Phase Diagrams
Tungsten-Cobalt-Carbon (W-Co-C) Phase Diagram
The transverse rupture strength of tungsten carbide (WC) tools bonded with cobalt is a function of the carbon content. The most preferred microstructure, formed on cooling from the sintering temperature, consists of just two phases: WC particles uniformly embedded in the matrix of face-centered cubic (FCC) cobalt. This microstructure is achieved when the ratio between tungsten and carbon atoms is close to the stoichiometric composition of WC. As Figure 1 shows, a deficiency of carbon leads to the formation of M6C carbides (or M12C carbides), whereas an excess of carbon leads to the formation of graphite. The presence of M6C carbides in cobalt-bonded WC tools is undesirable as it decreases significantly the rupture strength.
Figure 1. Vertical section (isopleth) of the tungsten-cobalt-carbon (W-Co-C) ternary system, calculated with Thermo-Calc coupled with SSOL4 thermodynamic database. The cobalt content is kept constant at 10 wt. %.
A stoichiometric mixture of WC + 10 wt. % Co corresponds to 5.519 wt. C; the value which is placed inside the desirable two-phase region (FCC + WC) for the temperatures below the solidus. In practice, carbon is often added to the mixture of WC and Co, since some carbon inevitably reacts with oxygen during sintering.
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• Vertical section (isopleth) of tungsten-cobalt-carbon ternary system
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