“Refractory metals” is used in the art to refer to a group of materials with very high melting points. They are able to retain their shape and usefulness at high temperatures and have very high melting points. In general, refractory metals meet two basic criteria:
- Their melting points exceed 2200 °C (4000 °F) and
- Creep strength exceeds 1500 °C (2700 °F).
This article examines the properties, types, and common uses of refractory metals.
Table of Contents
What are Refractory Metals?
Refractory metals are a wide variety of metals with excellent heat resistance, extreme wear resistance, and very high hardness at room temperature. There are 5 metals that are undisputed refractory metals. They are:
- Tungsten (W): mp 3420 °C, BCC
- Rhenium (Re): mp 3185 °C, HCP
- Tantalum (Ta): mp 3017 °C, BCC
- Molybdenum (Mo): mp 2623 °C, BCC
- Niobium (Nb): mp 2477°C, BCC
There are several other metals that share similar properties and are commonly referred to as refractory metals. They are:
- Osmium (Os): mp 3027°C, HCP
- Iridium (Ir): mp 2447°C, FCC
- Ruthenium (Ru): mp 2250°C, HCP
- Hafnium (Hf): mp 2227° C, HCP
- Technetium (Tc): mp 2200 °C, HCP (Radioactive)
- Rhodium (Rh): mp 1963 °C, FCC
- Vanadium (V): mp 1902 °C, BCC
- Chromium (Cr) : mp 1857°C, BCC
- Zirconium (Zr): mp 1852°C, HCP
- Titanium (Ti): mp 1670°C, HCP
mp refers to melting point.
Physical Properties of Refractory Metals
Refractory metals are distinguished by the following main physical properties.
- Very high boiling point. All refractory metals, except carbon, osmium, and iridium, have higher melting points than other metals.
- High creep resistance.
- High Density
- Thermal and electrical conductivity.
- All refractory metals have a close-packed or nearly close-packed crystal structure.
Chemical and Mechanical Properties of Refractory Metals
Some of the chemical and mechanical properties of refractory metals are:
- Thermal shock resistance.
- Refractory metals are generally acid resistant.
- Refractory metal has high strength and extreme hardness.
- Excellent wear and abrasion resistance
- Low diffusivity
- They oxidize easily and form a stable outer oxide layer that gives them high corrosion resistance.
Refractory Metal Applications
The most commonly used refractory metals are tantalum, niobium, molybdenum, and tungsten. They are combined with other materials to create a wide range of refractory masses. The main uses of refractory metals are:
- Refractory metals are used to produce long-lasting electrical components by alloying them with copper, gold, or silver because of their high electrical conductivity. They are commonly used in the manufacture of cases, anodes and cathodes.
- Refractory metals are suitable for applications with extreme thermal and mechanical stress. So it is widely used in glass melting electrodes, furnace boats, sintering bowls, rods, sheets, crucibles, shields, tubes and nozzles.
- Refractory metals are used to manufacture a variety of medical devices. For example, tantalum is used in dental instruments, surgical clips, bone grafts and cranioplasty plates. Molybdenum is used in medical scan tools, tungsten is used in radiation shields, and tantalum and niobium are used in his MRI scanner.
- Niobium, Molybdenum and Tungsten are widely used in the nuclear industry. Tungsten is used for radiation shielding and niobium-zirconium alloys are used for structural components in nuclear reactors.
- Refractory metals are widely used in the defense and aerospace industries due to their extremely high heat and mechanical resistance. Molybdenum, tungsten, niobium and tantalum have many aerospace applications such as forging tools, jet engines, shields and balance weights.
- Tantalum is commonly used in heat exchangers, reaction vessels and manifolds to protect against the corrosive effects of various acids and chemicals.
- Tantalum and Niobium are widely used in cryogenic superconductor applications, mass spectrometry, NRM, particle accelerators, medical MRI imaging, and various analytical and experimental equipment.
- Refractory metals are very popular in powder metallurgy. They are processed into specific powder sizes and shapes and blended to obtain the desired blend of properties.
- Rhenium is an additive used in the manufacture of superalloys. The creep strength of superalloys is greatly enhanced by the addition of rhenium in combination with iron, cobalt, nickel, tungsten and molybdenum, making these alloys suitable for producing gas turbine and jet engine components.
- Tungsten is widely used in lighting applications, industrial furnaces and various probes.
Sum Up
Refractory metals and alloys have attracted the attention of researchers due to their excellent properties and potential practicability.
The physical properties, strength and high temperature stability of refractory metals such as molybdenum, tantalum and tungsten make them suitable materials for hot metal processing and vacuum furnace technology. Many special applications take advantage of these properties. For example a tungsten lamp filament can operate at temperatures up to 3073K and a molybdenum furnace winding withstands temperatures up to 2273K.
However, the biggest drawback of refractory metals is that they are poorly workable at low temperatures and extremely prone to oxidation at high temperatures. Interaction with the environment can have a significant effect on high temperature creep strength. Application of these metals requires a protective atmosphere or coating.