Mechanism and Characteristics of the Hot Pressing Sintering

Sintering is the process of heating a powder to a certain temperature and holding it for a certain amount of time, then cooling the material to obtain a product with the desired properties. The entire process of special heat treatment is called sintering.

Sintering turns porous powder briquettes into products with certain textures and properties. From the process characteristics of the sintering process, the sintering of cemented carbide can be done by hydrogen protection, vacuum sintering, hot press sintering, hot isostatic pressing, etc. This article will describe the characteristics of the hot-press sintering process.

Densification mechanism

The process of putting the powder of the final additive into the graphite press mold and sintering and pressing at the same time is called hot pressing.

Although hot pressing is carried out simultaneously with both pressing and sintering processes, there is no difference in principle with ordinary cold pressing molding and sintering. Effectively, hot pressing greatly reduces the time for phase change and formation of alloy. Therefore, it can be said that the hot pressing process is an activated sintering process.

The sintering process during hot pressing is a highly unbalanced process. When the mixture powder is heated continuously under pressure, the oxide film on the powder surface is broken under pressure and can be reduced by carbon. The shrinkage of hot pressed products is different from that of the normal sintering method. In addition, powders with crystal defects can achieve greater alloy density because the presence of defects activates the hot pressing process.

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Characteristics of the hot pressing process

The hot pressing process gives unique advantages to its products. Therefore, this process plays a major role in the production of cemented carbide. The main characteristics of this process are.

a. High density and good performance of hot pressed products.

The porosity of the hot-pressed products is extremely low and the density is almost the theoretical value. At the same time, due to the short hot pressing time, the carbide grains will not grow due to the recrystallization process, so the product carbon is finer than tungsten grains, which makes the hot pressed products have higher hardness and better wear resistance, especially suitable for manufacturing precision tools such as precision rolls and high precision wear parts.

b. Small pressing pressure, can produce large products.

The unit pressing pressure required for hot pressing is only about one-tenth of that for cold pressing. In addition, the size of the product can be enlarged by welding, so the size of the hot pressed product is limited by the capacity of the press less than that of the cold press.

At present, the weight of cemented carbide products produced by hot pressing can reach tens or even hundreds of kilograms.

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c. Products with complex shapes can be produced.

Because of hot pressing, the powder material has thermoplasticity and good fluidity, so it can be made into complex shaped shaped products, such as thin-walled tubes and ball and pillar products. However, for long parts, there is still uneven density, and long parts of welding are often unevenly welded.

d. Large size non-deformed products can be produced.

Hot pressing method overcomes the shortcomings of cold pressing and sintering deformation, and can obtain long pieces without bending and hollow products with relatively accurate dimensions. However, due to hot pressing, cobalt is extruded from the product surface, which not only causes cobalt loss, but also makes the surface of hot pressed products rough and difficult to clean and process.

e High operation technology requirements.

In the hot pressing process, the combination of temperature and pressure, the speed of heating up and cooling down, and the control of shrinkage play a decisive role in the performance and defects of the product. Therefore, a high level of operator skill is required.

It is worth noting that the development of equilibrium and isostatic hot pressing has overcome some of the drawbacks of the hot pressing process.

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