This stage is from the sintering temperature to room temperature. At this stage, the structure of the alloy and the composition of the bonding phase have some changes depending on the cooling conditions. After cooling, the final structure of the alloy is obtained.
If the entire cooling process is in equilibrium, the composition conforms to Figure 10-7. When the sintered body is cooled, tungsten carbide should be precipitated from the liquid phase first, and WC+y binary eutectic will be formed after it is lowered to the eutectic temperature. Therefore, the structure of the alloy should be: the original WC+ WC+ eutectic (WC+y) precipitated from the liquid phase. The actual alloy structure is always WC+y, and has nothing to do with the cooling rate. However, the cooling rate may affect the composition of the alloy γ phase (related to the final carbon content of the sintered body), thereby affecting the properties of the alloy.
If the composition of the sintered body does not conform to Figure 10-7, when the amount of carbon is excessive, graphite will also precipitate from the liquid phase and solidify due to the formation of ternary eutectic WC+y+graphite; when the amount of carbon is insufficient, there will also be The η phase precipitates and is completely solidified when the ternary eutectic WC+γ+η is formed. The structure and properties of the alloy are mainly determined by the results of the first two stages of sintering, and the cooling rate has a small effect. The cooling rate mainly affects the composition of the alloy Y phase, especially the state of the alloy surface. Figure 10-8 is the comparison between cooling with the furnace and rapid cooling. With the furnace cooling, the alloy surface is obviously rich in diamonds, while the rapid cooling surface is much less rich in diamonds.
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