Refractory metals are widely used in various applications because of their unique and desirable properties and behaviors, not least of which is their resistance to corrosion. This group of metals also has extraordinary resistance to wear and heat. These metals are mostly used in the field of engineering, science and metallurgy. The five main elements that belong to this class of metal include the following:
- Molybdenum (Mo)
- Rhenium (Re)
- Niobium (Nb)
- Tungsten (W)
- Tantalum (Ta)
All of these five elements share a few key properties, such as a high level of hardness at room temperature and a high melting point, especially when subjected to temperatures higher than 3,600°F (2,000°C). (For another method to increase hardness, read Nitriding for Corrosion and Wear Fatigue Resistance.) These metals have high density and are chemically inert. The high melting points of these metals give way to powder metallurgy as the choice for manufacturing different components.
A few of the most common applications from refractory metals include wire filaments, tools, chemical vessels within corrosive atmospheres, casting molds and more. With their very high melting point, refractory metals are highly stable.
The Key Features of Refractory Metals
One major identifying factor of refractory metals is heat resistance. All five metals under this class are characterized with a melting point higher than 3,632°F (2,000 °C). They are strong under extremely high temperatures and are innately hard, making these metals perfect for drilling and cutting tools. Also, these metals are highly resistant to factors like thermal shock. This means that they will not experience cracking, expansion or stress when cooled and heated repeatedly.
Moving forward, all five metals under this group have high density levels along with good heat and electrical properties. They are also resistant to creep, so they undergo slow deformation when exposed to very stressful environments or conditions. This is because these metals have the ability to structure a layer of protection, making them corrosion-resistant despite the fact that they can undergo oxidation under high temperatures.
The Physical and Chemical Properties of Refractory Metals
Refractory metals can be classified by their unique physical and chemical properties. It's vital to have a thorough understanding of these properties to get the most out of these elements.
Refractory metals are marked by the following key physical features:
- A high boiling point. The melting points of refractory metals are higher than those of all other metals except carbon, osmium and iridium.
- They all have a body-centered cubic crystal structure.
- They are highly resistant to creep.
That said, the specific characteristics of these metals vary pretty widely because they come from different groups in the periodic table. For instance, of all the five metals, rhenium has the highest melting point at 5,757°F (3,186°C), while niobium has the lowest at 4,491°F (2,477°C). In terms of boiling point, niobium is at 8,572°F (4,744°C) while rhenium is at 10,105°F (5,596°C).
The same goes for creep resistance; there are metals that begin to form creep at 1,832°F (1,000°C), while there are metals that start at less than 932°F (500°C).
Refractory metals come with a wide array of chemical properties since each belongs to a different classification in the periodic table of elements. Generally, these metals oxidize easily, but the reaction can be controlled through the creation of stable layers of oxide on top of a metal. This is especially the case with rhenium because it's highly volatile. Thus, it can lose its stability against oxygen attack under high temperatures as the oxide layer undergoes evaporation. However, all refractory metals are comparatively stable against substances such as acids.
What Are the Benefits of Using Refractory Metals?
Thanks to their unique qualities, refractory metals are highly beneficial to a variety of applications and industries. Their key benefits include:
A Super High Melting Point
Refractory metals such as tungsten, molybdenum and tantalum have very high melting points, making them beneficial in glass making.
They have outstanding strength even at ultra-high temperatures. For instance, rocket cones that are made from tungsten have double the tensile strength of iron under normal temperatures. (For more on this topic, read What is the Difference Between Strength and Toughness?)
Outstanding Abrasion and Wear Resistance
Refractory metals, especially alloys, can extend the lifespan of valve seats, seals, nozzles and other areas prone to high wear.
Excellent Corrosion Resistance
The piping in chemical plants is typically made of refractory metals, allowing for a high degree of corrosion resistance compared to stainless steel.
Thermal Shock Resistance
Metals like tungsten can resist the stress brought by quick expansion because of heat. Tungsten has a very high melting point and can withstand a series of on and off cycles without affecting its intactness.
Heat and Electrical Conduction
Apart from the many electronic or electrical applications, refractory metals such as tungsten and molybdenum are also good as heat sinks.
Most of the cutting tools widely used these days are fabricated out of materials like tungsten carbide. Refractory metals are also great for forming metals and steel as well in mining and drilling gas or oil wells.
High Density and Specific Gravity
Most refractory metals have high density and these can be very useful in making golf club heads and aircraft gyroscopes.
There are refractory metals that have ultra-useful capabilities such as acting as outstanding radiation shields as well as chemical catalysts.
Types of Refractory Metals and Their Applications
There are five refractory metals, and each come with varying properties and applications.
Tungsten is the most abundant among the refractory metals. It has the highest melting point and one of the highest metal densities among refractory metals. It can be very hard when combined with other elements like carbon. It is also highly resistant to corrosion. This metal is widely used in wire filaments, such as those in most of the incandescent lighting used in houses, but it is also common in industrial arc lamps and lighting.
Molybdenum is the most used refractory metal of all because it is less expensive than most others and, when made into an alloy, can be very resistant against creep and high temperatures. It also does not create amalgams, making it corrosion-resistant. It is mostly used in strengthening steel alloys, particularly in structural piping and tubing. This metal also has excellent anti-friction qualities, making it an ideal component of oils and greases used in automobiles.
This refractory metal is the most resistant against corrosion. It is often used in the field of medicine and surgery as well as in environments where there is high acidity. Tantalum is also the major component of computer and phone circuits or capacitors.
This metal always comes with another refractory metal, tantalum. It is highly unique and can be worked on easily to obtain high elasticity and strength. It can be used in making electrolytic capacitors and superconductors. Niobium can also be found in nuclear reactors and vacuum tubes.
Rhenium is the most recently discovered refractory metal. It can be found with other metals in ultra-low concentrations. It is also present in other refractory metal ores. This metal is known for its high tensile strength and ductility. It is widely used in nuclear reactors, gyroscopes and other electric components. Due to its rarity, it can be really expensive.
Despite the many applications and benefits of refractory metals, there are many that we do not yet fully understand. Thus, ample research is required in the field of metallurgy in order to improve a variety of processes.