How does a vacuum furnace work?
The working principle of the vacuum furnace mainly includes the following key steps:
1. **Evacuation**:
- After the vacuum furnace is started, it first starts working through the vacuum system. The vacuum system is generally composed of a vacuum pump, a vacuum measuring device, a vacuum valve, etc. The vacuum pump begins to extract air and other gases in the furnace, gradually reducing the pressure in the furnace. As the gas is continuously extracted, the vacuum degree in the furnace continues to increase until the set vacuum degree requirement is reached. This process is to create an environment with almost no air to reduce or avoid the reaction of the workpiece with oxygen and other gases during the heating process.
2. **Heating**:
- **Select the heating method**: When the required vacuum degree is reached in the furnace, the heating element begins to heat up to heat the workpiece. Common heating elements include resistance wire, silicon carbon rod, molybdenum wire and molybdenum rod, silicon molybdenum rod, graphite, induction coil, etc. Different heating elements are suitable for different temperature ranges and process requirements. For example, resistance wire is often used for heating at lower temperatures, while materials such as graphite and molybdenum are suitable for high temperature environments.
- **Achieve heat transfer**: In a vacuum environment, heat is mainly transferred to the workpiece by radiation and conduction. Since there is no air convection, radiation becomes the main heat transfer method. The heat emitted by the heating element is radiated to the surrounding space in the form of electromagnetic waves, and the temperature of the workpiece increases after absorbing this radiation energy. At the same time, heat conduction is also carried out between the heating element and the workpiece through direct contact, but this conduction method accounts for a smaller proportion than radiation.
3. **Insulation and treatment**:
- **Temperature control**: During the heating process, the temperature control system will monitor and control the temperature in the furnace in real time. The temperature control system is mainly composed of thermocouples (temperature measurement) and temperature controllers (temperature control), which can be divided into PID programmable control, touch screen control and PLC full-automatic control. Through the set heating curve and insulation time, ensure that the workpiece is kept at a certain temperature for a certain time to achieve the desired heat treatment effect, such as annealing, tempering, aging treatment, sintering, etc.
- **Material treatment**: Under vacuum and specific temperature conditions, the internal structure of the workpiece will change, thereby obtaining the desired performance. For example, during vacuum sintering, the powder material particles are more tightly bonded, and the density and strength of the material are improved; during vacuum annealing, the internal stress of the workpiece is eliminated and the organizational structure is more uniform.
4. **Cooling**:
- **Natural cooling or forced cooling**: After the workpiece completes the heat treatment, it needs to be cooled. The cooling method can be natural cooling, that is, turning off the heating element and letting the workpiece cool slowly in a vacuum environment; or forced cooling can be used, such as introducing inert gas (such as nitrogen) or starting a water cooling system to speed up the cooling speed. The water cooling system is usually used to cool components such as furnace shells, furnace covers, and electric heating elements to ensure the normal operation and service life of the equipment.
- **Maintaining vacuum (optional)**: During the cooling process, if the quality and performance of the workpiece surface need to be maintained, a certain vacuum may continue to be maintained. However, for some processes that do not require high vacuum, a small amount of gas can also be appropriately introduced during the cooling process to adjust the cooling speed and prevent problems such as workpiece deformation.
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