Vacuum sintering process
Vacuum sintering is mostly batch sintering at present. Sintered parts are heated, sintered and cooled with the furnace. The position of products does not change during sintering process, so all stages of sintering can only be carried out by adjusting the heating rate, temperature and time. Therefore, it must be set according to sintering requirements.
1. Degreasing stage
The first stage is the deburring or forming agent stage, which can also be called the pre-burning stage. At this stage, the decomposition temperature of lubricant or forming agent time should be slowly raised. The decomposition temperature of lubricant or forming agent time should be around 300 C. Therefore, the temperature should be raised as slowly as possible at 300 C, and there should be enough time to make the lubricant clean. In the first stage, the temperature should be kept at a certain temperature for a period of time. Its purpose is to remove lubricants sufficiently and to carry out its own oxidation-reduction reaction. If carbon is contained in sintered parts, 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. The decomposition of gases and oxygen from lubricants or forming agents should be fully eliminated by pre-firing in the first stage. Whether these gases are sufficiently excluded can be observed by the degree of vacuum. If the degree of vacuum is stable at a certain value, it means that they have been excluded.
2. Sintering stage
The temperature set in the sintering stage is the temperature required for sintering. Because vacuum sintering has the function of activating sintering, the sintering temperature of vacuum sintering is lower than that of atmosphere sintering by 50-100 C. If liquid phase sintering is carried out, the sintering temperature should be specified at a temperature slightly higher than the melting point of liquid metal. In this stage, sintering between powder particles and alloying between alloying elements will occur. At the same time, this stage should not use too high vacuum, because the higher the vacuum, the greater the loss of liquid metals. In order to reduce the volatilization loss of metals, some gases, such as nitrogen, argon and hydrogen, are often filled in the sintering process.
3. Cooling stage
Vacuum sintering cooling has direct power-off cooling or phase-down current cooling, depending on the cooling requirements. Because it is cooled with furnace, the cooling rate is slower than that of atmosphere sintering. Filling with protective gas can improve the cooling rate.