We continue to review some of the most important materials in heat treatment and metallurgy.
Tungsten (chemical symbol: W)
Tungsten is a silver-gray to white lustrous metal (Fig. 1) that tarnishes in air, forming a protective oxide coating. Pure tungsten is soft and ductile and is readily extruded, forged, saw-cut, spun and drawn. But when impurities exist, the metal becomes brittle and difficult to work.
Tungsten is one of the five refractory metals that are known for their extraordinary corrosion resistance, wear resistance and high melting points. The other refractory metals are niobium, molybdenum, tantalum and rhenium. They are all adjacent to tungsten on the periodic table. Perhaps tungsten’s most impressive characteristic is its temperature resistance. It has the highest melting point of any metal, 3410°C (6170°F). This led to its use as an incandescent light filament.
In 1783, brothers José and Fausto Elhuyar were Spanish chemists at the Royal Basque Society in Bergara, Spain. They were the first to isolate tungsten, by reduction of tungstic acid with charcoal, and they are credited with the discovery of the element tungsten. Fausto Elhuyar was also commissioned by the king of Spain to build the distinguished building known as the "Palacio de Minería," or Palace of Mines, in Mexico City (Fig. 2). The name tungsten was derived from the Swedish tung sten, meaning "heavy stone.” Since tungsten is most commonly found in the mineral wolframite, the element is referred to as “wolfram” in many European countries, hence its chemical symbol W.
Approximately 60% of current tungsten consumption is used in the manufacture of tungsten-carbide materials. It is one of the hardest carbides and is used to make highly wear-resistant abrasives and cutting tools such as knives, drill bits, saw blades and milling tools used by the metal fabrication, woodworking and construction industries. Some jewelry is made of sintered tungsten carbide, tungsten carbide/metal composites and metallic tungsten. Due to tungsten's high melting point, it is used in arc-welding applications in combination with another electrically conductive metal such as silver or copper.
Superalloys containing tungsten, such as Hastelloy and Stellite, are used in high-performance applications such as turbine blades and rocket nozzles for applications such as the UGM-27 Polaris submarine-launched ballistic missile.
Here are some interesting facts about tungten.2
- Element classification: Transition metal
- Density (g/cc): 19.3
- Melting point (K): 3683
- Boiling point (K): 5930
- Atomic radius (pm): 141
- Atomic volume (cc/mole): 9.53
- Specific heat (@20°C J/g mole): 0.133
- Fusion heat (kJ/mole): (35)
- Evaporation heat (kJ/mole): 824
- Oxidation states: 6, 5, 4, 3, 2, 0
- Lattice structure: Body-centered cubic
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