Refractory metals

Refractory metals are a class of metals extraordinarily resistant to heat, wear, and corrosion. These properties make them useful in many applications. Household incandescent bulbs contain refractory metals in their tungsten filaments, and nearly all manufactured goods, particularly those containing metal or electronics, contain or were produced using refractory metals.

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The five refractory metals are:

• Tungsten (W)
• Molybdenum (Mo)
• Niobium (Nb)
• Tantalum (Ta)
• Rhenium (Re)

Refractory metals are used in lighting, tools, lubricants, nuclear reaction control rods, as catalysts, and for their chemical or electrical properties. Because of their high melting point, refractory metal components are never fabricated by casting. The process of powder metallurgy is used. Powders of the pure metal are compacted, heated using electric current, and further fabricated by cold working with annealing steps. Refractory metals can be worked into wire, ingots, bars, sheets or foil.

Tungsten was discovered in 1781 by the Swedish chemist, Karl Wilhelm Scheele. Tungsten is both the most abundant of the refractory metals, and has the highest melting point of all metals, at 3,410C(6,170F). Tungsten wire filaments provide the vast majority of household incandescent lighting, but are also common in industrial lighting as electrodes in arc lamps. TIG-welding (Tungsten Inert Gas welding) or GTAW-welding (Gas Tungsten Arc welding) equipment uses a permanent, non-melting tungsten electrode. The most common use for tungsten is as the compound tungsten carbide in drill bits, machining and cutting tools. It also finds itself serving as a lubricant, antioxidant, in nozzles and bushings, as a protective coating and in many other ways. Tungsten can be found in printing inks, x-ray screens, photographic chemicals, in the processing of petroleum products, and flame proofing of textiles. Tungsten is also used by virtue of its strength and density, in applications ranging from weights in helicopter rotors and weapon projectiles to the heads of golf clubs. The largest reserves of tungsten are in China, with deposits in Korea, Bolivia, Australia, and other countries.

Molybdenum is the most commonly used of the refractory metals. Its most important use is as a strengthening alloy of steel. Structural tubing and piping often contains molybdenum, as do many stainless steels. Its strength at high temperatures, resistance to wear and low coefficient of friction are all properties which make it invaluable as an alloying compound. Its excellent anti-friction properties lead to its incorporation in greases and oils where reliability and performance are critical. Automotive constant-velocity joints use grease containing molybdenum. The compound sticks readily to metal and forms a very hard, friction resistant coating. Most of the world's molybdenum ore can be found in the USA and Canada.

Niobium is nearly always found together with tantalum, and was named after Niobe, the daughter of the mythical Greek king Tantalus for whom tantalum was named. Niobium has many uses, some of which it shares with other refractory metals. It is unique in that it can be worked through annealing to achieve a wide range of strength and elasticity, and is the least dense of the refractory metals. It can also be found in electrolytic capacitors and in the most practical superconducting alloys. Niobium can be found in aircraft gas turbines, vacuum tubes and nuclear reactors.

Tantalum is one of the most corrosion resistant substances available. Many important uses have been found for tantalum owing to this property, particularly in the medical and surgical fields, and also in harsh acidic environments. It is also used to make superior electrolytic capacitors. Tantalum films provide the most capacitance per volume of any substance, and allow miniaturization of electronic components and circuitry. Cellular phones and computers contain tantalum capacitators.

Rhenium is the most recently discovered refractory metal. It is found in low concentrations with many other metals, in the ores of other refractory metals, platinum or copper ores. It is useful as an alloy to other refractory metals, where it adds ductility and tensile strength. Rhenium alloys are being found in electronic components, gyroscopes and nuclear reactors. Rhenium finds its most important use as a catalyst. It is used as a catalyst in reactions such as alkylation, dealkylation, hydrogenation and oxidation. However its rarity makes it the most expensive of the refractory metals.

The creep behavior of refractory metals

Refractory metals and alloys attract the attention of investigators because of their remarkable properties and on account of promising practical prospects.

Refractory metals are characterized by their extremely high melting points, which range well above those of iron and nickel. When the refractory metals are considered to be those metals melting at temperatures above 2123 K, twelve metals constitute this group: tungsten (the melting point 3683 K), rhenium, osmium, tantalum, molybdenum, iridium, niobium, ruthenium, hafnium, zirconium, vanadium, and chromium.

Physical properties of refractory metals, such as molybdenum, tantalum and tungsten, their strength, and high-temperature stability make them suitable material for hot metalworking applications and for vacuum furnace technology. Many special applications exploit these properties: for example, tungsten lamp filaments operate at temperatures up to 3073 K, and molybdenum furnace windings withstand to 2273 K.

However, a poor low-temperature fabricability and an extreme oxidability at high-temperatures are shortcomings of the most refractory metals. Interactions with environment can significantly influence on their high-temperature creep strength. Application of these metals requires a protective atmosphere or a coating.

The refractory metal alloys of molybdenum, niobium, tantalum, and tungsten have been applied for the space nuclear power systems. These systems were designed to operate at temperatures from 1350 K to approximately 1900 K. An environment must not interact with the material in question. Liquid alkali metals as the heat transfer fluids are used as well as the ultrahigh vacuum.

The high-temperature creep strain of alloys must be limited for them to be used. The creep strain should not exceed 1-2%. An additional complication in studying creep behavior of the refractory metals is interactions with environment, which can significantly influence the creep behavior.

References

Levitin, Valim (2006). High Temperature Strain of Metals and Alloys: Physical Fundamentals. WILEY-VCH. ISBN 978-3-527-31338-9. 

See also

• Refractory ceramic