Refractory metals are metal elements on the periodic table with several distinct characteristics. They have melting points above 3,632° Fahrenheit (2,000° Celsius) and are solid at room temperature. They are also extraordinarily resistant to elements of wear such as creep deformation, which is a change in shape that ordinary metals undergo when exposed to stress. There are five established refractory metals, including niobium, molybdenum, tantalum, tungsten, and rhenium. Other metal elements and compounds are sometimes included in this group, however, including chromium, tungsten alloys, and a alloy of molybdenum that contains titanium and zirconium, known as TZM.
The refractory metals heat resistance of tungsten is a good example of one of these properties. It is so great that, even when its heated to a temperature of 1,832° Fahrenheit (1,000° Celsius), it still displays two times the strength that the element iron has at room temperature. This makes it useful for such applications as rocket nose cones, as long lasting filaments in incandescent light bulbs, and as an additive to steel used in welding and other high-temperature applications. Stainless steel also contains tungsten to increase its property of refractory metals corrosion resistance, for industrial piping use where strong, caustic chemicals are processed.
Since refractory metals wear at a greatly reduced rate, they are also widely used in the manufacture of components that must demonstrate long-term abrasion resistance, such as bushings and nozzles. Many of these components are used in high-performance machinery, such as in the aerospace industry or semiconductor electronics manufacturing. Tungsten alloys, such as TZM as well as niobium and chromium, are most often used for these applications. Chromium is included in the refractory metals oxidation resistance category as well, because it is a highly durable coating for bearings.
Other properties of refractory metals target their uses to several specific industries. Molybdenum has a very predictable expansion coefficient, which makes it a key metal in thermocouples and heat sinks in computers, while rhenium's unique chemical reactivity gives it applications in processes such as hydrocracking, the breaking down of petroleum into simpler molecules. Niobium is employed in the design of nuclear power stations and is an ideal metal for superconductors, as it has a very low level of neutron absorption. Tantalum is used in aerospace and in surgical equipment due to its inert nature when in contact with body fluids and tissues. Tungsten and its compounds are widely used as a strengthening metal in everything from mining to the oil drilling industries, with such a demand for it that much of the metal is reclaimed from scrap.