Abstract

The four most important hard carbides for the production of cobalt-cemented (hard metal) carbide compositions are tungsten carbide (WC), and the cubic carbides titanium carbide (TiC), tantalum carbide (TaC), and niobium carbide (NbC). Of these, tungsten carbide is of paramount importance, in most cases forming a foundation-alloy to which smaller quantities of the cubic carbides are added to affect changes in metalcutting insert and other tool properties. In cemented carbide compositions for machining applications, preformed binary and ternary solid solutions (mixed crystals), made up of the primary carbides and based on the solven powers of the cubic carbides for WC, comprise important portions of the hard phases in cutting-insert microstructures. WC–TiC and WC–TiC–TaC(NbC) are of great importance among carbide solid solutions. WC, essentially unalloyed, is the paramount hard phase in metal-cutting tools for short-chipping materials, forming tools, and in tools used by the mineral, transportation, and construction industries. Auxiliary hard carbides of chromium, molybdenum, vanadium, hafnium, and zirconium are used to achieve secondary property enhancement. In general, the hard carbides are produced by reaction of elementary carbon or hydrocarbons and metals and metal compounds at high temperatures. Methods include carburization by thermal diffusion, by menstruum process, by thermochemical reaction, by reduction of metal halides and, to a minor extent, by fusion. Several categories of refining services support manufacturers of cemented carbides: extraction of tungsten as tungstic acid, WO₃, or ammonium paratungstate from tungsten-mineral concentrates, and extraction of oxides of titanium, tantalum, and niobium from mineral concentrates of those metals. Integrated carbide manufacturers carry out reduction of the metal oxide to metal powder, preparatory to carburization. Equipment and process departures to produce submicron and ultrafine tungsten carbide powders include carburization by gaseous fluid-bed processes and chemical vapor reaction.