When sintering, it is necessary to determine the state of the system at various temperatures according to the specific content of the component to make the corresponding process. For example, the different sintering process paths are set by the eutectic temperature.

 

 

Binary phase diagram is a concise graphical representation of the relationship between the two components in the thermodynamic equilibrium state and the relationship between temperature and pressure. With the binary phase diagram, phase state at specific content and specific temperature of WC-Co tungsten carbide can be obtained, providing a theoretical basis for sintering and heat treatment.

 

Because of the presence of free carbon, WC-Co tungsten carbide is actually a W-C-Co triad. The binary phase diagram of WC-Co tungsten carbide is actually the cross section of the W-C-Co phase diagram when the content of the C is 6.1% (WC with C 6.1%), so it is also called the pseudomorph binary phase diagram of WC-Co.

 

In industry, the cobalt content (weight) of WC-Co cemented carbide is generally 3%-25%. Therefore, the composition of WC-Co tungsten carbide is in the right of the pseudomorph binary phase diagram eutectic point (the figure is E point), which is in the hypereutectic phase in phase diagram. The sintering temperature of WC-Co tungsten carbide decreases with the increase of cobalt content. It generally exceeds 1350℃-1500℃, which exceeds the eutectic temperature（1340℃）and 100-200℃.

 

Seeing from the phase diagram, we can find that the Co-WC solid solution contains about 4% (atomic fraction) at 1000℃, while it contains about 10% (atomic fraction) at the eutectic temperature. In addition, it was found that the content of WC in solid solution based on Co was about 1.5% (atomic fraction) at about 700-750℃. On the contrary, the solubility of Co in WC is very low, which ensures the existence of the liquid phase when the WC-Co tungsten carbide is sintered.

 

Because of the formation of WC+γ+C ternary eutectic at the sintering temperature, the melting point of WC-Co tungsten carbide is lower than that of WC+γ binary eutectic (1340℃), which is about 1280℃.