Tungsten and Tungsten Alloys

Abstract

Tungsten, discovered in 1755 and produced as a metal in 1783, is notable for having a very low vapor pressure, the highest melting point of any metal, and the highest tensile strength of any metal above 1650°C. It has been used formerly in the manufacture of high speed steels and as a metal filament in electric lamps, where it was fabricated by powder metallurgy using the Coolidge process. The largest modern application is in the manufacture of cemented carbide cutting tools. Tungsten is rated as a strategic material in the U.S. economy. Tungsten occurs in a number of mineral forms, the most important being scheelite and wolframite. The beneficiation and hydrometallurgy are described, together with the principal intermediate compounds used to make the metal powder by hydrogen reduction. Technically it is possible to process compacted powder by arc or electron‐beam melting, but commercially the power metallurgy process of compaction, sintering, and working is used exclusively. The largest reserves of ores occur in China, which has, in more recent years, become the principal source of supply not only for ores but for metal powder and intermediates at prices that have caused most U.S. mines to be closed. About 65% of tungsten usage is as tungsten carbide in cutting and mining tools. The instability of the market as well as economic and environmental factors have favored development of recycling processes. Used cutting tools can be treated with molten zinc and both are recovered tungsten carbide and cobalt binder as mixed powders after vacuum‐distilling off the zinc. It is estimated that 25% of cutting tools are recycled in this way in the United States. Tungsten alloyed with nickel and iron is used for a range of high density compositions that find applications as counterweights in aircraft, as armor‐piercing penetrators, and for collimators and shielding in x‐ray machines. These alloys are characterized by high density and high modulus, which make them suitable for use in critical dynamic structures. Tungsten forms only a few alloys; tungsten–rhenium is important for thermocouples and as a tough lamp filament alloy. Insoluble tungsten has relatively low toxicity, but aerosols with cobalt and with impurities found in mining can have some effect and require controls of exposure level.