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Method for producing tungsten carbide
Release time:
2019-12-23 16:16
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Method for producing tungsten carbide
Using tungsten metal and carbon as raw materials, an average particle size of 3–5 μm tungsten powder is dry-mixed with an equal molar amount of carbon black using a ball mill. After thorough mixing, the mixture is pressed into shape and then placed in a graphite crucible. It is subsequently heated to 1400–1700℃ in a graphite resistance furnace or an induction furnace, with the temperature carefully controlled within the range of 1550–1650℃. In a hydrogen gas flow, W2C is initially formed, and further reaction at high temperatures leads to the formation of WC. Alternatively, tungsten powder can first be prepared by thermal decomposition of hexacarbonyl tungsten in a CO atmosphere at 650–1000℃; this tungsten powder is then reacted with carbon monoxide at 1150℃ to produce WC. At temperatures higher than this, W2C may also be formed.
Chemical equation:
2W + C = W²C
W + C = WC
Tungsten powder (with an average particle size of 3–5 μm) is prepared by hydrogenating and reducing tungsten trioxide WO3. The tungsten powder is then mixed with carbon black in an equimolar ratio (dry-mixed using a ball mill for about 10 hours), after which the mixture is compacted under a pressure of approximately 1 ton/cm². The compacted material is placed into a graphite disk or crucible and heated in a hydrogen atmosphere—using a graphite resistance furnace or an induction furnace—in pure hydrogen with a dew point of –35℃—to a temperature ranging from 1400 to 1700℃ (**specifically 1550–1650℃**), causing carbon infiltration and forming WC. The reaction starts around the tungsten particles; initially, W2C is formed. Due to incomplete reaction—primarily because of the relatively low reaction temperature—unreacted tungsten and intermediate product W2C remain alongside WC. Therefore, it is essential to heat the material to the aforementioned high temperature. The optimal heating temperature should be determined based on the particle size of the raw tungsten material: For coarse particles with an average size of around 150 μm, the reaction should be carried out at the higher temperature range of 1550–1650℃.
Chemical equation:
WO3 + 3H2 → W + 3H2O
2WO₃ + 3C → 2W + 3CO₂
2W + C = W²C
W + C = WC
The particle size requirements for tungsten carbide (WC) in cemented carbides vary depending on the specific application. For cemented carbide cutting tools—such as V-cut blades used in trimming machines—ultrafine and submicron-sized tungsten carbide particles are employed for fine finishing alloys; medium-sized tungsten carbide particles are used for roughing alloys. Medium-to-coarse-sized tungsten carbide particles serve as raw materials for alloys used in heavy-duty and high-impact cutting applications. For mining tools designed to handle high rock hardness and significant impact loads, coarse-sized tungsten carbide particles are preferred. When dealing with rocks that experience low impact and minimal shock loads, medium-sized tungsten carbide particles are used as raw materials for wear-resistant components. When emphasis is placed on wear resistance, compressive strength, and surface finish, ultrafine, submicron, and medium-sized tungsten carbide particles are selected as raw materials. For impact-resistant tools, medium-to-coarse-sized tungsten carbide particles are primarily used as the base material.
The theoretical carbon content of tungsten carbide is 6.128% (50% by atom). When the actual carbon content of tungsten carbide exceeds the theoretical value, free carbon (WC + C) will appear in the material. The presence of free carbon during sintering causes the surrounding tungsten carbide grains to grow larger, resulting in uneven grain sizes in the cemented carbide. Generally, tungsten carbide is required to have a high proportion of combined carbon (≥6.07%) and low levels of free carbon (≤0.05%). The total carbon content, meanwhile, depends on the specific manufacturing process and application range of the cemented carbide.
Under normal circumstances, the total carbon content in tungsten carbide produced by vacuum sintering using the paraffin process is primarily determined by the amount of combined oxygen present in the compacted powder before sintering. For each percentage point of oxygen, 0.75 percentage points of carbon must be added. Thus, the total carbon content in WC can be calculated as follows: Total carbon in WC = 6.13% + (Oxygen content % × 0.75). (Note that this calculation assumes a neutral atmosphere within the sintering furnace; in reality, most vacuum furnaces operate under a carburizing atmosphere, so the actual total carbon content in the tungsten carbide produced will be lower than the calculated value.)
The total carbon content of Chinese tungsten carbide can be roughly divided into three categories: For tungsten carbide sintered under vacuum using the paraffin process, the total carbon content is approximately 6.18 ± 0.03% (with an increase in free carbon); for tungsten carbide sintered in hydrogen using the paraffin process, the total carbon content is 6.13 ± 0.03%; and for tungsten carbide sintered in hydrogen using the rubber process, the total carbon content is 5.90 ± 0.03%. Since these processes sometimes overlap, the determination of the total carbon content of tungsten carbide must be based on the specific circumstances.
The total carbon content of WC used in alloys with different application ranges, different cobalt contents, and different grain sizes can be adjusted slightly. For low-cobalt alloys, tungsten carbide with a relatively higher total carbon content can be selected; for high-cobalt alloys, tungsten carbide with a relatively lower total carbon content is more suitable. In short, the specific application requirements of cemented carbides dictate different requirements for tungsten carbide grain size.
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SX-80 plasma equipment is being used at a customer site in Russia.
SX-80 plasma equipment is being used at a customer site in Russia.
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