Vacuum Sintering Furnace Equipment and Process

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Introduction of Vacuum Sintering Technology and Vacuum Sintering Furnace Equipment and Process

Vacuum sintering is a method of sintering ceramic blanks under vacuum conditions. The water vapor, hydrogen, oxygen and other gases contained in the pores of the oxide ceramic blanks are dissolved and diffused along the grain boundaries of the blanks or pass through the sintering process. The particles can escape from the pores. However, carbon monoxide and carbon dioxide, especially nitrogen, are not easy to escape from the pores due to their low solubility, resulting in pores in the product and a decrease in density. If the green body is sintered under vacuum conditions, all gas will escape from the pores before the green body is completely sintered, so that the product does not contain pores, thereby increasing the density of the product.

Under high temperature and a certain degree of vacuum, the ceramic body with a certain shape becomes a dense, hard, stable volume and sintered body with certain properties through physical and chemical processes. The physical and chemical processes include viscous flow, diffusion, evaporation, condensation, new phase formation, dissolution and precipitation, and solid solution generation. During the process, the total surface energy of the powder decreases, macroscopically shows the shrinkage of the green body, and increases the strength; microscopically shows the decrease in the number of pores, changes in the shape and size of the pores, changes in the size and morphology of the crystal grains, and the growth of crystal grains. The boundary is reduced and the structure is densified. The porcelain blank contains a certain amount of pores, in which water vapor, hydrogen, and oxygen can escape from the closed pores through the process of dissolution and diffusion. Carbon monoxide, carbon dioxide, and nitrogen are not easy to escape from the closed pores due to their low solubility. The sample is placed in a vacuum condition and a certain driving force is applied to make the gas escape from the closed pores, which can increase the density of the porcelain piece.

Sintering under the condition of negative pressure in the furnace is called vacuum sintering. There are batch vacuum sintering and continuous vacuum sintering. Continuous vacuum sintering is rarely used because the furnace is more complicated, the vacuum degree is not guaranteed, there is no way to pass gas, and the sintering process curve is not easy to adjust. Intermittent vacuum sintering has a special vacuum sintering furnace equipped with a matching dewaxing furnace; there is also a dewaxing-sintering integrated furnace. At present, the mainstream vacuum sintering is an integrated furnace in which dewaxing (forming agent) and vacuum sintering are completed at one time in the same furnace.

Advantages of vacuum sintering：

1. It is easy to control the carbon content of the alloy under vacuum sintering conditions. At the sintering temperature, the pressure in the furnace is only tens of Pascals (Pa) or even lower, O2, N2, H2 and H2O molecules are very few, many reactions can be ignored, and the influence of the medium is small. As long as the dewaxing process is strictly controlled, the carbon content of the alloy changes very little during the vacuum sintering process, and the performance and structure are quite stable.

2. Under vacuum sintering conditions, the purity of cemented carbide can be improved. Vacuum sintering is conducive to the reduction of metal oxides; the entire sintering cycle does not need to open the furnace door, no air enters, and almost no reaction of N2 and O2 takes place.

3. Under vacuum sintering conditions, there are less impurities adsorbed on the surface of the hard phase, which improves the wettability of the drill to the hard phase and improves the strength of the alloy, especially the alloy containing TiC.

4. Under vacuum sintering conditions, the process is easy to operate. Since the filler can be omitted during vacuum sintering, this not only simplifies the operation, but also avoids the adverse effect of the filler on the surface of the sintered body.

5. The integration of dewaxing and sintering can reduce product oxidation and reduce the difficulty of carbon control; reduce equipment area and reduce labor intensity.

6. Multi-atmosphere dewaxing-sintering integration, temperature, atmosphere and furnace pressure can be controlled separately in temperature sections, and isothermal sintering (heat preservation) at any temperature can be realized, and multiple functions, such as gradient alloy sintering.

Vacuum sintering equipments:

In the cemented carbide manufacturing process, molding agents, such as paraffin, rubber, PEG, etc., must be mixed into the mixture powder, especially for extrusion molding and injection molding, where the content of molding agents is relatively high. Degumming furnace (also called degreasing furnace or dewaxing furnace) is an electric furnace dedicated to removing forming agents. Independent degumming furnaces have been gradually reduced, but some products require semi-processing before final sintering. Large products or extrusions with high molding content The degreasing time of pressed or injection molded products is long, or in order to improve the utilization rate of the pressure sintering furnace, a special dewaxing pre-burning furnace is still useful. The dewaxing furnace for producing cemented carbide is batch type. According to the structure, it is divided into vertical and horizontal furnace types; according to the removal method, it is divided into hydrogen H2 degreasing, low-pressure carrier gas degreasing or vacuum degreasing. The hydrogen degumming pre-burning furnace is widely used because of its high dewaxing efficiency and suitability for various forming agents.

The horizontal furnace type adopts a structure with an inner circle and an outer square. It is made of heat-resistant stainless steel as a circular inner tank. The square outer furnace shell is lined with alumina fiber insulation layer. Nickel-chromium resistance wires are hung on the outside and bottom of the inner tank, usually in a horizontal direction. It is divided into three zones. For large furnaces or when extremely high temperature uniformity is required, 9 temperature zones are also used (three temperature zones on both sides and the bottom horizontal direction). The front end of the furnace tank has a single door, the thermocouple is inserted into the furnace tank from the rear end, and the wax discharge pipe is led out from the middle and lower part of the rear end. When H2 is used for degumming, the H2 glue steam passes through the wax discharge pipe to the wax collecting tank, most of the glue is left in the tank, and the rest is directly ignited and burned with H2. When vacuum or low-pressure carrier gas is used for dewaxing, it is necessary to use a high-efficiency wax collecting tank to collect more than 97% of the paraffin to prevent the forming agent from damaging the vacuum pump. There are also water-ring vacuum pumps (various forming agents can be pumped out, but the vacuum degree is less than 4000Pa ). In order to speed up the cooling rate, an external air cooler is equipped to blow the indoor air into the furnace, and the hot air is exhausted from the top exhaust pipe to the outside.
Whether the temperature uniformity and atmosphere uniformity are good is the main index to measure the function of the dewaxing furnace.

Vacuum sintering technology
1. Degreasing stage
The first stage is the delubrication or forming agent stage, which can also be referred to as the burn-in stage. At this stage, the temperature should be raised slowly. The decomposition temperature of both the lubricant and the forming agent is usually around 300°C. Therefore, the temperature should be as slow as possible at about 300°C and have a long enough time to remove the lubricant. The first stage should be kept at a certain temperature for a period of time, the purpose of which is to fully remove the lubricant, and to carry out its own oxidation-reduction reaction. If the sintered part contains carbon, carbon-oxygen reaction will occur above 700°C. The time required for the first stage depends on the amount of lubricant added to the part and the size of the part. Through the first stage of pre-firing, the lubricant or forming agent decomposition gas and oxygen should be fully eliminated. Whether these gases are fully eliminated can be observed by the degree of vacuum. If the degree of vacuum is stable at a certain value, it means that it has been eliminated.

2. Sintering stage
The temperature set in the sintering stage is the temperature required for sintering. Because vacuum sintering has the function of activation sintering, its sintering temperature is 50-100 ℃ lower than that of atmosphere sintering. If liquid phase sintering is performed, the sintering temperature should be specified at a temperature slightly higher than the melting point of the liquid phase metal. Sintering between powder particles and alloying between alloying elements will occur at this stage. At the same time, excessive vacuum should not be used at this stage, because the higher the vacuum, the greater the loss of liquid metal. In order to reduce the volatilization loss of metals, certain gases such as nitrogen, argon and hydrogen are often filled in sintering.

3. Cooling stage
The cooling of vacuum sintering includes direct power-off cooling or stepwise current reduction cooling, which depends on the cooling requirements. Since it is cooled with the furnace, the cooling rate is slower than that of atmosphere sintering. Filling with protective gas can increase the cooling rate.

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