Cobalt doped ammonium paratungstate (Co-doped APT) precursors for the fabrication of cemented carbides were synthesized by co-precipitation, where W and Co are mixed on the atomic scale. The effect of cobalt content on the reduction of W–Co precursor was studied. W–Co precursors with different ratios of tungsten to cobalt were prepared and calcined at 220 °C for 5 h in air, followed by a reduction in hydrogen gas at 600 and 650 °C for different times. The resulting powders were characterized with respect to phase constituent, specific surface area, particle size, density, and morphology of the reduced powders. It has been found that the addition of cobalt results in a higher reduction rate toward the metallic phases. However, the particle size of the reduced powders increases with increasing cobalt content, while its relative density decreases. The specific surface area of reduced powders at 650 °C increases for short reduction times, due to reduction of oxides to metal powder, and decreases for long reduction time due to agglomeration of metal powder. Cobalt plays the role as a catalyst during the reduction processing. On the other hand, it promotes the agglomeration of small particles. However, the completely reduced powders were well nanostructured with an average particle size between 20–50 nm.

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Ammonium paratungstate hydrates (hereinafter referred to as APTs) are known intermediates for producing tungsten metal, tungsten-containing catalysts or hard materials based on tungsten, for example tungsten carbides.

A process for the continuous preparation of ammonium paratungstate hydrate directly in the reextraction of a tungsten-laden organic phase with an ammonia-containing aqueous solution in a mixer-settler apparatus, which comprises carrying out the reextraction at an NH3:W molar ratio of from 0.83 to 1.30, and a volume feed ratio of the tungsten-laden organic phase to the ammonia-containing aqueous solution of from 5 to 25. Directing ammonium paratungstate decahydrate containing at least 75% of crystals having a length of at least 200 μm and having a ratio of length to width of less than 4.5:1.

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The possibility of mechanochemical decomposition of ammonium paratungstate (APT) has been studied, and compared with thermal decomposition method. For this purpose, APT powders were milled using a planetary ball mill up to 36 h and under air atmosphere. For thermal decomposition, APT powders were heated for 30 minutes at 300 and 450 °C in air atmosphere. X-ray diffraction (XRD), differential scanning calorimeter (DSC), and thermo gravimetric analyzer (TGA) were used to study the decomposition progress, and products. The XRD results showed that APT completely decomposed to WO₃ by thermal decomposition, while the final product of mechanochemical decomposition was WO₃ (H₂O)_0.5. According to DSC and TGA results, during thermal decomposition, ammonia and water released in four steps, and leaved WO₃. By mechanochemical decomposition crystal water and ammonia liberated from APT structure, but structural water of APT remained. In both methods, an X-ray amorphous phase was the intermediate product of APT decomposition.

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Thermal plasma process has been applied to the synthesis of nanosized tungsten carbide powder with ammonium paratungstate (APT) as the precursor. The reduction and carburization of vaporized APT produced nanosized tungsten carbide (WC1−x) powder, which sometimes contained a small amount of W2C phase. The effects of reactant gas composition, plasma torch power, the flow rate of plasma gas, and the addition of secondary plasma gas (H2) on the product composition and particle size were investigated. The produced tungsten carbide (WC1−x) powder was <20 nm in particle size. The synthesized powders were also subjected to a hydrogen heat treatment to fully carburize the WC1−x and W2C phases to the WC phase as well as to remove excess carbon. Finally, WC powder of particle size <100 nm was obtained.

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Violet tungsten oxide was prepared with pure ammonium paratungstate in argon gas. Tungsten powder was obtained from violet tungsten oxide through deoxidation in hydrogen gas (rate of purity: 99.99%, dew point: –40 °C), and tungsten powder was oxidized in the air. Tungsten oxide was reduced to tungsten powder in hydrogen gas. The samples were characterized with the laser particle size distribution measuring instrument and the electron probe scan instrument. The results show that submicron tungsten powder can be obtained through circulatory oxidation twice and reduction three times. The percentage of the particle size distribution of submicron spherical tungsten powder is 95.73% between 0.1 μm and 1.0 μm, and its BET is 1.082 m2/g.

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After decades of development, the hardness of tungsten carbide cutting tools has reached 89 ~ 93HRA, at a high temperature of 1000 ℃ still has good red hardness, durability is several times its high speed steel cutting tools. tungsten carbide cutting tools material is WC, TiC, TaC, NbC, VC and other refractory metal tungsten carbides and prepared by powder metallurgy from iron group metals as a binder. Compared with the high-speed steel, it has high hardness, wear resistance and red hardness; compared with super hard material, it has a higher toughness. Because of tungsten carbide cutting tools has a good overall performance, so in the tools industry has been widely used.

However, under certain circumstances, tungsten carbide cutting tools diffusion wear occurs. Diffusion wear occurs at high temperature effects. tungsten carbide at the time of cutting temperature 800 ~ 900 ℃, W and C atoms will diffuse to chip in, chip in Ti and Al atoms will diffuse into the tool, after mutual displacement between atoms, it will reduce the tools interatomic bonding strength and wear resistance and the formation of diffusion wear. When cutting titanium, due to the high temperature of the cutting area, and the surface is always in contact with each other just cutting out a fresh surface, chip, before the workpiece and tools flank greater chemical activity, so the tools material and titanium tungsten carbide element alloy contact surfaces will spread to the other side to each other, and cause diffusion wear, changing the chemical composition of both the direct impact on its performance.




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A refractory metal tungsten carbide cutting tools hard metal compound and a binder made by powder metallurgy method, its hardness (HRA) is 89 to 94, much higher than the high-speed steel; at 540 ℃ hardness (HRA) 82 to 87, and the hardness of high-speed steel normal temperature (HRA83 ~ 86) fairly, but also has good chemical stability, heat resistance advantages. tools materials currently used for a lot of variety though, the new tools materials are also emerging, but still very popular tungsten carbide cutting tools material kind.

When cutting titanium, tungsten carbide cutting tools cutting when the temperature reaches 700 ~ 800 ℃, tungsten carbide materials WC, TiC, and Co compounds and plays the role of oxygen in the air, forming a lower hardness and strength oxide film. As air enters the cutting area is not easy, it is easy to form an oxide film on the flank position after tungsten carbide cutting tools. When cutting the workpiece surface by the oxide, chilled layer and hard impurity continuous friction points on the oxide film, resulting in oxidation of flank wear, also called boundary wear. When a large amount of wear and tear, will form oxidation wear grooves. After tungsten carbide cutting tools oxidation wear mainly as tools and vice flank flank have emerged oxidation wear phenomenon. And this phenomenon is more pronounced in high-speed cutting.




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Qualified tungsten blanks is one of the key timber production, a good system to choose a qualified first blank tungsten powder. Feature powder (average particle size, particle size distribution, chemical composition), mixing, forming and sintering process on the blank composition, density and microstructure have a direct impact, and strongly influences the processing and performance. Does not sag tungsten added silicon, aluminum, potassium is tungsten trioxide or "Blue W" (for a variety of low-cost tungsten oxide mixture) in the form of oxides added, the mixture containing hydrofluoric acid solution used washed to remove impurities in the powder.

Production of silk and small blank sheet more in the press forming, may also isostatic press forming. Powder compact size is generally 12 × 12 × 400 mm, there is also a larger-sized round bar, square bar or rectangular bar. First obtaining 1200 ℃ powder compact in a hydrogen atmosphere for 1 hour so as to have prefired certain strength and electrical conductivity, and then perform a power on self resistance sintering. Self-powered resistance sintering commonly known as "vertical melt" is a method developed tungsten processing. The principle is the current directly through the sintered compact, due to the resistance of the blank itself, Joule heat is generated, with this heat the billet sintering, the sintering current is typically 90% of the fusion current. The resulting self-resistance sintered blank strips (also known as a sintered article).

Processed into a general standard vertical wire melting section shall control the number of grains per square millimeter of about 10,000 to 20,000, a density of 17.8 ~ 18.6 g / cm 3. For pipe, sheet, or other large-sized products, often using isostatic pressing (pressure at 2500 kg / mm 2 or more) formed and sintered at 2300 ~ 2700 ℃ high temperature in a vacuum or hydrogen protection.

Swaging the blank is produced and the fine tungsten wire rods commonly used plastic working method, different sizes of rods in a hydrogen atmosphere is heated to 1400 ~ 1600 ℃, be swaged on a different model of the swaging machine. Start track deformation of the times is not too large, then be appropriate to increase the amount of deformation. Swaging deformation between the workpiece and the die graphite lubrication process. Tungsten rod density after processing up to 18.8 ~ 19.2 g / cm 3. Because forging billet into a round billet, deformation of each part is different, so the organization is uneven, you should recrystallization annealing.




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Tungsten carbide powder is conventionally produced by carburization of tungsten powder by carbon black in a solid–solid reaction.  Carburization of ammonium paratungstate by methane to yield tungsten carbide at low temperature in short times. The carbide powder is fine grained and has specific surface area around 35m²/g. This makes the powder interesting for catalysis and fine grained hard metals. The influence of reaction parameters such as temperature, gas flow, composition of the reaction gas and powder charge are investigated by means of gas chromatography and XRD with Rietveld refining. The overall reaction time is the sum of reduction and carburization times of the sequential reactions. Some parameters, like gas composition affects only the carburization rate, other parameters, such as gas flow affects both reactions.

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Ammonium paratungstate is produced separating tungsten from its ore. Once the ammonium paratungstate is prepared, it is heated to its decomposition temperature, 600°C. Left over is WO3, tungsten oxide. From there, the oxide is heated in an atmosphere of hydroge, reducing the tungsten to elemental powder, leaving behind water vapor. From there, the tungsten powder can be fused into any number of things, from wire to bars to other shapes.

I.E. mechanical activating caustic decomposition and removing molybdenum from tungstate soluton by ion exchange method are used in this technological process for treating low grade tungsten concentrate with a low concent of WO3 and a high content of calcium and impurities to produce APT of high purity. The combination of whole technological process is reasonable and has the advantages of shorter high recovery of tungsten and high purity of product.

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