Sintering is a special heat treatment process, whose purpose is to transform porous powder into alloy with certain structure and properties. Sintering is the last procedure in the production process of cemented carbide. The structure and properties of the alloy depend on many process factors before sintering, but under some conditions, the sintering process has a significant, even decisive impact on it.

The sintering of cemented carbide belongs to the liquid phase sintering of multiple systems, and its process is relatively complex. There are both physical and chemical changes, but the main physical changes include the densification of the sintered body, the growth of carbide grain, the change of the composition and structure of the bonding phase, and the formation of the alloy structure.

For sintering to be effective, the powder particles must be in close contact. Therefore, sintering is usually applied to pressed or molded powder rather than loose powder. Although most powder parts need to be sintered in the manufacturing process, in this case, sintering is mainly used to achieve metallurgical bonding between particles. This is usually the case for mold parts because their size control is extremely important.

There are usually three processes that rely on the sintering process to achieve high density. The use of very fine powder particles can shorten the volume diffusion path at the later stage of sintering, and has a high initial surface area, which can promote the sintering process at the initial stage. This is one reason why injection molding uses fine powder, because sintering is the main mechanism to obtain high performance in the injection molding process. When the liquid phase appears in the sintering process, whether the liquid phase is instantaneous or permanent, it can promote the rearrangement of particles. The use of high temperature sintering can be accompanied by local melting during the sintering process.

Liquid phase sintering

When the mild powder with low melting point powder and high melting point powder is used, the liquid phase will appear in the sintering process. This liquid phase is conducive to the rearrangement of particles and the densification of pressed embryos. The cermet is usually produced by this method, and the structure with almost no holes can be obtained by this method.

Instantaneous liquid phase sintering

One branch of liquid phase sintering is instantaneous liquid phase sintering. In this case, the powder mixture contains low melting point phase, and the liquid phase only exists for a short time. This process is not the main densification method. The second is to further improve the density. For example, in the production of high alloy steel or tool steel, the density can be increased from 80% to 95% of the density of the pressed embryo.

High temperature sintering

If the temperature of the press embryo is raised to the solid line very close to the equilibrium phase diagram, the liquid phase pool will be formed due to the smiling segregation of alloy elements. These small amounts of liquid phase can promote the densification of the sintering process. Although the high temperature sintering conditions are a little extreme, this method is applicable to many alloy systems, especially tool steel and other high alloy steel.

Reaction sintering

Reactive sintering makes full use of the flexibility of powder metallurgy, in which the powder mixed powder compactor will have a self-sustaining reaction during the sintering process. If this reaction can be properly controlled, the compactor can be rapidly densified and new alloys can be produced. If it is not properly controlled, the alloy with the required composition will not be obtained.