Coarse Tungsten Powder

Single crystals, due to their high cost are used only in special cases like basic investigations. The outstanding properties are a lack of grain boundaries and a very small number of lattice defects. They are therefore easy to deform, show great ductility, and possess no embrittlement up to the highest temperature. 

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FIGURE 5.28. Particle size distribution of fine and coarse tungsten powders. 

Lithium Doping. Lithium compounds are preferred to sodium for very coarse tungsten powder grades (50-100 pm) Fig. 5.29. Reduction conditions are the same as above. Due to the high lithium boiling point, it is not volatilized during reduction like... 

Carburization By Fusion. This method is used for the preparation of tungsten carbide for the mineral industry, ie, for coarse-grained powder or castings for welding (qv) onto oil drills and wear-resistant parts. 

This section deals primarily with the carburization of tungsten powder, the most widely used process for producing WC on a, technical scale. Also, the carburization performed in melts is discussed briefly owing to historical reasons and the production of hard facing alloys. Finally, the formation of coarse tungsten carbide crystals in auxiliary melts ( Menstruum WC) is discussed. 

Preparation of Coarse Tungsten Carbide Powder in an Auxiliary Melt... 

Coarse tungsten carbide powder. Coarse WC powders are available in the particle size range of >8 to 50 pm. Their crystallite sizes vary from <10 pm up to about 50 pm and mainly depend on the carburization temperature (strictly speaking, the maximum crystallite size is determined by the particle size). 

The particle size of the metal powder does not depend on that of the starting WO3, but (principally) on the reaction temperature, as well as the heating time and the Hg flow rate. An especially fine tungsten powder is obtained when the above directions are followed a very coarse powder results from reduction with moist Hg above 1500°C. The product is pure if the starting WO3 is also pure. 

Low Surface Area, Reducing Sintering Atmosphere The material selected for this formulation was a relatively coarse tungsten carbide powder. The binder system was a standard acrylic polymer. Since the WC was very coarse (16 pm average particle size) and had such a high density (15.7 g/cc), the formulation was difficult to prepare. The tape casting had to be done immediately after the slurry was poured from the preparation jar. 

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