Tungsten carbide is a compound composed of tungsten and carbon, with a molecular formula of WC and a molecular weight of 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 of electricity and heat. Tungsten carbide is insoluble in water, hydrochloric acid and sulfuric acid, but easily soluble in a mixed acid of nitric acid and hydrofluoric acid. Pure tungsten carbide is brittle, but if a small amount of metals such as titanium and cobalt are added, the brittleness can be reduced. Tungsten carbide used as a steel cutting tool often adds titanium carbide, tantalum carbide or their mixture to improve its explosion resistance. The chemical properties of tungsten carbide are stable. Tungsten carbide powder is used in cemented carbide production materials.

Brief History of Development
Since 1893, German scientists have used tungsten trioxide and carbon to heat to high temperatures in an electric furnace to produce tungsten carbide, and tried to use its high melting point, high hardness and other characteristics to make wire drawing dies, etc., in order to replace diamond materials. However, due to the brittleness, easy cracking and low toughness of tungsten carbide, it has not been industrially applied. In the 1920s, German scientist Karl Schroter found that pure tungsten carbide could not adapt to the intense stress changes formed during the drawing process. Only by adding low-melting-point metals to WC can the blank have a certain toughness without reducing the hardness. Schroter first proposed a patent for the production of hardness alloys using powder metallurgy in 1923, that is, mixing tungsten carbide with a small amount of iron group metals (iron, nickel, cobalt), then pressing and forming, and sintering in hydrogen at a temperature above 1300°C.

Production Method
Using metal tungsten and carbon as raw materials, tungsten powder with an average particle size of 3-5μm and an equal amount of carbon black are dry mixed in a ball mill. After being fully mixed, they are pressed and formed into a graphite plate, and then heated to 1400-1700°C in a graphite resistance furnace or induction furnace, preferably controlled at 1550-1650°C. In a hydrogen flow, W2C is initially generated, and continues to react at high temperature to generate WC. Alternatively, firstly, tungsten hexacarbonyl is thermally decomposed at 650-1000℃ in a CO atmosphere to obtain tungsten powder, and then reacted with carbon monoxide at 1150℃ to obtain WC. W2C can be generated at a temperature higher than this temperature.
Tungsten trioxide WO3 is hydrogenated and reduced to obtain tungsten powder (average particle size 3-5μm). Then, a mixture of tungsten powder and carbon black in an equal molar ratio (dry mixed in a ball mill for about 10h) is pressed and formed at a pressure of about 1t/cm2. The pressed block is placed in a graphite plate or crucible, and heated to 1400-1700℃ (preferably 1550-1650℃) in a hydrogen flow (using pure hydrogen with a dew point of -35℃) using a graphite resistance furnace or an induction furnace to carburize and generate WC. The reaction starts around the tungsten particles, because W2C is generated in the early stage of the reaction. Due to the incomplete reaction (mainly due to the low reaction temperature), in addition to WC, there are still unreacted W and intermediate products W2C. Therefore, it must be heated to the above high temperature. The maximum temperature should be determined according to the particle size of the raw tungsten. For example, if the average particle size is about 150μm, the reaction is carried out at a high temperature of 1550-1650℃.
The requirements of cemented carbide for the particle size of tungsten carbide WC, according to the different uses of cemented carbide, different particle sizes of tungsten carbide are used; cemented carbide cutting tools, such as V-CUT knives for cutting machine blades, etc., use ultra-fine and sub-fine particles of tungsten carbide for finishing alloys; medium particles of tungsten carbide are used for roughing alloys; medium and coarse particles of tungsten carbide are used as raw materials for gravity cutting and heavy cutting alloys; coarse particles of tungsten carbide are used for mining tools with high rock hardness and large impact load; medium particles of tungsten carbide are used as raw materials for wear-resistant parts with small rock impact and small impact load; when emphasizing its wear resistance, pressure resistance and surface finish, ultra-fine and sub-fine particles of tungsten carbide are used as raw materials; impact-resistant tools mainly use medium and coarse particles of tungsten carbide as raw materials.
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. The presence of free carbon causes the surrounding tungsten carbide grains to grow during sintering, resulting in uneven cemented carbide grains. Tungsten carbide generally requires high combined carbon (≥6.07%) and free carbon (≤0.05%), and the total carbon is determined by the production process and scope of use of cemented carbide.
Under normal circumstances, the total carbon of tungsten carbide used in paraffin process vacuum sintering is mainly determined by the combined oxygen content in the block before sintering. One part of oxygen requires 0.75 parts of carbon, that is, WC total carbon = 6.13% + oxygen content% × 0.75 (assuming that the sintering furnace is a neutral atmosphere, in fact, most vacuum furnaces are carburizing atmospheres, and the total carbon of tungsten carbide used is less than the calculated value).

The total carbon content of tungsten carbide in China is roughly divided into three types: the total carbon of tungsten carbide for vacuum sintering in paraffin process is about 6.18±0.03% (free carbon will increase); the total carbon content of tungsten carbide for hydrogen sintering in paraffin process is 6.13±0.03%; the total carbon of tungsten carbide for hydrogen sintering in rubber process = 5.90±0.03%; the above processes are sometimes carried out crosswise, so the determination of the total carbon of tungsten carbide should be based on the specific situation.

The total carbon of WC used in alloys with different application ranges, different cobalt contents, and different grain sizes can be adjusted slightly. Low-cobalt alloys can use tungsten carbide with a relatively high total carbon, while high-cobalt alloys can use tungsten carbide with a relatively low total carbon. In short, the specific use requirements of cemented carbide are different, and the requirements for the particle size of tungsten carbide are also different.

Application Field

It is used in large quantities as high-speed cutting tools, kiln structure materials, jet engine parts, metal ceramic materials, resistance heating elements, etc.

It is used to manufacture cutting tools, wear-resistant parts, melting crucibles for metals such as copper, cobalt, and bismuth, and wear-resistant semiconductor films, for example Mulching Carbide Teeth Type C for Caterpillar HM315B.
Used as superhard tool material and wear-resistant material. It can form solid solutions with many carbides. WC-TiC-Co cemented 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.
Tungsten carbide (WC) powder is synthesized by using tungsten anhydride (WO3) and graphite at a high temperature of 1400-1600℃ in a reducing atmosphere. Then, dense ceramic products can be obtained by hot pressing or hot isostatic pressing.