Properties of refractory metals
The main characteristic of refractory metals is their heat resistance. They are durable at extremely high temperatures and are hard in nature. Also, these metals are highly resistant to factors such as thermal shock. This means they will not crack, expand, or experience mechanical stress when subjected to repeated cooling and heating.
All elements of this group have high density, as well as good thermal and electrical properties. They are also creep resistant, so they deform very slowly when exposed to external stress. Minimal creep is explained by the ability to form a protective anti-corrosion layer that maintains integrity even at high temperatures.
The specific temperature characteristics of these metals vary quite a bit because they come from different groups of the periodic table.
For example, the most refractory metal is rhenium, whose melting point is 3186°C, with a boiling point of 5596°C. Creep resistance usually varies between 500...10000C.
The chemical properties of refractory metals are also varied. In general, these metals oxidize easily, but the oxidation reaction can be controlled by creating stable oxide layers on top of the base metal. However, all refractory metals are resistant to acids.
The chemical elements in question have not only high melting points, but also a high level of hardness at room temperature.
Advantages and disadvantages
The properties of heat-resistant steels make this material indispensable in such areas as rocket science and the space industry, complex engine construction, the aircraft industry, the production of key elements of gas turbines and many others. Their share in rolled high-tech steel reaches 50%. Some alloys are capable of operating at temperatures above 7000° C.
This difficult-to-produce material, the production of which is impossible without special equipment and qualified personnel, has a high cost. The use of such steels cannot be universal, therefore, for its effective use it is necessary to have a developed scientific and technical base.
Production technologies
High melting temperatures make powder metallurgy the main method for obtaining final products.
Usually the question of which metals are refractory is decided by which of three categories they belong to:
- Hard alloys;
- Oxide or carbide dispersion-strengthened materials;
- Materials with alloyed properties.
Thus, all tungsten-based products and most molybdenum products are processed using powder metallurgy because, due to their high melting point, they cannot be produced by atomization. Thus, chemical reduction processes and electrolysis are the only practical methods.
Powders obtained electrolytically from aqueous electrolytes or in a molten state have high purity and are active during sintering. However, they have the following disadvantages:
- To remove residual impurities from the electrolyte, cleaning is necessary;
- The process is often expensive, consuming a lot of electricity due to low current efficiency;
- The process is limited to the production of pure metal powders, as it is not possible to produce alloy powders with this method.
The reducing agents used in most processes are carbon and hydrogen - in their elemental form or as gaseous compounds or mixtures (hydrocarbons, cracked ammonia or carbon monoxide).
Until recently, vacuum casting was used to produce molybdenum and its alloys, but in modern production powder metallurgy is also used here. The effectiveness of metal powder recovery depends on:
- Required activation energy;
- Chemical composition and degree of dispersion of metal powder;
- Flow rates of the reducing gas - hydrogen.
Powder metallurgy is the only way to produce alloys whose components are refractory metals, including at the level of nanopowders. Among the various methods being explored for the production of refractory nanoalloys is the traditional gas-solid reaction route, the implementation of which allows the production of nanostructured powders in significant quantities and with broad quality assurance capabilities.
It is important to emphasize that high-temperature powder metallurgy technologies make it possible to produce custom-made materials. By choosing the right starting material, you can control the composition of the final product accordingly.
The third way is to use 3D printing. This is an actively developing technology that is ideal for the production of lightweight, highly stable components from tungsten, molybdenum, niobium, tantalum and their alloys with complex geometric structures.
Nanopowders of refractory metals are obtained in the following ways:
- Selective laser sintering.
- Selective laser melting.
- Electron beam melting.
- Direct deposition.
- Rapid prototyping.
The key advantages of gas atomized metal powders are perfectly spherical shape, excellent reproducibility, low oxide content.
Obtaining refractory materials
The main difficulty encountered in the production of refractory metals and alloys is their high chemical activity, which prevents the element from being present in its pure form.
Installation for producing refractory metals
The most common production technology is powder metallurgy. There are several ways to obtain refractory metal powder.
- Reduction with hydrogen trioxide. This method includes several stages; the processing equipment is multi-tube furnaces, with a temperature range from 750 to 950 ° C. This method is used to produce molybdenum and tungsten.
- Reduction with hydrogen from ammonium perrhenate. At a temperature of about 500 °C, at the final stage, the resulting powder is separated from alkalis using acids and water. Used to produce rhenium.
- Salts of various metals are also used to obtain molybdenum powder. For example, use ammonium salt of the metal and its powder, no more than 15% of the total mass. The mixture is heated to 500-850 °C using inert gas, and then the production technology provides for reduction with hydrogen at a temperature of 850-1000 °C.
Production of refractory metals
The powder obtained by these methods is subsequently sintered into special forms for further transportation and storage.
Today, these methods for obtaining pure refractory metals continue to be refined and new techniques for extracting material from rocks are being applied. With the development of nuclear energy, the space industry, and metallurgy, we will soon be able to observe the emergence of new methods, perhaps cheaper and simpler.
Types and applications
Due to their unique properties, refractory metals are very useful for various applications and industries. Their main advantages:
- Ultra-high melting point. In particular, refractory metals include tungsten, molybdenum and tantalum, which are used in the production of glass;
- Strength at ultra-high temperatures. For example, rocket cones made of tungsten have twice the tensile strength of iron at normal temperatures;
- Excellent abrasion and wear resistance to extend the life of valve seats, seals, injectors and other high wear areas;
- Excellent corrosion resistance, which is why critical pipelines in chemical plants are usually made of refractory metals;
- Resistance to thermal shock. In particular, tungsten products can withstand stress caused by rapid expansion due to sudden temperature changes;
- Thermal and electrical conductivity, as a result of which radiator parts are made from tungsten and molybdenum;
- Extreme hardness, therefore highly resistant cutting stamping and drilling tools are made from tungsten carbide;
- The high density of refractory metals is the reason for their use in golf club heads and aircraft gyroscopes.
In addition, these materials are used as catalysts for chemical reactions, in nuclear fusion processes, etc.
Refractory metals include the particularly widespread tungsten, molybdenum, niobium, tantalum, rhenium and chromium. The specifics of their use are discussed below.
Tungsten
Tungsten is the most common refractory metal. It has the highest melting point and one of the highest densities. It is also highly resistant to corrosion. Widely used in wire fibers, such as most incandescent lamps used in homes, as well as industrial arc lamps and other lighting equipment.
Molybdenum
Molybdenum is the most used refractory metal of all because it is cheaper than most others and, when made into an alloy, can be very resistant to creep and high temperatures. It also does not form amalgams, making it resistant to corrosion.
Molybdenum is used to strengthen steel alloys, especially in structural piping and tubing. This metal also has excellent anti-friction properties, making it an ideal component in oils and lubricants used in automobiles.
Niobium
It has an optimal combination of ductility and strength. It can be used in the manufacture of electrolytic capacitors, superconductors, nuclear reactors and vacuum tubes.
Tantalum
It is more resistant to corrosion than others, therefore it is used in medicine (especially surgery), as well as in environments with high acidity. Tantalum is also a major component of computer, telephone and capacitor circuits.
Rhenium
Known for its high tensile strength and ductility. It is widely used in nuclear reactors, gyroscopes and other electrical components. Due to its rarity, rhenium is very expensive. The concept of corrosion resistance is especially relevant for rhenium because it is very volatile. May lose resistance to oxygen at high temperatures as the oxide layer actively evaporates.
Austenitic-ferritic and austenitic steel alloys
The most significant features of austenitic steels are that their internal structure is formed due to nickel in their composition, and heat resistance is associated with chromium.
Alloys of this category, characterized by low carbon content, sometimes contain the alloying elements titanium and niobium. Steels, the basis of the internal structure of which is austenite, are included in the category of stainless steels and, under prolonged exposure to high temperatures (up to 1000°C), they resist the formation of scale well.
The most common steels with an austenitic structure today are dispersion-hardening alloys. In order to improve quality characteristics, carbide or intermetallic hardeners are added.
The most popular brands , the basis of the internal structure of which is austenite:
- Dispersion-hardening Х12Н20Т3Р, 4Х12Н8Г8МФБ, 4Х14Н14В2М, 0Х14Н28В3Т3УР.
- Homogeneous 1X14N16B, 1X14N18V2B, X18N12T, X18N10T, X23N18, X25N16G7AR, X25N20S2.
Steel alloys based on a mixture of austenite and ferrite are distinguished by very high heat resistance, which exceeds that of even high-chromium materials. Heat resistance characteristics are also achieved due to the high stability of the internal structure of steels in this category. Products made from them are successfully used even at temperatures up to 1150°C.
Heat-resistant steels with an austenitic-martensitic structure are characterized by increased brittleness, and therefore cannot be used in the production of products that are operated under high loads.
Products for the following purposes are made from heat-resistant steels of this category:
- Heat-resistant pipes, conveyors for furnaces, tanks for cementation (Х20Н14С2 and 0Х20Н14С2).
- Pyrometric tubes (Х23Н13).
Features of materials with heat-resistant properties
Heat-resistant steels and alloys, as mentioned above, can be successfully operated under conditions of constant exposure to high temperatures, without showing a tendency to creep. The essence of this negative process, to which ordinary steel grades and other metals are subject, is that the material, which is exposed to constant temperature and constant load, begins to slowly deform, or creep.
Creep, which they try to avoid when creating heat-resistant steels and other types of metals, comes in two types:
- long-term;
- short-term.
To determine the creep of alloys, research centers use a set of testing machines
To determine the parameters of short-term creep, materials are subjected to special tests, for which they are placed in an oven heated to a certain temperature and a tensile load is applied to them. This test is carried out for a limited period of time.
It will not be possible to test a material for its tendency to long-term creep and determine such an important parameter as the creep limit in a short period of time. To do this, the test product placed in the oven must be subjected to prolonged load. The importance of such an indicator as the creep limit of a material lies in the fact that it characterizes the greatest stress, which leads to the destruction of a heated product after exposure for a certain period of time.
How was this metal discovered?
The most refractory metal in the world was discovered by the Swedish scientist K.V. Scheele (in 1781). He managed to synthesize tungsten trioxide (this is what the lightest of metals was called) by dissolving the ore in nitric acid. A couple of years later, the purest metal was obtained by chemists from Spain - F. Fermin and J. José de Eluard, who isolated it from wolframite. However, at that time, this discovery did not particularly impress humanity, and all because the necessary technologies did not exist to process the resulting metal.
Where is tungsten used?
Tungsten compounds are widely used. They are used in the engineering and mining industries, for drilling wells. Due to its high strength and hardness, this metal is used to make parts of aircraft engines, filaments, artillery shells, high-speed gyroscope rotors, bullets, etc. Tungsten is also successfully used as an electrode in argon-arc welding. Such industries cannot do without tungsten compounds - textile, paint and varnish.
