Carbide sintering is the most basic and critical process in the production process, and it is also the last process. Sintering methods and equipment have a huge impact on product quality. Traditional sintering methods include hydrogen sintering, vacuum sintering, hot isostatic pressing, vacuum subsequent hot isostatic pressing, sintering hot isostatic pressing, etc., and then microwave sintering appeared.
1. Hydrogen sintering
Put the compact in a graphite boat, then fill it with alumina filler or graphite particle filler with a certain carbon content, put it into a continuous pusher molybdenum wire furnace, and sinter it under the protection of hydrogen. But there are many deficiencies, such as inaccurate furnace temperature control, large changes in the atmosphere in the furnace, and easy carburization and decarburization of products. Moreover, the pores inside the product cannot be fully eliminated, leaving residual pores, and oxide impurities cannot be volatilized and removed well.
2. Vacuum sintering
Vacuum sintering is the process of sintering and pressing in a negative pressure gas medium. Compared with hydrogen sintering, vacuum sintering can improve the purity of furnace gas, and the negative pressure can improve the wettability of the bonded relative hard phase. Its advantages are: (1) Improve the purity of cemented carbide; (2) Ensure the carbon content of the final alloy and control the structure of the alloy; (3) Reduce the sintering temperature or holding time to prevent the uneven growth of carbide grains. The disadvantage is that there are a small number of pores and defects inside the product.
3. Hot isostatic pressing
Put the powder compact and the powder body (that is, the powder package) in the special container into the high-pressure container of the hot isostatic press, and apply high temperature and high pressure to make the powder be pressed and sintered into dense parts or materials. It improves the grain structure of the product, eliminates the defects and pores between the particles inside the material, and improves the density and strength of the material.
4. Vacuum follow-up hot isostatic pressing
After the cemented carbide products are sintered in vacuum (or hydrogen), the holes in the compact can be eliminated, and the densification process is basically completed. In order to further increase the density and flexural strength of cemented carbide, subsequent hot isostatic pressing treatment can be carried out to eliminate micropores. Its advantages are that the produced cemented carbide products have good surface finish, can reduce or eliminate pores, have uniform composition and hardness distribution, and improve bending strength.
5. Sintering hot isostatic pressing
It is a process of simultaneous hot isostatic pressing and sintering of the workpiece under a pressure lower than conventional hot isostatic pressing. It combines the removal of the forming agent, sintering and hot isostatic pressing of the product in the same equipment. Put it into a vacuum sintering isostatic pressing furnace, dewax the low-pressure carrier gas (such as hydrogen, etc.) at a lower temperature, then carry out vacuum sintering at 1350-1450 ° C, and then carry out hot isostatic pressing in the same furnace, using argon as the The pressure medium is kept warm for a certain period of time and then cooled.
6. Microwave sintering
Microwave sintering is one of the effective means to prepare fine-grained materials. It mainly uses the dielectric loss of the material in the microwave electromagnetic field to heat the whole to the sintering temperature to achieve rapid sintering of densification. It mainly relies on the material itself to absorb microwave energy and convert it into the kinetic energy and potential energy of the internal molecules of the material. The material is heated inside and outside at the same time, and the internal thermal stress of the material can be minimized. Under the action of microwave electromagnetic energy, the sintering activation is reduced and the diffusion coefficient is increased. Carry out low-temperature rapid sintering, so that the fine powder is sintered before it grows up.
