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Vacuum Furnaces: Everything You Need To Know

Vacuum furnaces have short cycle times. Heating 1,000 pounds of charge in a heating chamber that is already at 1,000°C takes 60 minutes, whereas heating the same chamber with 1,000 pounds of charge from cold would take 90 minutes (1.5 times longer). Advantages in cycle times and energy usage result from the fact that only the charge is heated.

The charge operation of the vacuum furnace has both loading and unloading methods and direct passing methods. There can be two, three or more furnace chambers. Under normal circumstances, the basic operation of a vacuum furnace is as follows.

The vacuum pump system is started and the charge is ready. The operator uses the loading machine to place the charge on the lifting device in the front chamber/quenching chamber. When the outer furnace door is closed, automatic operation begins. The front chamber is evacuated, and when the two furnace chambers reach the same pressure, the inner furnace door automatically lifts. The charge is moved into the heating chamber under vacuum: a telescopic transfer mechanism moves the charge from the antechamber/quenching chamber into the heating chamber and then places it on the furnace base plate.

The charge remains in the high temperature zone until the end of the treatment. You can choose to use oil quenching, gas quenching or oil/gas quenching in the same front chamber.

If oil quenching is selected, the transfer mechanism moves the charge out of the heating chamber, and the lifting device lowers the charge into the quenching tank and immerses it in the quenching oil. If gas quenching is used, the operation method is exactly the same, except that the charge is always in a lifted position on the quenching lifting device. The front chamber/quenching chamber is backfilled to the desired pressure and a cooling fan located in the upper part of the furnace chamber circulates gas through the charge and cooling coils.

The inner furnace door that separates the furnace chambers is only opened when the charge is transferred between the two furnace chambers, and is closed at all other times to ensure that the furnace chambers are tightly sealed and to prevent the heating chamber from being contaminated. Vacuum slow cooling can also be carried out. At this time, the charge is left in the high temperature zone, but the heating element is turned off.

The vacuum holding capacity and service life of a vacuum furnace depend on the quality and sealing of the furnace shell and inner furnace. Planned repairs include comprehensive leak detection and leak rate measurements. This is true regardless of whether the water-cooled furnace shell or the insulated lined furnace shell (hot wall).

The advantage of the vacuum furnace is that the heating chamber is not repeatedly evacuated and recharged, so the furnace shell will not withstand such dynamic conditions, which is beneficial to maintaining the quality of the furnace shell and extending the life of the vacuum furnace. Since the furnace chamber is always in a vacuum state, it will not be exposed to oxidizing moisture, so that the surface of the workpiece can remain bright or eliminate the temper color.

Multi-chamber vacuum furnaces have many moving parts, as well as seals that separate the vacuum chamber from the charging/discharging chamber, as well as seals for the furnace door and transfer mechanism. However, they are commonly used components in the heat treatment industry and do not cause any operational problems.

For comparison, consider a batch-type integral quenching furnace with a front chamber and an inner furnace door, as well as their atmosphere and nitrogen return lines. The supporting structure and maintenance knowledge of traditional atmosphere heat treatment equipment are applicable to multi-chamber vacuum furnaces.