Direct carburization is advantageous mainly in the production of submicron tungsten carbide powders. The idea behind the process is to circumvent the expensive hydrogen reduction of fine tungsten powder by excluding water vapor during the reduction step, which is responsible for grain growth reactions and low capacity. Roughly speaking, the grain size of the final carbides is related to the size of the internmediate oxides.
Only recently was a new carbothermal reduction process developed in which the WC is synthesized by a rapid carbothermal reduction of tungsten oxides in a vertical graphite transport reactor (RCR entrainment process). Rapid heating of the WO3/C mixture driven by thermal radiation allows conversion of the mixture into a carbide precursor WC1-x) within very short reaction times (a matter of seconds). In a second step, additional carbon is added to the carbide precursor to form a mixture, which then undergoes a second heat treatment to convert the precursor into substantially pure WC.
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