Refractory metals are referred to metals with melting points over 3632°F and certain amounts of reserves, including tungsten, tantalum, molybdenum, niobium, hafnium, chromium, vanadium, zirconium and titanium. Usually refractory metals have great densities and weigh heavily. With refractory metal as matrix, the alloys added with other elements are called refractory metal alloys. Refractory metal has good high-temperature strength as one of its most important strengths. Besides, it also has good corrosion resistance to molten alkali metal and steam. However, bad oxidation resistance at a high temperature is the major weakness of refractory metal.
Refractory metals will not easily crack or break under high temperatures and can bear repeated heating or thermal shock. Tungsten, molybdenum, chromium and other refractory metals at low temperatures are likely to become brittle, while turn into ductile under high temperature conditions. Ductile-Brittle transition temperature (DBTT) is an important index for ductility processing and usage of refractory metals. DBTT can be influenced by many factors, like material’s purity, ingredients of alloys, processing methods and structures. There are two ways to reduce DBTT. One is to add alloy elements in refractory metals. For example, rhenium can be added into tungsten. The other way is choosing more reasonable processing methods, like technology of plastic processing.
Refractory metals of high density are very stable at room temperature and not easy to be oxidized in air. However, refractory metals will be rapidly oxidized under high temperatures. Tungsten and molybdenum begin to oxidize at about 752°F . They will be oxidized and generated respectively into WO3 and MoO3 with the temperature going up. When the temperatures reach 1562°F and 1112°F, the materials will be sublimate markedly. Rhenium starts to oxidize at 572°F and turns into Re2O7 at the temperature of 662°F . Tantalum and niobium begin to oxidize at the temperatures of 536°F and 392°F. When the temperature is over 932°F, they will generate into Ta2O5 and Nb2O5. Titanium and zirconium can be oxidized rapidly at temperatures above 1112℉ to 1292℉. The powder of zirconium and titanium can self-ignite in air and even can burn with explosions. In order to fix the oxidation problem, there are two measures. The first one is producing antioxidant alloys and the second one is covering the refractory metals with antioxidant coatings. However, the problem of refractory metal’s oxidation under high temperatures has not been totally solved so far.
Tungsten, molybdenum, rhenium do not react with hydrogen but their oxides can be reduced to metal with hydrogen at certain temperature. Tungsten, molybdenum and rhenium can become brittle when absorbing hydrogen. When the temperature reaches between 572°F to 932°F, those metals will absorb large quantity of hydrogen and generate into brittle metal hydride. In high vacuum condition, hydrogen will be released. Therefore, this feature of refractory metals can be used for producing the alloy powder of titanium, zirconium, tantalum and niobium.
Refractory metals have good corrosion resistance. When temperature is under 302°F, the surface of tantalum has a dense and stable oxide film. Therefore, the chemical properties of tantalum are very stable. Tantalum has excellent resistance towards sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, organic acids and nitric acid hydrochloride but will be melting in hydrofluoric acid, concentrated alkali solution and molten base. The corrosion resistance of niobium is similar to that of tantalum, but not as good as Ta. Tungsten is very stable at room temperature in hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid and aqua regia, but it will be easily corroded sodium nitrate. Molybdenum is similar but not as good as tungsten in corrosion resistance. In general, tantalum, niobium, titanium, zirconium and other refractory metals are excellent anti-corrosion materials to work as protective layers.
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