Why does tungsten carbide have porosity after sintering？

Tungsten carbide is a compound composed of tungsten and carbon. Its hardness is similar to that of diamond. Tungsten carbide is a "double high" cemented carbide material with high toughness and high hardness. The key technology is the preparation of raw material powder and sintering of alloy. It is very important to sinter tungsten carbide. If it is not well controlled, it is easy to have pores after sintering. Today we will talk about the related knowledge of tungsten carbide sintering.

Tungsten carbide powder and tungsten powder are mixed in a certain proportion to make mixture, briquetting, sintering, and then mechanical crushing / sieving classification, and then various specifications of sintered tungsten carbide powder are obtained. The production process of powder metallurgy: powder preparation → powder mixing → blank pressing or forming → sintering → post-treatment. The sintering process mainly includes four stages: removing molding agent and pre sintering stage, solid phase sintering stage, liquid phase sintering stage and cooling sintering stage.

Each stage must be carefully controlled to obtain the desired microstructure and chemical composition. As for the most critical problem, the sintering temperature is very important. On the one hand, too fast heating program or too high sintering temperature will lead to uneven grain growth and movement, resulting in pores. On the other hand, if the binder is used, the binder should be discharged before sintering. If this step is not done well, the binder will volatilize during the high temperature process of sintering, resulting in the existence of pores.

There are two basic methods in common sintering process, one is hydrogen sintering, that is, controlling the components of parts by phase reaction kinetics in hydrogen and under normal pressure, the other is vacuum sintering, that is, controlling the components of cemented carbide by slowing down the reaction kinetics in vacuum environment or reducing the ambient gas pressure.