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Vacuum Heat Treatment Furnace

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Aerospace Vacuum Heat Treating Furnaces

Vacuum furnaces are widely used in a variety of heat treatment processes required for the manufacture of aerospace parts, such as brazing, aging, and solution heat treatment of various materials. Typically, vacuum furnaces are used to ensure that materials are not contaminated by oxidation or nitridation during the heat treatment process. This article will discuss the origins, theory, and main equipment structures of vacuum technology and how it is used in aerospace.

Aerospace Vacuum Heat Treating Furnaces Theory
The core of vacuum technology is a vacuum pump system that is able to evacuate a pressure vessel (vacuum chamber) to different vacuum levels. Vacuum is the opposite of pressure value: high vacuum means low pressure.
Vacuum changes the vapor pressure of the material. The so-called vapor pressure of a material is the pressure exerted at a given temperature when the material is in equilibrium with its own vapor. Vapor pressure is material- and temperature-dependent, so this can cause potential process challenges when processing certain materials in a heat treatment furnace. For example, a furnace temperature uniformity test is performed at 4 temperature points: 1000°F, 1500°F, 1800F, and 2250F. This type of TUS usually takes 6-8 hours, and as the furnace heats up at the test temperature, the vacuum reading is likely to increase to a higher vacuum level. If a consumable K-type thermocouple is used, there is a high probability that the test thermocouple will fail at high temperatures due to vapor pressure.

Vacuum Furnace Pumping System
Vacuum heat treatment aims to eliminate contact between the heat treated product and oxidizing elements. This is achieved by connecting a vacuum pump to the pressure vessel and pumping out the atmosphere. The vacuum pump system operates in different stages and must be operated in sequence to achieve the required vacuum. Each pump within the vacuum pump system is capable of pumping to a different vacuum and requires the pumps to work in conjunction with each other. At startup, the mechanical pump is the initial stage of pumping. The pump can pump from 105 to 10 torr. At pressures below 20 torr, the efficiency of the mechanical pump begins to decline. This is when the booster pump is activated. The booster pump has two two-blade impellers mounted on parallel shafts that rotate in opposite directions. The diffusion pump is activated at vacuum levels of 10 to 1 micron. The diffusion pump allows the system to be pumped down to high vacuum or lower vacuum. The diffusion pump has no moving parts. The pump works on the principle of vaporization of oil, condensation of the oil as it falls, and capture and extraction of gas molecules through the vacuum system. The maintenance pump generates more pressure in the foreline to ensure that when the crossover valve between the mechanical pump and the diffusion pump is activated, the oil in the diffusion pump does not leak into the pressure vessel.

Vacuum Furnace Heating Zone Design
The heating zone in a vacuum furnace is simply an insulated chamber away from the inner cold wall, where heating is required. The vacuum itself is a good insulator, so the space between the cold wall and the heating zone ensures that the heat flow from the furnace to the outside can be reduced. There are two types of vacuum furnace heating zones: insulated and radiant.

Vacuum Furnace Quenching System
Quenching is defined as the rapid cooling of metal to achieve the desired properties. Different alloys may require different quenching rates to achieve the desired properties. When quenching is required, vacuum furnaces use inert gas for quenching. As the gas passes through the loaded parts, it absorbs heat, then leaves the chamber and cools the gas through the quenching pipe. The cooled gas is then pumped back into the chamber to repeat the process.