Tungsten Carbide Rod Powder Extrusion Molding

Tungsten carbide rod powder extrusion molding is developed on the basis of metal materials and polymer materials. With the widely usage of advanced manufacture technology, large production line and precision manufacture center, the need for the high property tools increased a lot, thus drive the need for the tungsten carbide rod. Tungsten carbide rod powder extrusion molding has many advantages. It can produce tungsten carbide rod of equal section and complex shapes. And there is no limitation for the length of the rod, especially for the rod with internal coolant holes. The rod is of even thickness. The size is approximately to the finished product which can reduce the following processing, thus it can cut the producing cost and raw material. Compared to mold and isostatic pressing technology, it can greatly promote producing efficiency. As a result, powder extrusion molding has become the most important method to produce tungsten carbide rod and it has been widely used.


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Tungsten Carbide Rod Hard Facing

A hard facing tungsten carbide rod is made by mixing particles of tungsten carbide, particles of a metal binder, and a temporary organic binder. Tungsten carbide rod hard facing relates to a rod of wear-resistant hard facing for application to wear or cutting surfaces such as teeth on bits for drilling oil wells or the like. The hard facing is applied by melting an end of the rod on the surface to be hard faced. The mixture is pressed, preferably by extrusion, into the form of a rod. The organic binder is removed and the rod is heated to a sufficient temperature to sinter the metal particles together without melting so that the rod has a residual porosity in the range of from 5 to 20%. Such a rod is used for hard facing a metal surface in lieu of conventional tube-rods. The technique permits a high proportion of tungsten carbide relative to the metal matrix and provides uniform distribution of carbide particles in the hard facing.


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First Tungsten Carbide Rod in China

On March 1958, China gave the mission to factory 601 to make φ2 mm tungsten carbide rod. After a long and hard research, on September 7^th, 1958, the first φ2 mm tungsten carbide rod was made successfully in factory 610. China had the ability to produce tungsten carbide rod since then and no longer need to be controlled by foreign companies.

Tungsten carbide rod is used to made mining tools which does great benefit to ore mining. Before factory 601 made it, our country mainly get the tool from other countries to operate mining.

Binder, mould and cutting is the hardest part during the experiment.  Compression moulding and semi-finished product sintering is the most important part in the process. The traditional binder gasoline colloidal solution is no longer suitable for the tungsten carbide rod extrusion method. Finally they use the paraffin as binder and also the perfect type and amount. In order to reduce the friction between the binder and diaphragm wall, lubricant is also added in it. To solve the pressing mould problem, they decided to insert a small device like a tower in the mould to distribute pressure.  Also after a long exploration they find a new method which makes the cutting efficiency 8 times faster then before. 


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Tungsten Carbide Rod Extrusion Affecting Factors

Tungsten carbide rod extrusion affecting factors includes mixing, carbon, binder, extrusion temperature, extrusion mould and extrusion speed.
1.Mixing. Mixing is a complex process of completing the powder liquidity and dispersion. In this process the binder must be completely mixed with the powder and becomes a perfect powder which has an excellent liquidity property.
2.Carbon. Carbon content plays an important role in the process of tungsten carbide rod extrusion Tungsten carbide is made of WC and Co, other phase may cause adverse affects to the property of the tungsten carbide rod, thus it is a vital problem to control carbon content in the extrusion process. The decarburization and carburization will affect both the density and property of the tungsten carbide rod.
3.Binder. Binder is used for enhance liquidity and maintain the shape of the tungsten carbide rod in the extrusion process. The require for the binder is good wettability and adhesion, no chemical reaction with the powder and high strength.
4.Extrusion temperature. If the extrusion temperature is too high or too low, it will cause the blanks being out of shape, affects its density and strength, and quality of the tungsten carbide rod.
5.Extrusion mould. If the mould is improper, it will cause the density of the rod being uneven. Thus affect the later transformation in sintering.
6.Extrusion speed. The extrusion speed is various from the productive efficiency and easy to operate. Either the speed is too high or too low will affect the density and shape of the tungsten carbide rod.


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During tungsten oxide reduction process the size of tungsten powder particle is usually varies. At present, there are two main theories to explain tungsten powder particle size changing at the reduction process change.

First is growth mechanism by chemical vapor migration. Tungsten oxide in contact with steam, will generate produce a volatile hydrated tungsten oxide (WO2 (OH) 2), and evaporated to gaseous phase react with H2 occurring homogeneous gas reduction reaction. The reduction product is deposited on the nucleation of tungsten grain, so that tungsten powder particles grow. This process reaction rate is quick, and the reduction product forms will change significantly compared to the raw materials.

Second is solid-phase local chemical reaction mechanism. The solid tungsten oxide contacts with H2 gas causing gas- solid reaction. With oxygen atom removing the lattice will rearrangement. This process is slow reaction rate, and reduced product shape does not change.

In tungsten powder reduction process, the above two mechanisms exist simultaneously, what kind of mechanism play a role part depending on the temperature and moisture content of the atmosphere. 

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Hydrogen reduction – carbonization method produces tungsten powder having a simple operation, coating reasonable, stable and reliable advantages. the process of hydrogen reduction is: tungsten oxide →hydrogen reduction →sieving → blending → tungsten powder.

With the improvement of tungsten powder manufacturing level, has made great progress in the selection of tungsten oxide raw materials. Starting with tungstate (H2WO4) and yellow tungsten oxide (WO3) and now widely used blue tungsten oxide (referred to WO2.90) and purple tungsten oxide (referred to WO2.72 ) as a raw material. Structure, components and their physical and chemical properties of raw materials have great impact on tungsten powder particle size and uniformity. During the restore process, by regulating the various process parameters can control the particle size of the tungsten powder, such as the temperature of reduction, material thickness, hydrogen flow, ventilation direction.

The method for producing tungsten carbide powder is using tungsten oxide producing tungsten power then tungsten powder mix with carbon back making tungsten carbide powder. At the first using  method mixed tungsten powder and carbon black at 1400~1600℃high temperature carbonization and get WC powder. Since the method can not make tungsten powder and carbon full contact, easily lead to incomplete carbonation. Then switch to using hydrogen reduction, firstly hydrogen will react with black carbon and producing hydrocarbon. And then carbon-containing gas will carbonize tungsten powder to tungsten carbide powder.

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When blue tungsten oxide has same quality, the reduction process plays an important role at tungsten powder particle size. In producing fine tungsten powder must be strictly controlled reduction of temperature, hydrogen flow rate, hydrogen gas humidity, the thickness of the material layer, through hydrogen speed and direction, as they will influence fine tungsten powder producing.

Firstly, hydrogen-way requirements, generally speaking, there are two ways through hydrogen: cis hydrogen and inverse hydrogen. When the hydrogen reduction reaction at low temperatures and low humidity conditions, following hydrogen flow direction to push the boat along will obtain fine tungsten powder which is better than using inverse hydrogen way produce tungsten powder.

For another requirement is the reduction temperature, the higher reducing temperature the faster response, the more intense vapor transport, resulting in crystalline powder. And tungsten powder will agglomerate and coarse. Therefore, during the reaction the temperature should use low starting point, warmed gently heating system. Because the reduction temperature is low, heating will not so fast, so the water for the reaction is not too much which is in favor of the producing fine tungsten powder.

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Tungsten target is also called tungsten sputtering target，which is made from raw ground tungsten powder. Tungsten target owns high melting point, elasticity, low coefficient of thermal expansion, resistivity and fine heat stability and other properties.
Pure tungsten and tungsten alloy targets have been widely applied in electrical industries, chemical equipment (X-Ray tube, thin-film material, semi-conductor integrated circuit), medical equipment, smelting equipment, rare earth smelting, aerospace and many other fields. For example, Mo alloys and   tungsten X-ray targets which can withstand extreme thermal stress and temperatures over 2,000℃, are used for the general medical diagnostic and amigo- cardiograph. All tungsten targets have superior quality and durable, guaranteeing longer life.
Tungsten target become one of the main source for X-ray targets because of the electronic structure of tungsten. The production process of tungsten target is that pressing the tungsten powder into form and sintering, then hot isostatic pressing, bloomed, cold rolling, annealing, cutting, machining and bonding with a copper backing plate. 

 

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Application of nano powder composite can improve hardness, toughness and other comprehensive performance of tungsten carbide cutting tools which is an important research direction of tungsten carbide cutting tools.

Mechanical properties hardness and relative density of nano-SiCp modified tungsten carbide cutting tools are better than tungsten carbide cutting tools free of SiCp. This is due to nano SiCp in nano-SiCp modified tungsten carbide cutting tools are mainly in the hard phase WC particles at grain boundaries, and play a role in pinning grain boundaries and the grain refinement. Intensity of tungsten carbide cutting tools is related to relative density.the higher the intensity, the presence tends to diffuse spheroidal form, the anti-chipping performance also increases. Flank wear area of Nano-SiCp modified tungsten carbide cutting tools will not fall off, showing a more stable abrasive wear.
 

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Tungsten resources in tungsten ore belongs to non-renewable resources, and tungsten resources in tungsten carbide cutting tools material belongs to the secondary resources and renewable resources. As tungsten resource supply increasingly tense, people used the resources of tungsten carbide cutting tools increasingly strong awareness of recycling. Tungsten carbide cutting tools saltpeter method is an important method of recycling tungsten resources in tungsten carbide cutting tools.

Steps of tungsten carbide cutting tools saltpeter method are mainly as follows: the waste and saltpeter placed 900-1200 ° C melted, and then flooding tungsten Na2WO4 form into the solution. Then according to the general method of preparation of WO3 or APT from the solution, cobalt remain in the leached recovered.

Advantages of tungsten carbide cutting tools saltpeter method are wide adaptability, low investment costs, fast response. Disadvantages of tungsten carbide cutting tools saltpeter method are long process, low recovery rate, environment pollution.

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