Material of the month – tungsten

Materials World magazine
,
7 Apr 2015

This month, Anna Ploszajski explores the hard and rare metal known as wolframite

In 1556, De Re Metallica was post humously published by German scholar and metallurgist Georg Pawer, better known under the pen name Georgius Agricola. This extensive work defined him as the father of metallurgy, although he unfortunately died the year before its publication. In it, Agricola made reference to a mineral lupi spuma, meaning wolf’s froth, in reference to the way it consumed tin in its extraction, like wolves eat sheep, and to the froth produced in the process. In 1747, Johan Gottschalk Wallerius, an agricultural chemist from Sweden, translated the name into German, ‘wolf rahm’ and hence coined the term ‘wolframite’. 

In 1779, Peter Woulfe, a chemist from Ireland, perceived that a new element might exist in wolframite. Two years later, Carl Wilhelm Scheele isolated it as tungsten trioxide (WO3), by making tungstic acid from scheelite, a different tungsten ore from wolframite and one that was, confusingly, called ‘tungsten’ at the time, from the Swedish ‘tung sten’, meaning heavy stone. Scheele lacked a suitable furnace to reduce this oxide to its metal, but together with Torbern Bergman suggested it may be obtained by reducing the tungstic acid. Scheelite was later named in his honour. 

The isolation of pure tungsten was eventually achieved by brothers Juan José and Fausto de Elhuyar in 1783, who reduced tungstic acid by strongly heating it with powdered charcoal. They named the new metal wolfram, after its parent ore. Many languages, including English, have adopted ‘tungsten’ from the Swedish name for the ore scheelite. 

Tungsten metal is sourced from four different ores – wolframite ((Fe,Mn)WO4), scheelite (CaWO4), ferberite (FeWO4) and hübnerite (MnWO4). China holds around 75% of the world’s tungsten reserves and produces around 85% of tungsten metal worldwide, followed by Russia, Canada, Austria, Bolivia and Portugal. One of the largest tungsten and tin deposits in the world sits in Devon, UK, called the Hemerdon Mine. This source was exploited during the First and Second World Wars but has remained mostly unused since 1944, although drilling and exploration projects since 1960 promised vast tonnage in the deposit. Excitingly, a recent acquisition by Australian company Wolf Minerals will see around 3% of global tungsten supplied by the mine, set to commence this year. This will make the UK a significant player on the global tungsten scene and represents a historic reversal of the trending demise of the British mining industry.

Processing 

Modern tungsten ore processing uses the same method as that of the de Elhuyar brothers. First, the ore must be crushed and concentrated. Then, the non-tungsten-containing minerals can be removed by gravimetric, magnetic, flotation or characterisation methods. Ammonium paratungstate (APT) is the standard tradable commodity of tungsten precursor. It is produced by removing sulphides and arsenides from ore concentrate by heating the mixture to 800°C and then decomposing it by chemical methods, using acid leaching or the autoclave-soda process. This yields different tungsten oxides, which can be reduced by hydrogen or carbon to create pure, powdered tungsten metal.

Together with niobium, molybdenum, tantalum and rhenium, tungsten is part of the group of refractory metals, due to its high-melting temperature. Tungsten has the highest melting point of all metallic elements at 3,422°C, and is therefore very difficult to work with. It is not feasible to cast liquid tungsten, so instead, powdered tungsten is usually mixed with nickel and then sintered to form an alloy. The metal can also be worked by forging, drawing or extruding. 

Switching on

The most popular everyday application of tungsten is the incandescent lightbulb. In December’s Material of the Month, I answered the question, ‘How many scientists does it take to change a lightbulb?’ the answer was a great many. 

Tungsten’s role in bringing about the lightbulb began in 1904, when Sandor Just and Franjo Hanaman patented its use as a filament material, recognising its superior functionality at high temperatures. But it was found that at such temperatures, the filaments suffered from severe creep, quickly sagging and short-circuiting. This problem was solved by doping the wire with potassium. The microscopic potassium inclusions are gaseous at the lightbulb’s operating temperature, and these bubbles of pressurised gas pin the grain boundaries during operation, preventing creep and filament sag and significantly increasing its functional lifetime. 

Properties

Other high-temperature electronic applications for tungsten filaments include cathode ray and vacuum tube filaments and heating elements. Tungsten is often used for the emitter tips in electron beam instruments, due to its ability to conduct electricity together with chemical inertness. Tungsten inert gas (TIG) welding is an arc welding technique in which current is passed through a tungsten electrode, forming an arc through a plasma, shielded by an inert gas such as argon or helium. 

As we have seen, tungsten is a material with extreme properties. It is 1.7 times denser than lead and has the highest tensile strength of all metallic elements and the lowest co-efficient of thermal expansion. These properties are due to the strong covalent bonds formed between tungsten atoms by the 5D electrons, giving tungsten metal applications as military projectiles, weights, counterweights and ballasts in aircraft and Formula 1 vehicles.

The density of tungsten differs from that of gold by just 0.3%. As such, tungsten has been used to counterfeit gold bars and coins, by plating tungsten blocks with a thin layer of gold or drilling holes in pure gold bullion and filling with tungsten rods. These fakes defy visual and weight tests, and the traditional way to detect a bogus bar would be to bite into the gold. Alloying elements would harden the metal, and the tester may penetrate through a gold-plated layer to the cheaper metal underneath. These days, non-destructive analytical techniques such as X-ray fluorescence and conductivity measurements using eddy currents can quickly identify phoney investments.

Alongside chromium, vanadium and molybdenum, tungsten adds high strength to steels by solid solution strengthening and the formation of intermetallic compounds and carbides. These carbides form because tungsten has the effect of lowering the eutectoid concentration of carbon. As such more carbon in the alloy precipitates out to form carbides and complex carbides than would do so in steel without tungsten. The inclusions increase the hardness and wear-resistance of the steel, and a fine-grained-structure then gives enhanced toughness. High-speed steels can contain up to 18% tungsten and are used in cutting tools such as drill bits and blades, offering superior performance at temperatures up to 500°C. 

Tungsten also has an important role to play in the superalloys responsible for carrying us around the world – adding tungsten to nickel in Hastelloy and cobalt-chromium alloys for Stellite. These superalloys find use in unforgiving environments that turbine blades and industrial machinery operate in. Professional quality darts are made with more than 90% tungsten, for superior weight at a narrow diameter. 

Heating powdered tungsten with equal measures of carbon forms tungsten carbide (WC). Together with an approximate hardness of 9mohs, WC is extremely dense (15.63g/cm3) and stiff (Young’s Modulus 550GPa). These properties make WC a popular component in composite materials for high-temperature cutting tools – it is often embedded in a cobalt metallic matrix. This application accounts for around 60% of current global tungsten consumption. Other applications include sports equipment, the rotating ball in ballpoint ink pens, and jewellery. Sintered WC or WC/nickel composites are gaining popularity in wedding jewellery due to WC’s high resistance to scratching and its hypoallergenic nature. 

Tungsten-based technologies have truly moulded the way we live in the modern world. The metal’s extreme properties brought us electrical lighting, air transport and superior steels, but those same characteristics rendered the metal elusive to the early metallurgists who sought it. The rewards of their efforts have been worth their weight in gold.