For its unique properties and irreplaceable use within the broad area, tungsten becomes one of the rare metal mineral resources with particular strategic importance, as well as one of the most internationally dominant mineral resources for China. Tungsten and its alloys are widely used in iron and steel industry, equipment manufacturing, information industry, electric power industry, petrochemical industry, aerospace and military fields. Carbide blades and tools with tungsten carbide as raw material are extremely important functional materials in the modern industry, national defense and high-tech applications, with a very important position in China’s national economy.
Properties and Standard of Tungsten Metal Powder
Tungsten metal powder has to control not only certain requirements for impurity content, but also the oxygen content within a certain range. Tungsten powder commonly used has average Fisher particle size of 2~10 μm. Tungsten powder is almost polygonal particle shape. In addition, specific surface area, bulk density and tap density of tungsten powder vary within a certain range. Tungsten powder properties have a direct impact on the production of tungsten materials and the quality of tungsten powder metallurgy products, in particular on the purity and grain size. Tungsten powder is classified by the purity, grain size and different uses. The company which firstly produced tungsten powder in America divided tungsten powder by the hydrogen reduction into three levels based on the chemical purity; Japan had enacted the industry standard (JISH2116-1979) of the tungsten powder and tungsten carbide powder; and countries such as Britain, France and the former Soviet Union were provided with uniform national standards for tungsten powder. And China had established the technical conditions of hydrogen reduction of tungsten powder (GB3458-82) for industrial production tungsten powder in 1982. This standard not only stipulates properties and classification grades of tungsten powder, but also has specific requirements for test methods, acceptance rules, packing, transportation and storage, etc. For tungsten powder with special purposes and military special, manufacturers can process the trial production based on user requirements.
Applications of tungsten metal powder
Tungsten metal powder is a major raw material to process tungsten products by the powder metallurgy and tungsten alloys. Pure tungsten powder can be made into processing materials such as wires, rods, tubes, plates as well as products with certain shape. Tungsten powder mixing with other metallic powders can be made into various tungsten alloys, such as tungsten-molybdenum alloys, tungsten-rhenium alloys, tungsten-copper alloys and high-density tungsten alloys. Another important application of tungsten powder is made into tungsten carbide powder to produce cemented carbide tools, such as turning tools, milling cutters, drills and molds, etc.
Preparation methods of tungsten metal powder
This is to prepare tungsten metal powder by hydrogen reduction of tungsten trioxide or ammonium paratungstate, generally with two stages: the first stage is to reduce tungsten trioxide to tungsten dioxide at temperatures within 500~700°C; the second phase is to reduce tungsten dioxide to tungsten powder at temperatures within 700~900°C. And the reduction reaction usually occurs in a tube furnace or rotary furnace. Properties of reduction tungsten powder (such as purity, particle size and particle size composition) depend primarily on the reduction process. When reducing tungsten powder in tube furnace, major technological parameters which affect the reduction speed are reduction temperatures, amount of tungsten oxide mounted in the burning boat, movement speed of burning boat, flow rate of hydrogen and moisture content of hydrogen. With the rise of temperature, grain size of tungsten powder coarsens. Apart from the hydrogen reduction, there is tungsten oxide carbon reduction to prepare tungsten powder, with reduction temperature higher than 1,050°C. And tungsten powder obtained by this method has a lower purity. In addition, the study about reducing tungsten oxide by aluminum, calcium, zinc and other metals is being in progress. Tungsten powder in special applications requires high purity and ultrafine particle size, so hydrogen reduction of tungsten chloride is developed to obtain tungsten powder with particle size less than 0.05 μm.
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