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Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

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What Is The Pressure On A Vacuum Furnace?

Vacuum solid solution treatment requires a high degree of vacuum (1.33×10^-2~1.33×10^-3Pa); in order to prevent the evaporation of alloy elements, vacuum partial pressure treatment should be used, that is, the vacuum in the heating chamber should be evacuated to 1.33×10^-2~1.33×10^-3Pa, and then back-fill with high-purity neutral or inert gas to maintain the inflation pressure at 133~13.3Pa or higher. High-purity neutral or inert gas must always be refilled into the heating chamber. First, the gas can fully play a protective role and increase the pressure to reduce the evaporation of metal elements. Second, it can increase the convective heat transfer effect of the gas, which is more conducive to uniform heating of parts. Vacuum aging can use a higher vacuum degree (1.33×10^-2~1.33×10^-3Pa). Due to the long aging time, the pressure rise rate of the vacuum furnace is preferably less than or equal to 0.67Pa/h. Otherwise, it will be difficult to ensure the surface brightness of the parts.

When partial pressure heating is used for vacuum heat treatment, the return gas should be high-purity neutral or inert gases, such as hydrogen, helium, nitrogen, argon and other common gases. The cooling speeds of the four gases from fast to slow are hydrogen, helium, Nitrogen, argon. If the cooling rate of air is 1, the ratios of the cooling rates of hydrogen, helium, nitrogen, and argon to air are 7, 6, 0.99, and 0.7 respectively.

Hydrogen has the greatest heat conductivity and maximum cooling rate at any pressure, and can be used in vacuum furnaces where graphite is used as a heating and insulation element. However, for steel types with high carbon content, slight decarburization may occur during the high temperature stage of cooling (above 1050°C); when used for high-strength steel, there is a risk of hydrogen embrittlement. The gas supply system using hydrogen as the cooling medium should be airtight and reliable. After the cooling operation is completed, the hydrogen gas should be discharged in time and filled with nitrogen before the furnace door can be opened. Otherwise, there is a risk of explosion.

Helium has the highest price. Since it also has a certain cooling capacity under low pressure, it is generally used at an inflation pressure below 1×10^-4Pa.

Nitrogen is the most commonly used gas and the cheapest. When forced circulation cooling is performed at a pressure slightly lower than atmospheric pressure, the cooling intensity value can increase by about 20 times. In the range of 200 to 1200°C, nitrogen is neutral to general steel and has certain activity to titanium alloys, stainless steel, high-temperature alloys, etc. If vacuum heating and cooling are performed under nitrogen partial pressure, Cr2N will be formed on the surface of the part, resulting in surface The performance deteriorates, that is, the tensile strength increases, the area shrinkage decreases, and the corrosion resistance will also be affected to a certain extent. So it is not suitable to use nitrogen.

Argon is more common and cheaper than helium, but more expensive than nitrogen. In situations where nitrogen is not suitable, such as vacuum heating and cooling of stainless steel, high-temperature alloys and titanium alloys, argon has a better effect.

Therefore, during the vacuum heat treatment process, it is necessary to refill the heating chamber with high-purity neutral or inert gas to increase the vacuum working pressure in the heating chamber, reduce the evaporation of metal elements, and avoid causing the depletion of alloy elements on the surface of the parts and affecting the corrosion resistance of the parts after heat treatment. The performance, partial pressure and partial pressure time must be selected appropriately to reduce evaporation and obtain a bright surface.