Tungsten carbide (WC) is an inorganic non-natural compound composed of tungsten (W) and carbon (C). Mono-tungsten carbide (WC), which is stable at room temperature, is of major technical relevance. WC exhibits a hexagonal structure made of a grid of tungsten and carbon. It’s most striking properties are a high density and a high melting point of 2600 °C, a high hardness as well as the metal-like, high values of electric and thermal conductivity.
The usage of carbide in the Industry
The WC grid causes a certain plasticity and high breaking ductility while maintaining the high hardness of tungsten carbide. Exposed to air, WC corrodes only at temperatures above 600 °C. Furthermore, besides its high hardness at ambient temperature and at temperatures above 1000 °C, the ability to bind metals such as cobalt, nickel and iron is responsible for its economic importance. WC is mainly used for the production of hard metals. Hard metals are alloys consisting of tungsten carbide and metals belonging to the iron group, particularly cobalt as plastic metal-binder.
Because WC decays during melting, the production of compact pieces is only possible via sintering (comparable to the burning of clay). Hard metals are used for the production of tools for turning, milling and drilling, as well as cutting and stamping. Furthermore, they are used for wear-resistant parts and reinforcements. By varying the cobalt content and the carbide grain size, important properties such as hardness and ductility can be adapted to the desired specification for its application. Tungsten carbide is not self-igniting. As a mixture with air (dust) under the influence of an ignition source, carbide is possibly flammable (dust explosion).
Occurrence and Production
WC do not occur naturally. WC is technically produced by converting powdery tungsten metal or tungsten oxide with carbon black or graphite at temperatures between 1400 °C and 2000 °C in a graphite lined furnace (carbothermic reaction). WC is always produced in powdery form. The particle size depends on the size of the starting material and process conditions. In the traditional production process it can be varied between 100 nm and 100 µm.
Tungsten can be found as tungstenate in tungstenite and scheelite in the earth crust. The tungsten content of these ores is low and mostly below 1 mass per cent. In mediaeval times in the Saxon Ore Mountains, tungstenates contained in tin ores interfered with tin production by “consuming” the tin “like a wolf“. The by far biggest tungsten deposits are located in China. In Europe, mines in the Russian Federation and in Austria are of major relevance. In the Ore Mountains, there are tungsten ore deposits as well. The worldwide output volume is above 70.000 t/a.