The main usage of tungsten


Tungsten and its alloys are widely used in the industries of electron and electric light source. It is used in producing various lighting bulbs. The tube filament uses the doped tungsten wire which has the property of sagging resistance. Usually, the doped tungsten wire is added with rhenium. The thermocouples made by the tungsten-rhenium alloy wire with low content of rhenium and tungsten-rhenium alloy wire with high content of rhenium have an extremely wide temperature measurement range (0~2500 degrees celsius), good linear relationship between temperature and thermoelectromotive force, quick measuring reaction rate (3 seconds) and relatively cheap price. It is an ideal thermocouple which can measure in the hydrogen atmosphere.

Tungsten filament has not only triggered a revolution in lighting industry, but also become a thermionic emitter of electrons because of its high melting point on the premise of not losing its mechanical integrity. For example, it can be used as the electron source of scanning electron (micro-) microscope and transmission electron (micro-) microscope. It can also be used as the filament of X-ray tube. In the X-ray tube, the electron generated by tungsten filament is speeded up to make it collides with tungsten and tungsten- rhenium alloy anode before emitting X-ray from the anode. In order to produce X-ray, it requires very high energy of electron beam generated by tungsten filament. As a result, the spots on the surface collided by the electron beam are very hot. Therefore, rotating-anode is used in most of the X-ray tubes. Besides, the large size of tungsten filament can be used as the heating elements of vacuum furnace.

The density of tungsten is 19.25 g/cm3 which is around 2.5 times of iron (7.87 g/cm3). It is one of the metallic elements which have the heaviest periodic system. Based on this characteristic of tungsten, the manufacture of high-density tungsten alloys (high specific gravity tungsten alloy) has become an important application field of tungsten. The high-density tungsten alloys are made by using liquid-phase sintering process which adds nickel, iron, copper, and small amounts of other elements into the tungsten powder. The high-density tungsten alloys can be divided into two alloy systems according to their different components, which are tungsten-nickel-iron and tungsten-nickel-copper. Through the liquid-phase sintering process, the density of high-density tungsten alloys can reach 17~18.6 g/cm3. The so called liquid-phase sintering process is a sintering process of which a certain amount of liquid phase exists when the mixed powder compact is under the sintering temperature. Its advantage is that the liquid phase can wet the solid particles and can dissolve small amounts of solid matters, which greatly accelerates the process of densification and grain growth and achieve very high relative density. For example, the nickel iron powder which is often used in the liquid-phase sintering process will be smelted when it is in sintering. Although the solubility of nickel iron liquid is minimum in the solid phase tungsten (which occupies 95 percents of volume fraction), the solid tungsten is easily to dissolve in the nickel iron liquid. Once the nickel iron liquid wets the tungsten flux and dissolves a part of tungsten powder, the tungsten particles will change the shape and its internal pores will disappear when the liquid flow entering. If this process continues, the tungsten particles will coarsen and grow constantly and will generate the final products which are close to 100 percents compact and have the best microsturcture. Except for the high density, the high-density tungsten alloy produced by the liquid-phase sintering process has better impact property than pure tungsten. Its main usage is to make the military penetrator of high penetration.

Tungsten carbide can keep good hardness at above 1000 degrees celsius. It is an ideal tool for cutting and grinding. In 1923, the German Schroter invented WC-Co hard alloy by using this property of WC. As the WC-Co hard alloy brought huge business opportunities for being the cutter and stretching and stamping mould, it realized the industrial production soon in 1926-1927. In short, the mixture of tungsten metal powder (or W03) and carbon black is carbonized in hydrogen or vacuum under a certain temperature before forming the carbide tungsten (WC). With a certain proportional ingredients of WC and metallic colbat binger and by the process of milling, shaping and sintering, the cemented carbide products of tools, molds, rollers and percussive rock drills can be made.