Tungsten

Topics Covered

Background

Key Properties

Applications

Tungsten Metal

Tungsten Compounds

Background

In 1779 Peter Woulfe examined the mineral commonly know today as wolframite and concluded in his examination there to be a new substance within it. It was not until 1783 that the de Elhuyar brothers found an acid in wolframite (tungstic acid) that was identical to another acid found in the mineral known today as scheelite. In that same year they succeeded to obtain the element by reduction of this acid with charcoal.

Tungsten (W) occurs in wolframite ((Fe, Mn)WO₄), scheelite (CaWO₄), huebnerite (MnWO₄) and ferberite (FeWO₄). The metal is obtained commercially by the reduction of tungsten oxide with hydrogen or carbon.

Pure tungsten is a steel-grey to tin-white metal. The pure metal has the ability to be cut with a hacksaw, may be forged, spun, drawn, and extruded. However, the impure metal is brittle and can be worked with some difficulty.

Natural tungsten has five stable isotopes, with a further twenty-one other unstable isotopes are recognised.

Key Properties

The main properties of interest for tungsten are:

^•        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

•         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 (4.4x10^-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 temperaratures

•         Its corrosion resistance is excellent, and it is not attacked by nitric, hydrofluoric, or sulphuric acid solutions

Applications

Tungsten Metal

Tungsten metal is used in:

•         Filaments for electric lamps, electron and television tubes.

•         Inert gas welding electrodes.

•         Metal evaporation work.

•         Tungsten alloys (steels): high-speed tool steels (e.g. Hastelloy^® and Stellite^®), weights and counterbalances, radiation shielding, grinding tools.

•         Magnets.

•         Heavy metals.

•         Electronic applications such as electric contacts points for automobile distributors, heat sinks, electrochemical machining, electrodes for electrical-discharge machining (EDM).

•         X-ray targets.

•         Windings and heating elements for electric furnaces.

•         Electroplating.

•         Space missiles, rocket nozzles and high-temperature applications as a coating.

•         As tungsten yarn, it is used for reinforcement in metal, ceramic and plastic composites.

•         Its high density makes it useful for high inertia devices and for balancing masses such as balancing weights for flywheels.

Tungsten Compounds

Tungsten compounds also have a number of uses:

•         Tungsten Carbide - 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.

Primary author: AZoM.com