What is a vacuum furnace?

Vacuum Furnace Advantage

Vacuum furnaces are used in many applications and industries. Through vacuum heat treatment technology, material performance can be improved, and the service life and safety performance of materials can be increased.

Vacuum furnaces have the advantages of high temperature, low pressure, and uniform heating.

Materials in the vacuum furnace are not affected by oxidation, hydrogenation, nitridation, and other factors, so the quality of the materials after heat treatment is more stable and pure.

In addition, the vacuum furnace can also control the gas flow during heating to achieve material surface cleaning and modification (like nitriding, etc …).

Finally, due to the introduction of high pressure gas quenching technology, vacuum furnaces become preferred choice for heat treatment industry, as almost any heat treatment result can be achieved with a vacuum furnace in a more environmentally friendly way than previous methods like heat treatment inside atmosphere furnaces.

Vacuum Furnace Functions

The main function of the vacuum furnace is to heat-treat materials, including annealing, aging, quenching, solution treatment, and other processing methods. In addition, vacuum furnaces can also be used for high-temperature sintering, high-temperature evaporation, high-temperature melting, high-temperature carbonization, and other processes.

In general the the vacuum heat treatment profile will have the following steps:

• Loading of the charge (work-pieces) inside the vacuum furnace.
• Pumping down to create necessary degree of vacuum.
• Energizing the heating system when necessary vacuum level is reached to protect heated steel or metal parts from negative influence of an air atmosphere.
• Ramping up to desired temperature and then holding for a certain time period to make sure the temperature uniformity inside the work-piece is reached.
• Finally, the charge (work-pieces) will be cooled and depending on the cooling rate requirements it can be done slowly or very fast. Quenching can be done in a single chamber vacuum furnace or a multi chamber vacuum furnace where the heating and quenching are done in a separate chamber to achieve more effective quench. It is important to keep the work-piece under vacuum until it is sufficiently cooled to ensure the hot work-piece does not have an opportunity to react with air, water vapor, etc …
• Unloading the charge (work-pieces) from the furnace

Optionally, during the heat treatment cycle specific gases can be introduced into the vacuum environment, such as inert gases (nitrogen, argon, helium) or reactive gases (hydrogen, oxygen). The introduction of these gases can further adjust the heat treatment process to achieve specific material modifications or reactions.

Vacuum Furnace – Key Elements

Vacuum furnaces are composed of furnace body, vacuum system, heating elements, temperature control system, process gas mixing panels, thermal instrumentation and cooling components.

• The furnace body is usually made of stainless steel or other alloy materials, with strong corrosion resistance and high-temperature resistance. It is usually double skinned and water cooled pressure chamber that may enclose only the hot zone or maybe also additional components like heat exchanger and fan circulation motors in case of gas quench being done in the same chamber as the heat treatment process. A variation to that is a vacuum retort furnace, that normally works at lower temperature and has heat exchanger and circulation motor placed outside of the furnace body.
• The vacuum in the furnace is maintained by vacuum pumps. Depending on the application, a single or combination of mechanical booster, diffusion and turbo-molecular are used. If multiple chamber furnaces are used then each chamber would have its own vacuum pumping group.
• A heating system is realized through electrical resistance elements made out of graphite or metal and placed inside the hot zone as close to the charge as possible for high temperature furnaces. A variation on that would be a retort vacuum furnace, where the electrical resistance heaters are placed outside the retort, while the charge is placed inside the retort and kept under vacuum. This will be the case of indirect heating. Retort is heated by the electrical heaters and then it radiates the heat towards the charge. But the limitation here is that the heat treatment temperature will usually not exceed 850 °C, but a retort vacuum furnace will be a lot cheaper then the one made with a hot zone for high temperature application.
• The temperature control system mainly consists of thermometers, controllers, power supplies, etc., which can achieve precise control of the heating process.
• To achieve various thermo-chemical heat treatment effects on the work-pieces various process gases are mixed / created outside the furnace and released in a very controlled way into the hot zone to achieve various thermo-chemical effects like case hardening of the surface of the work pieces to make them more wear resistant, etc …
• Cooling / Quenching of the charge can be realized in a variety of ways, either by “vacuum” cooling, that means to leave the furnace to slowly cool itself or expose the charge to gas, oil or salt as a quench media to greatly shorten the quench time, control distortion and achieve required material properties. The cooling can be as simple as blowing ambient air on the hot retort from outside, to back-filling the hot zone with inert gas (like nitrogen or argon) and circulating this gas through a heat exchanger and the hot zone. To increase the quench effect gas can be circulated at higher velocity and pressure in a single vacuum chamber furnace or if more aggressive quenching is required, then the charge needs to be quenched in ever higher gas pressure or oil … but for that a transfer into a separate quench chamber would be needed.

Vacuum Furnace – Classifications

For the purposes of heat treatment the vacuum furnaces can be classified in a variety of ways:

• Temperature Range and Uniformity. Vacuum furnace can heat up to 3000 °C with select material and processes, though typical vacuum heat treatment process would require a temperature up to 1400 °C. The best hot zone would provide temperature uniformity of +/- 3 °C.
• Pressure Range and Vacuum Level. Depending on the furnace overall design, manufacturing and selection of key components like seals, valves, threaded connections, solder joints and of course selection of vacuum pumps the furnace can be classified as fine vacuum or high vacuum (requiring a diffusion pump to achieve it). For pressure range on the positive scale it would depend on the maximum desired quench pressure. That could be from 1.5 Bar to about 20 Bar in a single chamber vacuum furnace. With separate quench chamber much higher pressures can be achieved.
• Single or Multi Chambers. A vacuum furnace is referred to be as a single chamber when the heating and cooling is done in the same chamber. However, when more higher cooling rates are needed then multi chamber furnaces are used to separate the heating and quench chambers. Also if overall higher production output is needed from heat treatment equipment, then it is best to allocate heat treatment and quenching into different modular chambers and even better to create an automated production line where various heating chambers can be linked to gas, oil and salt quench chambers and other auxiliary equipment like washing machines and tempering furnaces through the use of vacuum heated automatic transfer trolley / chamber. That will create maximum flexibility and allow easy ramp up and down of the production with fluctuating requirements as only the modules that are needed are being used.
• Horizontal or Vertical Chamber Orientation. Depending on the work-piece geometry a vertical chamber orientation would be needed to load the furnace either from top or bottom. Horizontal orientation is much more popular because of convenience of loading and more affordable price. But in case of massive parts like air plane landing gears the vertical orientation would be the way to go to prevent distortion when quenching.
• Round versus Square Hot Zone. Depending on quantity and geometry of the work pieces in a charge a square or round hot zone shape may be more conducive to achieve uniform heat treatment and then a distortion free quench. Though, round hot zone is certainly the more flexible option as the positions of heaters and gas quenching nozzles are exactly same distance from the center of the hot zone. So that arrangement provides the best chance for great temperature uniformity and effective quench. But then again, it depends what specific work-pieces are heat treated and how they are arranged inside the charge / on the heat treatment fixture, so maybe a square hot zone is more appropriate in a particular case.
• Graphite or Metal Hot Zone. The choice of material greatly impacts the acquisition and operational cost of the vacuum furnace. Graphite hot zone is nowadays the most popular choice, but depending on the heat treatment process and material of work-pieces to be heat treated a metal hot zone maybe the right choice.