Refractory Metals: Properties, Types and Applications
Refractory metals have many extraordinary qualities compared to other metals—most notably a high resistance to corrosion.
These metals are mostly used in the fields of engineering, science and metallurgy. The five main elements that belong to this class of metal include the following:
These five of these 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 degrees Fahrenheit (2,000 degrees Celsiuis). (For more on increasing hardness, see: Nitriding for Corrosion and Wear Fatigue Resistance.)
Refractory metals also have high density and are chemically inert. Their high melting points give way to powder metallurgy as the choice for manufacturing different components.
A few of the most common applications for refractory metals include wire filaments, tools, chemical vessels within corrosive atmospheres and casting molds. With their very high melting point, refractory metals are highly stable.
Now, let's take a deeper look at refractory metals—including their features, properties, benefits and applications:
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 degrees Fahrenheit (2,000 degrees Celsius).
Refractory metals also remain strong under extremely high temperatures and are innately hard, making them perfect for drilling and cutting tools. Refractory metals are highly resistant to factors like thermal shock. This means they will not experience cracking, expansion or stress when cooled and heated repeatedly. (For more on metals under heat, see: Top 5 Considerations when Coating High-Heat Surfaces.)
Moreover, all five metals under this group have high density levels and 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 refractory metals can 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. And, to get the most out of these elements, it's vital to thoroughly understand them.
The Physical Properties of Refractory Metals
Refractory metals are marked by the following key physical features:
- A high boiling point. Refractory metals' melting points are higher than those of all other metals except carbon, osmium and iridium.
- A body-centered cubic crystal structure.
- A high creep resistance.
That said, refractory metals' specific characteristics vary pretty widely because they come from different groups in the periodic table.
For instance, of all five refractory metals, rhenium has the highest melting point—at 5,757 degrees Fahrenheit (3,186 degrees Celsius)—while niobium has the lowest—at 4,491 deF (2,477°C). In terms of boiling point, niobium is at 8,572 degrees Fahrenheit (4,744 degrees Celsius) while rhenium is at 10,105 degrees Fahrenheit (5,596 degrees Celsius). (For more on applications using high-heat materials, see: All Steamed Up: The Many Types and Uses of Steam.)
The same goes for creep resistance; there are metals that begin to form creep at 1,832 degrees Fahrenheit (1,000 degrees Celsius), while there are metals that start at less than 932 degrees Fahrenheit (500 degrees Celsius).
The Chemical Properties of Refractory Metals
Refractory metals come with a wide array of chemical properties since each belongs to a different classification in the periodic table.
Generally, these metals oxidize easily; but the reaction can be controlled by creating stable oxide layers on top of a metal. This is especially true 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 Very High Melting Point
Refractory metals such as tungsten, molybdenum and tantalum have very high melting points, making them beneficial in glass making.
Refractory metals are uniquely strong, even at ultra-high temperatures.
For instance, rocket cones made from tungsten have double the tensile strength of iron under normal temperatures. (For more on this topic, see: What is the Difference Between Strength and Toughness?)
Outstanding Abrasion and Wear Resistance
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
On top of their many electronic or electrical applications, refractory metals such as tungsten and molybdenum make good heat sinks.
Today, most widely used cutting tools are made from materials like tungsten carbide.
Refractory metals are also great for forming metals and steel as well in mining and drilling gas or oil wells. (For more on the oil and gas industry, see: Remote Corrosion Monitoring Systems in the Oil and Gas Industry.)
High Density and Specific Gravity
Most refractory metals have high density and thus can be very useful for making golf club heads and aircraft gyroscopes.
Some refractory metals that have ultra-useful capabilities—such as acting as outstanding radiation shields and chemical catalysts.
Types of Refractory Metals and Their Applications
There are five refractory metals; and each comes with distinct properties and applications.
Across the board, common applications for refractory metals include wire filaments, tools, chemical vessels within corrosive atmospheres and casting molds With their very high melting points, refractory metals are highly stable.
Here are more specific applications for each of the five refractory metals:
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.
Tungsten 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. (For more on industrial applications, see: 8 Things to Know About Polyurea Coatings.)
Stage lighting is used in television studios and on sound stages as well as in venues where theatrical productions, concerts and other live events take place. And, while stage lighting still uses incandescent technology, it is usually in the form of proprietary high-performance tungsten-halogen (or quartz-halogen) lamps with carefully calibrated color temperature values to control overall scene rendition.
Corona discharge is often done, and best accomplished, with tungsten wire. For air filtration, printers, copiers and other applications, only tungsten wire produces sufficient ionization and surface modification.
Tungsten wire is widely used to provide support or pulling action in industrial ovens.
This is due to tungsten wire’s non-sag properties and its heat resistance. For example, tungsten wire is often woven into mats used to position and hold objects in place in the engineered hot zone of industrial ovens, furnaces and kilns.
Tungsten wire's stiffness, even at very small diameters, is a crucial factor priming it for use in various types of probes, including cantilever semiconductor test probes used for silicon wafer testing in semiconductor manufacturing.
Stiffness is also an advantage in tungsten wire for neural probes used in medical diagnostics and treatment. (For more on materials science in healthcare, see: How to Get Started in a Career as a Materials Scientist.)
Cutting Tools and Parts
Cutting tools and parts require high abrasion resistance and are therefore made of tungsten carbide.
Tungsten-carbide inserts, with ground-cutting edges, are attached to the bodies of steel tools by brazing or mechanical fastening. Use of tungsten carbide tools ensures higher cutting speeds and longer tool life. Tungsten-carbide dyes are commonly used for drawing wire.
Molybdenum is the most-used refractory metal because it is less expensive than most others and, when made into an alloy, can be very resistant to creep and high temperatures.
This metal also does not create amalgams, making it corrosion-resistant. It is mostly used to strengthen steel alloys, particularly in structural piping and tubing. Molybdenum also has excellent anti-friction qualities, making it an ideal component of oils and greases used in automobiles. (For more on corrosion in the automotive industry, see: The Corrosion of Lightweight Materials Used in Automotive Applications.)
Molybdenum enhances this passive film by making it stronger and helping it to re-form quickly if it is disrupted by chlorides. Increasing the molybdenum content in stainless streels increases their pitting and crevice corrosion resistance.
Tools and High Speed Steel
Molybdenum in tool steels increases their hardness and wear resistance. By reducing the critical cooling rate, molybdenum promotes the formation of an optimal martensitic matrix, even in massive and intricate moulds which cannot be cooled rapidly without distortion or cracking.
Adding five to 10% molybdenum can effectively maximize the hardness and toughness of high-speed steels and maintain these properties at the high temperatures generated when cutting metals. (For more on cutting metals, see: How Hot Shortness and Welding Affect Corrosion in Metals.)
In electronic technology, wire and cable insulation represents a fire and smoke hazard to firefighters and those in the confines of aircrafts and hospitals. Ammonium octamolybdate has been used with PVC to suppress smoke formation.
As molybdenum has a modulus of elasticity of 47*10^6 pounds per square inch at room temperature, it is used for boring bars and quills for high-speed internal grinders. This ensures freedom from vibration and chatter.
Tantalum is the most corrosion-resistant refractory metal.
It is often applied in medical and surgical settings and in highly acidic environments. Tantalum is also the major component of computer and phone circuits or capacitors.
Although considerably more expensive than aluminum, tantalum has two key advantages over it:
- It has a higher dielectric constant, allowing capacitors to be smaller.
- Its oxide layer is more stable, giving tantalum capacitors an advantage in applications requiring high reliability.
Tantalum is also used in the electronics industry as a barrier to prevent copper from polluting silicon in products such as computer chips and storage devices. This separation is necessary because copper's presence in silicon leads to device degradation and failure. (For more on copper corrosion, see: Increased Rates of Copper Corrosion in the Zone of Aeration Explained.)
Tantalum is used in surgical and biomedical applications due to its bio-inertness—which, in turn, is due to the adherent oxide layer that forms spontaneously on the surface of tantalum metal when exposed to air.
Moreover, tantalum is inert in bodily fluids and has good mechanical properties. Examples of its applications in medicine include:
- Surgical clips.
- Bone grafts.
- Plates for cranioplasties.
- Mesh for abdominal wall reconstructions.
- Dental implants.
Chemical Plant Equipment
Tantalum's ductility and excellent corrosion resistance make it suitable for use in chemical process equipment operating under corrosive conditions at elevated temperatures.
The metal's corrosion resistance serves both to protect the equipment and to maintain the handled chemicals' purity. Tantalum and its alloys can be used to handle hydrochloric, hydrobromic, nitric and sulfuric acids. Examples of chemical plant equipment in qhich Tantalum may be used are:
- Heat exchangers.
- Pipes. (For more on pipe corrosion, see: 21 Types of Pipe Corrosion & Failure.)
Niobium is always used alongside tantalum. It is highly unique and can be worked on easily to obtain high elasticity and strength.
The leading use of niobium is in ferrous metallurgy.
About 75% of world niobium production is used by the steel industry to produce a variety of steel alloys that contain small amounts of other metals (microalloy and low-alloy steels) to improve—among other properties—corrosion resistance, strength and toughness.
Niobium acts as a grain refiner and precipitation hardener in high-strength low-alloy and microalloyed steel, simultaneously improving mechanical strength, high-temperature strength, toughness and corrosion resistance.
These steels are used in pipelines, transportation, and structural applications. (For more on transportation, see: Temporary Corrosion Protection During Storage, Transportation and Handling.)
Niobium alloys are also used to manufacture superconducting magnets. These superr conducting magnets are used in:
- Magnetic resonance imaging (MRI).
- Nuclear magnetic resonance instruments (NMRI).
- Particle accelerators, such as the Large Hadron Collider in Europe, which contains the world’s largest superconducting magnets.
- Electrolytic capacitors and superconductors.
Niobium alloys are preferred to other refractory metals up to 3,300 degrees Fahrenheit. Due to their low oxidation resistance, niobium alloys are limited to use in structural applications at elevated temperatures.
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. Rhenium is known for its high tensile strength and ductility.
Rhenium is also very rare and thus can be very expensive.
Rhenium is used as an additive to make superalloys, often in conjunction with iron, cobalt, nickel, tungsten and molybdenum. Adding rhenium improves a superalloy's overall creep strength, making it an excellent material for jet engine parts and gas turbine engines. (For more on jet engines and turbines, see: Hot Corrosion in Gas Turbine Components.)
Rhenium compounds are used as a catalysts for homogeneous and heterogeneous catalysis in many industries. These include:
- Organic synthesis processes, including isomerization, hydrogenation and alkylation.
Platinum-rhenium catalysts are essential in a chemical process called catalytic reforming in petroleum refineries. (For more on petroleum, see: The 6 Corrosive Components That Can Be Found in Crude Oil.)
Tungsten-rhenium and molybdenum-rhenium alloys are used mainly to manufacture thermoelements.
Their uses include semiconductors, heating elements, metallic coatings and welding rods.
Tungsten-rhenium alloys, in particular, are used to make rotating X-ray anodes.
At high temperatures, rhenium resists attack in hydrogen and inert atmospheres. It is resistant to hydrochloric acid and seawater corrosion and it is also resistant to the mechanical effects of electrical erosion.
Refractory metals have many applications and benefits. However, there are many we do not yet fully understand.
Ample research is required in the field of metallurgy to improve a variety of processes.