In several common coating (TiC, TiN, Al2O3, TIAiN), and because of their different properties, it is suitable for different processing conditions. When the low-speed cutting steel material, TiC, TiN coated tool compared with Al2O3 coated tungsten carbide cutting tools is good, it is because in this cutting conditions TiC, TiN coated tungsten carbide cutting tools in the anti-flank terms of wear and resistance to mechanical vibration better than Al2O3 coated tools.

In contrast, in the high-speed cutting Al2O3 coated tungsten carbide cutting tools are better than TiC, TiN coated tungsten carbide. When the temperature is high speed cutting cutting tool wear by friction, diffusion and oxidation caused, Al2O3 coated tungsten carbide has good chemical stability and high temperature oxidation resistance, and therefore anti-crater wear when its temperature, After the anti-flank wear, anti-edge plastic deformation are better than TiC, TiN coatings, carbide high durability.
In continuous cutting conditions, the wear resistance and heat resistance are the main factors affecting the tool life, Al2O3 coated tools have excellent oxidation resistance and high temperature hardness, so the cutting conditions under continuous performance is better than TiC, TiN coating. But under intermittent cutting conditions, due to an impact force, the toughness of the coating has become a major influence carbide durability factor, but poor toughness Al2O3 coated tools, impact resistance is far better than TiC, TiN coated layers.


 

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According to different uses, tungsten alloys are divided into tungsten carbide, heavy alloy, metal sweating materials, contact materials, electronic and electric light materials.

Tungsten doped tungsten powder is added in about 1% of silicon, aluminum and potassium oxide, in a sintered (since resistance sintering) process, volatile additives hydroxide, the formation of pores within the material, pores after processing edge axially elongated; after annealing, diffusion of pore-forming elongated parallel to the wire axis line of bubbles, this dispersion of bubbles bubble known as potassium.

Potassium tungsten bulbs impede lateral grain grow and improve high-temperature tungsten sag performance, but also to improve the room temperature ductility after recrystallization, help around the wire and transportation and storage. China doped tungsten values are in accordance with the high temperature creep WAl1, WAl2, WAl3 three kinds of grades.




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Tungsten carbide cutting tools in high-speed cutting, the blade tip temperatures up to 900 ° C, this time tungsten carbide cutting tools cutting tools wear is not only a mechanical friction and wear (behind the main form of tool wear), and adhesive wear, diffusion wear and oxidative wear (mainly in the form of the tool edge wear and crater wear), so that the cutting process can be considered as a process of physical and chemical changes of a micro-region.

Titanium tungsten carbide cutting tools is a high hardness and wear-resistant compound, has a good anti-friction and wear properties; titanium nitride lower hardness, but have a high chemical stability, and can greatly reduce the friction between the tool and the workpiece to be machined factor. From the coating process considerations, both of which are desirable coating material, but regardless of titanium tungsten carbide cutting tools or titanium nitride coating material, a single coating layer are difficult to meet the high-speed cutting tool coatings on integrated requirements. Antioxidant and anti-wear performance diffusion wear, no material can be compared with alumina (Al2O3). However, due to physical and chemical properties of the alumina matrix material with much difference, can not be made of a single alumina coating preferably coated tools. Emergence of multi-layer coatings and related technology, the coating and the substrate material can improve bond strength, while overall performance with a variety of coating materials.



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CVD technology since the late sixties, the coated tungsten carbide cutter has been very widely used. In the CVD technique, the preparation of the required metal source vapor deposition is relatively easy to achieve TiN, TiC, TiCN, TiBN, TiB2, Al2O3, such as single and multi-multi-layer composite carbide coating, the coating and substrate high strength, a film thickness of up to 7 ~ 9μm, CVD coating has good abrasion resistance.

In the late eighties, the United States 85% of tungsten carbide cutting tools with a coating, wherein the CVD coated tungsten carbide cutting tools accounted for 99%.

The mid-nineties, CVD coated tungsten carbide coated tungsten carbide inserts in the tool still account for more than 80%. For example, using the CVD method TiC single coat of WC-based cemented carbide substrate YW1 / TiC, and uncoated WC-based cemented carbide turning YT15 contrast. TiC single coated blade wear is much higher than YT15. CVD technology but also has its congenital defects, one process treatment temperature is high, the tool is easy to cause a decline in the flexural strength of the material; the second is the internal film tensile stress state, the use of micro-cracks easily lead; third CVD technology emissions, waste can cause industrial pollution, greater impact on the environment, and the current conflict to promote green industry, so the mid-nineties the development of high-temperature CVD technology subject to certain constraints.




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Ammonium paratungstate was prepared from tungsten alloy swarf employing anodic dissolution in sodium hydroxide using a packed bed configuration followed by chemical conversion into ammonium tungstate through tungstic acid. The alkaline tungstate solution was dialysed to recover sodium hydroxide, which could be recycled to the electrolytic dissolution. The influence of anodic current density and other parameters on dissolution/recovery was studied and discussed. It was observed that a current density of 600 mA/100 g scrap could be applied when the concentration of NaOH was maintained around 2.5 M. The energy consumption for anodic dissolution was 2.14 kWh/kg W with more than 90% recovery achieved. The exhausted scrap contained an alloy of nickel and iron with a very low percentage of tungsten. APT sample prepared through anodic dissolution in NaOH was characterized.

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The ammonium paratungstate is present in the ammoniated aqueous strip solution in an amount of from about 200 to about 300 grams per liter. Further, the ammonium paratungstate is substantially free of the impurities originally present in the brine.

The concentrated, purified ammonium paratungstate can be recovered from the ammoniated aqueous strip solution by any technique known in the art, such as evaporative crystallization, cooling the hot strip solution to allow the ammonium paratungstate to crystallize and the like. The crystallized ammonium paratungstate then can be recovered by filtration.

The filtrate of the filtration can be recycled to the two stage strip circuit of the secondary solvent extraction system where, after pH adjustment, the filtrate is reloaded with ammonium paratungstate at a temperature of from about 45C.to about 55° C. This procedure eliminates the need for additional evaporation stages and thus results in a substantial savings in equipment costs.

The ammonium paratungstate product recovered from the filtration of the strip solution can be dried using any suitable technique known in the art. Further, the ammonium paratungstate will be substantially free of impurities originally present in the brine, namely arsenic, boron, phosphorus, silica and other heavy metals.

The above described process provides an efficient and effective method for recovering concentrated, purified tungsten values (such as ammonium paratungstate) from a brine containing minor amounts of tungsten values (in the form of tungstic oxide) and substantial amounts of impurities without the use of substantial chemical costs, processing costs, or equipment modifications to obtain the desired result.

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Within the framework of a review of the fundamental processes applied for the manufacture of ammonium paratungstate (APT) the main byproducts and wastes generated in these processes and their environmental impact are summarized.

A subsidiary process based on the electrodialytic recovery of NaOH for the elimination of the salt byproduct, which is generated in the largest quantities, is outlined together with the main aspects to be taken into account for large-scale industrial implementation.

The present stage of development and implementation of an APT production line for the processing of typical scrap material from the lighting filament industry is shown, as well as the oxidizing and dissolving process of compact and lumpy tungsten alloy scrap (Cu-W, Fe-Ni-W) and hard metal scrap (Co-WC, Co-WC-Ta(Nb)C-TiC).

Several as yet unexploited potentials are sketched which could, from scientific and engineering standpoints, be interesting for the development of more efficient intermediate manufacturing processes for APT and tungsten or process operations for a wide variety of raw materials.

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Ammonium paratungstate tetrahydrate, APT·4H₂O dissolves rather slowly in aqueous ammonia at room temperature. Dissolution at higher temperatures increases the rate of dissolution, but the amount of dissolved tungsten decreases due to the lower solubility of ammonia at higher temperatures which results in a lower pH of the solution. The rate of dissolution was found to be independent of the crystal size. It is therefore concluded that the rate-controlling step occurs in the solution, and is probably H₂W₁₂O₄₂¹⁰⁻→2HW₆O₂₁⁵⁻. Calcination of APT·4H₂O in air, nitrogen and hydrogen/nitrogen mixtures, into an X-ray amorphous product, is shown to be one way to improve the dissolution rate and to increase the dissolved amount of APT·4H₂O. The highest solubilities can be obtained by calcining APT·4H₂O in air or nitrogen at temperatures ranging from 250 to 310°C for approximately 2 h. When the temperature is too low or the residence time is too short some crystalline APT·4H₂O will still exist; WO₃ is formed when the temperature is too high or the residence time too long. It is supposed that the large H₂W₁₂O₄₂¹⁰⁻ structure is broken during the calcination process.

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The crystallization of ammonium paratungstate tetrahydrate, ((NH₄)₁₀OH₂W₁₂O₄₂·4H₂O, APT·4H₂O) from aqueous ammonium tungstate solutions is a key unit operation in current tungsten powder production. Ammonium paratungstate hexahydrate (APT·6H₂O) can form under conditions where APT·4H₂O forms as well. Based on this information it was postulated that APT·6H₂O is metastable. Experiments were carried out to determine whether and under what conditions the hexahydrate is metastable. It was found that the hexahydrate is metastable at a solution concentration of approximately 300-230 g/kg WO₃ and from approximately 90° to 96°C. APT·6H₂O is not formed when APT·4H₂O seeds are added prior to crystallization. It was found that the recrystallization of APT·6H₂O into APT·4H₂O is prompted by a decreasing water activity in solution during crystallization. The increase in solution temperature during isobaric crystallization also stimulates the recrystallization process. In industrial practice some residual crystals are always present in the crystallizer. Therefore, APT·6H₂O will not form during industrial crystallization.

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Ammonium paratungstate tetrahydrate (NH4)10[H2W12O42]·4H2O (APT), a starting material of WO3 and tungsten production, has been subjected to a complex thermoanalytical study in air. The weight loss stages and the various heat effects have been followed by simultaneous thermogravimetry and differential thermal analysis (TG/DTA) in flowing air up to 600 C. Meanwhile evolution of gaseous products has also been analyzed and monitored by both online coupled mass spectrometer (TG/DTA-MS) and infrared gas cell (TG-FTIR), in comparison. Besides that of the previously known H2O and NH3, evolution of two new gaseous products, N2O and NO, which had not been reported earlier, has been detected and traced by both evolved gas analysis (EGA-FTIR and EGA-MS) methods. These oxides of nitrogen are considered as catalyzed oxidation products of the released ammonia in air at 260–350 and 400–450°C. Thus, the two exothermic heat effects observed by DTA at 342 and 443°C have been assigned to ammonia combustion. Furthermore solid intermediate products have been structurally evaluated by both FTIR spectroscopy and powder X-ray diffraction (XRD). Based on significant changes in XRD patterns, a consequent formation of hexagonal ammonium tungsten oxide bronze and monoclinic WO3 are confirmed.

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