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Popular Refractory Metals
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Refractory metals are not easy to crack or break at high temperature, and can withstand repeated heating or thermal shock. Refractory metals such as tungsten, molybdenum and chromium are likely to become brittle at low temperature and ductile at high temperature. The ductile-brittle transition temperature (DBTT) is an important index for ductile processing and application of refractory metals. DBTT will be affected by many factors, such as material purity, alloy composition, processing method and structure. There are two ways to reduce DBTT. One is to add alloying elements to refractory metals. For example, rhenium can be added to tungsten. Another method is to choose a more reasonable processing method, such as plastic processing technology.
High-density refractory metals are very stable at room temperature and are not easily oxidized in air. However, refractory metals will oxidize rapidly at high temperature. And tungsten and molybdenum begin to oxidize at about 752°F .. With the increase of temperature, they will be oxidized to WO3 and MoO3. When the temperature reaches 1562°F and 1112°F, the material will obviously sublimate. Rhenium begins to oxidize at 572°F and becomes Re2O7 at 662 F. And tantalum and niobium begin to oxidize at temperatures of 536°F and 392°F .. When the temperature exceeds 932°F, they will generate Ta2O5 and Nb2O5. And titanium and zirconium can be rapidly oxidized at a temperature higher than 1112 F to 1292 F.. Zirconium powder can spontaneously combust or even explode in air. In order to solve the oxidation problem, there are two measures. The first is to produce oxidation-resistant alloy, and the second is to cover refractory metals with oxidation-resistant coating.
Tungsten, molybdenum and rhenium do not react with hydrogen, but their oxides can be reduced to metal with hydrogen at a certain temperature. Tungsten, ferro molybdenum and rhenium become brittle when they absorb hydrogen. When the temperature reaches between 572°F and 932°F, these metals will absorb a large amount of hydrogen and generate brittle metal hydride. Under high vacuum conditions, hydrogen will be released. Therefore, this characteristic of refractory metals can be used to produce alloy powders of titanium, zirconium, tantalum and niobium.
Refractory metals have good corrosion resistance. When the temperature is lower than 302°F, there is a dense and stable oxide film on the surface of tantalum. Therefore, the chemical properties of tantalum are very stable. It has excellent resistance to tantalum sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, organic acid and nitric acid hydrochloride, but it will melt in hydrofluoric acid, concentrated alkali solution and molten alkali. The corrosion resistance of niobium is similar to that of tantalum products, but not as good as that of tantalum. Tungsten is stable in hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid and aqua regia at room temperature, but it is easily corroded by sodium nitrate. Molybdenum is similar to tungsten, but not as good as tungsten in corrosion resistance. Generally speaking, tantalum, niobium, titanium, zirconium and other refractory metals are excellent anticorrosive materials used as protective layers.
