We all know that metals are some of the toughest materials out there, used in everything from tiny electronics and everyday appliances to massive construction projects and even spacecraft.
However, we also know that conditions involving high temperatures and excessive mechanical stresses can cause weaker metals to break down and no longer fulfil their purpose. This is why the most resistant metals are relied upon when materials must endure high-intensity environments.
That’s where refractory metals come in – several elements with some of the most useful properties, especially when utilised in a range of alloys and other compounds. Read on to learn more about what refractory metals are, their benefits, and their common uses around the world.
What is a refractory metal?
Refractory metals are metallic elements with a selection of unique physical properties that make them ideal for use in various scientific fields, including metallurgy and engineering. Though their characteristics vary according to their different periodic table groups, refractory metals are all extremely resistant to high temperatures.
Generally, metal elements with a melting point higher than zirconium (1,852°C) can be counted as refractory metals – such as chromium, iridium, and titanium. However, the five elements that best fit the refractory metal category are widely accepted to be:
- Niobium (Nb)
- Molybdenum (Mo)
- Tantalum (Ta)
- Tungsten (W)
- Rhenium (Re)
These elements all have strong qualities that allow them to endure challenging environmental conditions, making them and their alloys ideal for use in many applications where the failure of a weaker material could have critical consequences.
Benefits of using refractory metals
The main refractory metal characteristics, which make them desirable for so many purposes, include:
- High density – all refractory metals are very dense, making them heavy and strong, and hard enough to resist wear.
- Heat resistance – extremely high melting points and boiling points allow them to maintain their state even in elevated temperatures.
- Thermal shock resistance – they don’t easily expand or crack even when repeatedly put through cycles of heating and cooling.
- Creep resistance – unlike other metals, which deform under far less stressful conditions, they’re more resistant to creep.
- Corrosion resistance – though they oxidise at high temperatures, they’re mostly chemically inert, making them stable even against acids.
- Good conductors – they can conduct heat and electricity well while resisting all of the stresses mentioned above.
All refractory metals have these qualities in varying levels, making some more suitable for particular uses than others. Let’s look into what each refractory metal is commonly used for.
Niobium applications
Formerly known as columbium, niobium is a soft and ductile metal with a shiny grey colour that turns blue when exposed to oxygen in the air. It was discovered in the first half of the nineteenth century, and has been frequently paired with tantalum due to their similarities.
While it is the least refractory metal, it is more abundant and easier to fabricate than the others due to its high ductility. The leading use for niobium is in steel production, refining and hardening steel alloys to improve their strength and corrosion resistance. These alloys are often used in applications such as transportation and structural steel.
Another common use of niobium is the manufacturing of superconducting magnets for low-temperature scenarios, such as magnetic resonance imaging machinery (MRIs), particle accelerators, spectrometers, and electrolytic capacitors.
Its low thermal neutron cross-section makes niobium useful for zirconium alloy components in nuclear reactors, while its lower density compared to other refractory metals – similar to nickel – makes it a good choice for alloys used in aircraft parts.
Molybdenum applications
Molybdenum was unknowingly used in compounds for centuries before the element was ‘discovered’ and named in the late eighteenth century. Due to its silvery-grey appearance, it was often mistaken with lead or graphite, despite being much harder with a far higher melting point.
Though it’s about as rare as tungsten in terms of abundance, molybdenum is one of the most frequently used refractory metals – being less expensive and therefore more economical. Like niobium, it’s often used to manufacture steels to increase their corrosion resistance and strength, typically in structural tubing and piping.
The most notable molybdenum alloy is TZM (titanium-zirconium-molybdenum) – which is harder, stronger, and has a higher recrystallisation temperature than molybdenum alone. It’s often used in applications involving heavy mechanical loads and high temperatures, from dies and metalworking tools to rocket nozzles and furnace heat shields.
With the best electrical conductivity of the refractory metals, molybdenum is also often used in electrodes, cathodes, and magnetron end caps, from medical equipment to microwaves.
Tungsten applications
Previously known as wolfram – hence the symbol for the element being W – tungsten was extracted from the mineral wolframite in the late eighteenth century. The name tungsten comes from the Swedish for ‘heavy stone’, as tungsten metal is the densest refractory metal.
Tungsten also has the highest melting point of the refractory metals, and when combined with carbon to form tungsten carbide, it’s almost as hard as diamond. Its superior heat resistance and thermal conductivity make tungsten useful for components used in furnaces and industrial ovens.
Of course, tungsten is most well-known historically for the wire filaments in incandescent light bulbs, and it’s still used in industrial lighting today. Now that alternative lighting options dominate that market, tungsten is more often used in the manufacture of mechanical parts and cutting tools.
Cutting tools need a high level of abrasion resistance to cut through very hard materials, so they’re often tipped with tungsten carbide to create a faster-cutting and longer-lasting product. Tungsten alloys such as this can be used in drilling and metal forming equipment.
Meanwhile, the high density and weight of tungsten makes it suitable for counterbalancing weights, used in everything from aircraft to clocks to sports equipment (such as golf clubs and darts). It’s also used in shotgun pellets and military munitions.
Additionally, tungsten’s high density also provides increased radiation absorption – ideal for radiation shields against X-rays and gamma rays, as well as nuclear reactors and uranium processing.
Tantalum applications
This shiny blue-grey metal was discovered in the early nineteenth century during analysis of a niobium tantalum ore (though tantalum-free niobium would not be produced for several more decades). Tantalum was named after the son of Zeus, Tantalus, from Greek mythology.
The primary benefit of tantalum is its extremely high resistance to corrosion, which makes it the ideal choice for acidic and surgical environments. Its inertness makes tantalum key in chemical equipment, from reaction vessels and heat exchangers to tubes, pipes, and valves.
This quality also means that tantalum is bio-inert (biologically inert). Combined with its strength and durability, it won’t degrade when used for surgical implants, and won’t react negatively with bodily fluids and tissues. Its applications in this area include dental implants, medical implants such as plates, mesh, and joints, and surgical clips.
Its oxide stability also gives it an edge over the cheaper aluminium when it comes to the electronics industry, where tantalum is used in capacitors and conductors. It’s often used to form a barrier between copper and silicon in electronic devices, as contact could cause degradation.
Rhenium applications
Rhenium is the rarest and most recently discovered refractory metal, first extracted in Germany in the 1920s (named after the Rhine/Rhein River). It also has the second-highest melting point of the refractory metals, but its scarcity makes rhenium more expensive.
It has one of the highest densities and does not transition from ductile to brittle at any temperature, and it’s the only refractory metal that doesn’t form carbides (carbon compounds). Its high electrical resistivity and low vapour pressure make rhenium ideal for filaments.
Rhenium is also used in alloys, typically to improve the tensile strength and ductility of other refractory metals. Tungsten-rhenium superalloys have the highest heat resistance of any metal, which is useful in aerospace and nuclear reactor applications. They can also be used in heating elements, semiconductors, metal coatings, and X-ray tubes.
Adding rhenium to molybdenum, iron, nickel, or cobalt alloys increases their creep strength, making them more suitable for use in jet engine and turbine engine parts. Other rhenium compounds can also be used in the petrochemical and pharmaceutical industries as catalysts for chemical processes.
Ordering refractory metals online
Most refractory metals are available to order in bulk quantities online, from reputable suppliers of metal parts such as Special Metals. These metals, including tungsten, are available to buy in a range of forms to suit a variety of uses – from wires, rods, and ingots to strips, sheets, and crucibles.
These high-performance refractory metal parts and fittings can be fabricated for all kinds of assemblies in many different sectors, in standard or bespoke sizes according to your specifications. Technical product advice and refractory metal quotes are available from the supplier upon request.