Refractory metals are a group of metals that show superior heat and wear resistance. Which exact metals belong to the group differs. However, the most common definition of refractory metals include the elements niobium, molybdenum, tantalum, tungsten and rhenium. A wider definition includes 12 elements, which includes titanium, vanadium and chromium, only to name a few.
The most common applications are glass-melting electrodes, furnace parts and heat sinks. Further, refractory metals are commonly applied in the aerospace and defense industry.
The definition of refractory metals
These elements share many properties. However, one characteristic stands out: the extremely high melting point. For instance, all elements belonging to the closer definition have a melting point higher than 2200 °C. Another common characteristic is the microstructure. Except for rhenium, all elements have a body-centered cubic structure. Other similarities are high density and the resistance to corrosion and wear.
Tungsten was discovered in 1781 by the Swedish chemist Karl Wilhelm Scheele, Therefore, the name is derived from the Swedish words “tung” (heavy) and “sten” (stone).
Tungsten is characterized by the highest melting point of all metals (3410 °C). Further, it is one of the most dense metals in the world (19.3 g/cm3) and shows the highest conductivity in comparison to other refractory metals. In addition, tungsten become one of the hardest man-made materials, when combined with carbon.
The metal is best known for its use in manufacture of filaments for light bulbs. Moreover, it is commonly used in industrial lightning as electrodes in arc lamps. Further use finds the element in metal cutting, mining and oil drilling tools.
Molybdenum was firstly used in 1894 by a French company as an armor plate and is the most popular of all refractory metals. The primary application of molybdenum is as an alloying element in steel, as it is widely used in the manufacture of pipelines, tools and stainless steel. Especially the alloy TZM, which stands for Titanium (0.5 %), Zirconium (0.08 %) and Molybdenum, is quite popular, due to its great creep resistance and strength at high temperatures. Further applications are molybdenum are high-heat applications such as furnace parts or elevator brakes.
Molybdenum shows superior electrical conductivity (2×107 S/m). In fact, it is the highest of all refractory metals. The heat conducting capabilities (138 W/(m*K)) are superior, as well. Other properties are good corrosion and wear resistance and high tensile strength.
Niobium is also known as Columbium (Cb) and was firstly discovered in 1801 in Connecticut. Usually, Niobium is found together with Tantalum and both elements share similar properties. The main difference is however the weight, as niobium is only half as heavy as tantalum.
Niobium is the least dense of all refractory metals and is the only one, which can be worked through annealing. Therefore, it shows great machinability at low temperatures and can achieve a wide range of strength and ductility.
The main application of niobium is as an alloying element of steel. Due to the low density, the element fits perfectly for the manufacture of high-performance refractory workpieces with low weight. Therefore, it is often applied in the aerospace industry, but also in gas turbines and nuclear reactors.
As already mentioned, tantalum was discovered at the same time as Niobium and share a lot of its characteristics. An outstanding property of tantalum is the superb corrosion resistance, especially in nitric, hydrochloric and sulfuric acid.
The main fields of application for tantalum are in medical industry. There, it used for surgical equipment. Due to its superior corrosion resistance, tantalum is applied as chemical equipment such as vapor condensers, orifices and reaction vessels. Further, it is used in the manufacture of electrolytic capacitors, as it shows the second-best capacitance of all substances.
Rhenium does not have a specific ore, as it was discovered by German scientists in platinum and columbium ores in 1925. Thus, it is the most recently discovered refractory metal of all. Due to the fact that the element occurs so rarely, it is the most expensive of all refractory elements.
Most often, rhenium is used as an alloy for other refractory metals. In this way, the ductility and tensile strength can be enhanced greatly. It is typically applied in the construction of nuclear reactors, gyroscopes and electronic components. However, the most common use of rhenium is as a catalyst for reaction such as oxidation or hydrogenation.