Tungsten, with the CAS number 7440-33-7 and the chemical symbol W, is a refractory metal with the highest melting point of all metallic elements, a density close to that of gold, and exceptional hardness. It also conducts electricity and heat well and has a low coefficient of thermal expansion. These properties make tungsten indispensable in alloy steels, tungsten carbide hard alloys, heat- and wear-resistant alloys, and electronic components ranging from lamp filaments to X-ray targets.
Key Properties of Tungsten
| Property | Value |
|---|---|
| CAS number | 7440-33-7 |
| Chemical symbol | W |
| Melting point | About 3422 °C, the highest of all metals |
| Boiling point | About 5555 °C |
| Density | About 19.25 g/cm3, close to that of gold |
| Hardness | Very high; tungsten carbide approaches diamond hardness |
| Electrical and thermal conductivity | Good, with a low thermal expansion coefficient |
Because tungsten retains its strength at very high temperatures, it performs where most metals would soften or creep. This combination of high melting point, high density, high hardness and good conductivity underlies nearly every use of the metal.
Tungsten in Alloy Steels
Tungsten is a major alloying element in high-speed steels, tungsten steels and tungsten-cobalt magnetic steels. Even a few percent of tungsten substantially increases the hardness, wear resistance and red hardness of steel, meaning the cutting edge retains its hardness at the elevated temperatures generated during machining. High-speed steel cutting tools, dies and wear parts rely on this behavior for long service life.
Tungsten Carbide Hard Alloys
Tungsten carbide (WC) is produced by carburizing tungsten powder with carbon at high temperature. Hard alloys are then made by powder metallurgy: tungsten carbide powder is mixed with a metallic binder such as cobalt, nickel or molybdenum, compacted into shape, and sintered at high temperature. The resulting cemented carbides have wear resistance and hardness approaching that of diamond and are used in cutting tools, mining and drilling tools, wear parts, dies and nozzles.
Heat- and Wear-Resistant Alloys
When combined with metals such as cobalt, chromium and carbon, tungsten forms alloys for high-strength, wear-resistant components exposed to extreme conditions. Typical applications include engine valves, turbine impellers, rocket motor nozzles and other aerospace and high-performance components, where the alloy must retain mechanical integrity at high temperature.
Tungsten in Electronics
Tungsten's high melting point, low vapor pressure and long service life make it the classic material for incandescent and halogen lamp filaments. Tungsten filaments are also used in electron tubes, hot cathodes and other electronic instruments, and tungsten and its alloys serve as X-ray targets, electrical contacts and heat sinks in power and electronic equipment.
Other Applications
In the chemical industry, tungsten compounds are used as catalysts and in specialty chemicals. Tungsten heavy alloys, with their high density, are used for radiation shielding, balancing weights and vibration-damping applications, and tungsten electrodes are standard in gas tungsten arc welding.
Frequently Asked Questions
What is tungsten used for?
Tungsten is used in alloy steels, tungsten carbide cutting and mining tools, heat- and wear-resistant alloys, lamp filaments, electronic components, radiation shielding, welding electrodes and catalysts.
Why does tungsten have the highest melting point of all metals?
The very strong metallic bonding and high cohesive energy of tungsten atoms give it a melting point of about 3422 °C, the highest among metallic elements.
Is tungsten carbide harder than steel?
Yes. Tungsten carbide has a hardness approaching that of diamond, far exceeding hardened steel, which is why it dominates cutting and wear applications.
How is tungsten carbide made?
Tungsten ore is processed to tungsten oxide, reduced to tungsten powder, carburized to tungsten carbide, then mixed with a binder metal and sintered by powder metallurgy into hard alloy parts.
Is tungsten radioactive or toxic?
Natural tungsten is not radioactive, and the metal has low acute toxicity; however, tungsten dust and certain tungsten compounds can irritate the respiratory tract, and occupational hygiene measures should be followed during processing.
Why is tungsten used in lamp filaments?
Tungsten's very high melting point, low evaporation rate and good electrical conductivity let a thin filament operate at white-hot temperatures for thousands of hours without sagging or failing.