation.
The total carbon content of WC in alloys with different application ranges, different cobalt contents and different grain sizes can be slightly adjusted. Tungsten carbide with high total carbon can be selected for low cobalt alloy and tungsten carbide with low total carbon can be selected for high cobalt alloy. In a word, different specific use requirements of cemented carbide have different requirements for tungsten carbide particle size.