What are Refractory Metals?

Generally speaking, metals that have an extremely high melting points are referred to as Refractory. Refractory metals are capable of retaining their shape and their overall usefulness in a range of extreme environments such as those that are highly acidic or basic, have other corrosive chemicals, or have high heat or high pressure.

Metals considered to be refractory typically meet these two thresholds:

• Melting point above 2200 C (4000 F)
• Creep resistance above 1500 C ( 2700 F)

There are 5 metals that are considered refractory metals:

1. Tungsten
2. Niobium
3. Molybdenum
4. Tantalum
5. Rhenium

Other metals that have similar properties and occasionally referred to as refractory metals are:

• Titanium
• Vanadium
• Chromium
• Zirconium
• Hafnium
• Ruthenium
• Rhodium
• Osmium
• Iridium

One example of a metal that is not a refractory metal is pure copper. If A bar of copper is heated at one end, the other end will become hot very quickly. It has very high thermal conductivity (see which metals conduct heat the best link here). At lower temperatures the copper bar will start to deform. The fact that it deforms readily under heat prevents its consideration as refractory.

Molybdenum, however, has a very high melting temperature (2623 °C / 4753 °F). If same heat is applied in the above example, the molybdenum’s shape will remain the same. It has one of the lowest coefficients of thermal expansion and the temperature for when it starts to creep is above 1500 °C / 2732 °F. The ability to retain its shape and its hardness at high temperatures makes molybdenum a refractory metal.

Refractory metals also all have high densities and hardness at room temperature.

Niobium

Niobium is the least dense of the refractory metals and is the only one that can realistically be annealed. Niobium can be found in aerospace and nuclear reactors.

Molybdenum

Molybdenum is mainly used over tungsten when cost is a factor. Molybdenum is cheaper than tungsten but still has comparable properties. It is commonly used as a strengthening alloy in steel.

Tantalum

Tantalum has superior corrosion resistance. It is found most often in the medical field as an allowing element in surgical tooling. Tantalum films can also be found in the capacitors of cell phones.

Rhenium

Rhenium is used in many of the aforementioned applications. It is commonly used as an alloy in other refractory metal alloys, adding ductility and tensile strength.

Tungsten

Tungsten has the highest melting point of all metals at an incredible 3410°C / 6170°F. Tungsten is most commonly known for being used as the filament in incandescent light bulbs. It is also known for its appearance in TIG welding (Tungsten inert gas) where it is used a permanent non-melting electrode.