Properties and Uses
Pure tungsten is a shiny white metal and in its purest form is quite pliant and can easily be processed. Usually, however, it contains small amounts of carbon and oxygen, which give tungsten metal its considerable hardness and brittleness.
Tungsten occurs in the natural state only in the form of chemical compounds with other elements. Although more than 20 tungsten bearing minerals are known, only two of them are important for industrial use, namely wolframite and scheelite.
Pure scheelite has blue-white fluorescence in ultraviolet light, a property which is utilised in prospecting.
Wolframite is a general term for iron and manganese tungstates where the iron/manganese ratio can vary. A mineral with more than 80% FeWO4 is called Ferberite and a mineral with more than 80% MnWO4 is called H¸bnerite.
Tungsten is the heaviest engineering material with a density of 19.25 g/cm3.
It has the highest melting point of all metals at 3410∞C with a boiling point of 5700∞C.
It has the lowest vapour pressure of all metals.
It has the highest modulus of elasticity of the metals (E = 400GPa).
It is the hardest pure metal.
Excellent high temperature strength characteristics.
It has the highest tensile strength at temperatures above 1650∞C.
It has a low thermal expansion co-efficient (4.4 x 10-6 m/m/∞C) similar to that of borosilicate glass, and therefore makes it useful for glass to metal seals.
It does not oxidise in air and needs no protection from oxidation at elevated temperatures.
Its corrosion resistance is excellent, and it is not attacked by nitric, hydrofluoric, or sulphuric acid solutions.
Due to the unique properties of tungsten, tungsten alloys and some tungsten compounds listed above, the metal cannot be substituted in many important applications in different fields of modern technology.
Today, the main use of tungsten is in the manufacture of cemented carbides. Cemented carbides, or hardmetals as they are often called, are materials made by “cementing” very hard tungsten monocarbide (WC) grains in a binder matrix of tough cobalt metal by liquid phase sintering which results in a hardness close to diamond.
Filaments for electric lamps – electrical and electronic contacts, wire, rods and so on.
Inert gas welding electrodes.
Metal evaporation work.
As an alloy (steels) it is used for high-speed steel tools, weights and counterbalances, radiation shielding, grinding tools.
Electronic applications such as electric contacts points for automobile distributors, heat sinks, electrochemical machining and electrodes for electrical-discharge machining (EDM).
Windings and heating elements for electric furnaces.
Space missiles, rocket nozzles and high-temperature applications as a coating.
As yarn itís used for reinforcement in metal, ceramic and plastic composites.
Magnetrons for microwave ovens.
Metalworking, mining, cutting tools bits, heat- and erosion-resistant parts, coatings, seal rings and petroleum.
Calcium and magnesium tungstates are widely used in fluorescent lighting.
Other tungsten salts are used in tanning industries.
Tungsten disulphide is used as a dry high temperature lubricant (stable to 500∞C).
Tungsten bronzes and other compounds are used as pigments for paints.