Properties and main uses of molybdenum
The properties of molybdenum and tungsten are very similar. Its boiling point (5560 °) and electrical conductivity are outstanding, and the coefficient of linear thermal expansion is small. The results show that the thermal conductivity [135 w / (m · K)] and specific heat [0.276 KJ / (kg · K)] are the best combination, which makes it a natural choice for thermal shock resistance and thermal fatigue resistance. Its melting point is 2620 ℃, inferior to that of tungsten and tantalum, but its density is much lower. Therefore, its specific strength (strength / density) is higher than that of tungsten and tantalum, so it is more effective in weight critical applications. Molybdenum still has high strength at 1200 ℃. The main disadvantages of molybdenum are poor high temperature oxidation resistance (rapid oxidation above 600 ℃) and poor room temperature ductility. In order to make full use of the advantages and avoid the disadvantages, coating (such as MoSi2 coating, nickel plating, chromium plating, etc.) is adopted to control the high-temperature oxidation problem; and the poor plasticity, which is commonly known as the lack of low-temperature embrittlement, is solved by means of alloy strengthening and carbide addition. Tungsten (W), rhenium (Re), tantalum (Ta), titanium (Ti) and zirconium (Zr) are common solid solution strengthening elements. Tungsten is the main solid solution strengthening element of molybdenum, and rhenium can reduce the ductile brittle transition temperature to - 200 ℃. The molybdenum lanthanum alloy composed of lanthanum shows outstanding creep resistance and high temperature deformation ability, especially at high temperature.
From the global consumption structure, molybdenum is indeed an ally of iron. In western developed countries, 80% of the demand for molybdenum comes from steel. Stainless steel absorbs 30% molybdenum, low alloy steel absorbs 30%, drilling bit and cutting tool account for 10%, and cast steel accounts for 10%. Another 20% of molybdenum consumption is in molybdenum chemicals, molybdenum based lubricants and petroleum refining. In 1998, the consumption proportion of molybdenum in steel production in the United States was 75%. In addition, molybdenum based alloys have been widely used in electronics, metal processing and aerospace industry.
1. Molybdenum alloy TZM alloy has excellent high temperature strength and comprehensive properties, which is the most widely used molybdenum alloy. The turbine disk made of TZM alloy in USA accounts for 15% of the total molybdenum consumption. The production of molybdenum materials including TZM molybdenum alloy in China is no less than 22 brands. In the early 1990s, the output of molybdenum and molybdenum products in China was nearly 200 tons. The high temperature mechanical properties of TZM and TZC molybdenum alloys are better than that of pure molybdenum. They are widely used in manufacturing high temperature tools, dies and various structural parts. They have been successfully made into hot piercing plugs of various seamless steel pipes in China as early as the 1920s. This kind of sintered molybdenum plug made by powder metallurgy technology can reduce the consumption of raw materials (50% of as cast) and increase the average service life by 1.5 ~ 2 times. The seamless tube made of mo re alloy (containing 50% RE) has excellent high temperature performance and can be used at the temperature close to its melting point. It can be used as the bracket, ring and grid of thermowell and electron tube cathode. In addition to high temperature strength, good electrical and thermal conductivity and low coefficient of thermal expansion (similar to glass for electronic tubes), molybdenum and molybdenum alloys also have the advantage of being easier to process than tungsten. Therefore, the molybdenum plates, molybdenum strips, molybdenum foils, molybdenum tubes, molybdenum bars, molybdenum wires and profiles produced by conventional processing methods are used in electronic tube (grid and anode), electric light source (supporting material) parts, metal processing tools (die casting and extrusion) Die, forging die, piercing plug, liquid metal filter screen) and turbine disk are widely used.
2. As the ally of steel, Mo, together with Ni and Cr, can reduce the embrittlement of alloy steel during heat treatment. In the aspect of solving the shortage of tungsten resources, the United States takes the lead in replacing tungsten with molybdenum in high speed steel. It is calculated that molybdenum has twice the "capacity" of tungsten. In this way, the steel containing 18% w can be replaced by steel containing 9% Mo (Cr and V are added at the same time), which greatly reduces the production cost of steel. The role of molybdenum in stainless steel is to improve corrosion resistance, high temperature strength and weldability. It can be seen that molybdenum plays an extraordinary role in the iron and steel industry.
3. Other applications molybdenum has very low vapor pressure under the working temperature and pressure of vacuum furnace. Therefore, molybdenum parts have the least pollution to the workpieces or working substances in the furnace, and the evaporation loss certainly will not restrict the service life of high temperature molybdenum parts such as heating elements and heat insulation encapsulation. In the manufacture of glass products, the high temperature strength of molybdenum makes it the most ideal electrode and processing equipment during rapid heating. Molybdenum is chemically compatible with most glass components, and will not cause harmful coloring effect due to the dissolution of a small amount of molybdenum in the glass bath. As a heating electrode in glass melting furnace, its service life can be as long as 3-5 years.
4. Emerging applications the main way to solve the problems of low temperature ductility and high temperature oxidation of molybdenum is to develop an advanced composite material based on molybdenum disilicide (MoSi2). MoO2 formed by contact between molybdenum and oxygen sublimates at 800 ℃ and forms a yellow white opal when condensed, which causes serious engineering problems for exerting the advantages of molybdenum in high temperature strength and creep resistance. For this reason, silicon rich coating with self-healing ability was used. However, the thermal cycling resistance of the coating was very poor. The high temperature strength and oxidation resistance of Mo-Si-B composite with molybdenum disilicide as matrix are excellent, but the ductility is poor, so it is limited to produce small batch commercial products. In order to solve the problem of ductility, the composition range of this kind of Mo Si B composite has been determined recently. Besides its excellent oxidation resistance, its high temperature mechanical properties are equivalent to that of TZM alloy. Mo5sib (T2) was used as the matrix phase and molybdenum as the second phase. The metal phase improves the ductility of the composite, and the matrix phase can form self-healing scale. At present, Mo-6Ti-2.2Si-1.1B composites with titanium addition can hardly be seen by naked eyes when exposed to air at 1370 ℃ for 2 hours, which is superior to TZM. This is a remarkable achievement of molybdenum based alloys.
The second new achievement of molybdenum is the lining of explosive filled warheads, which can penetrate and cut deep depths in military and industrial applications. In this kind of device, the explosive around the lining is detonated in a controlled way, which makes the lining deform in a very peculiar way. Deformation causes the lining material to produce rod-shaped fragments (ejectors) with extremely high speed and great tension, which can penetrate deep into the target or target. The development of molybdenum lined explosive liner liner is a new research field. The traditional liner material of warhead is copper, but the sound velocity of molybdenum is 5.12 km / S (3.94 km / s for copper), and the density is 10.2 g / cm3(8.93 g / cm3for copper). In order to obtain high-speed coherent jet, the tip must have high sound velocity. The velocity of the jet tip is more than 12 km / s when molybdenum is used, but less than 10 km / s when copper is used. The difference of velocity between the two is 20% - 25%. The reason is that the energy of the tip increases with high sound velocity, which leads to the increase of penetration. The new type of explosive liner is cone shaped and flared. Replacing copper with molybdenum will be an important reform in ordnance.
The third new achievement of molybdenum is the manufacture of flat panel display devices. In the electronic industry, active matrix liquid crystal display (LCD) technology is still used in flat panel display devices. However, LCD is competing with field emission display (FED), electroluminescent display (EL), plasma display panel (PDP), cathode ray emission display (CRT) and vacuum fluorescent display (VFD) in different development stages. In this display process, the display is realized by two pieces of glass separated by vacuum. The glass on the back serves as a cathode. On this glass, more than 500 million emitter tips are distributed in the form of field emitter arrays. The spacing between the emitters is much smaller than the pixels on the TV screen. The emitter tips are made of molybdenum, which can be controlled individually or in groups. In view of their wide viewing angle, fast response time, wide temperature range tolerance and low power consumption, they have become the main driving force for the development of panel display technology together with the trend of clear, bright, movable and durable requirements. It shows that there are more than 10 billion US dollars in the market. In flat panel display technology, molybdenum is deposited on the emitter tip by electron beam evaporation. Although the amount of molybdenum is small, it has an unlimited prospect for the development of large screen and high-definition TV.
