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Vacuum Heat Treatment Furnace

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Tips for setting partial pressure values in vacuum heat treatment

Metal elements in pure metals and alloys will evaporate under certain temperatures and vacuum levels. It has a harmful impact that cannot be ignored on the quality of vacuum heat treatment of parts and the pollution of the vacuum heating chamber. According to the phase equilibrium theory, the equilibrium pressure (vapor pressure) exerted by metal evaporation on the metal surface is different at different temperatures. As the temperature increases, the vapor pressure also increases. When the external pressure is less than the vapor pressure of the metal element at that temperature, the metal element will evaporate (sublimate). The smaller the external pressure, that is, the higher the degree of vacuum, the easier it is for metal elements to evaporate. Elements with high vapor pressure evaporate more easily than elements with low vapor pressure under the same conditions.

If the surface of the alloy (including part materials, structural materials of the vacuum heating chamber and tool fixture materials) or the wire used to bind the parts contains metal elements with high vapor pressure (such as Ag, Al, Mn, Cr, Si, Pb, Zn , Mg, Cu, etc.), when heated in a vacuum, when the vacuum degree is higher than the vapor pressure of the metal element, the metal element will evaporate, and the evaporated metal element will surround the solid metal in the form of gas, adhere and It pollutes the metal surface, causing mutual adhesion between parts or between parts and the basket during cooling. In severe cases, it may cause a short circuit between the heating element and the furnace body. When the evaporation is serious, it will cause the depletion of alloy elements on the surface of the parts, affecting the performance, and causing surface roughness, affecting the surface brightness of the parts.

Therefore, in heat treatment in vacuum equipment, reasonable control of vacuum degree is a very important issue, and strict process and program setting control must be formulated.
Low-pressure gas protection can reduce the evaporation of metal elements. In order to reduce or avoid the evaporation of alloy elements, the pressure can be increased by backfilling high-purity neutral or inert gas (such as nitrogen, argon, helium, etc.) into the vacuum heating chamber. Adjust the pressure in the vacuum chamber within the range of 0.1 to 650Pa. This vacuum protective atmosphere heat treatment method is also called low-pressure gas protection method, which can prevent the evaporation of alloy elements and obtain a bright surface. Another benefit of this method is that it increases convective heat transfer, which is more conducive to even heating of the parts. The purity of the gas used in the vacuum protective atmosphere heat treatment method should generally be greater than 99.99%. If the purity of the recharged neutral or inert gas is not enough, it will not only not reduce the evaporation rate of alloy elements, but will increase it, resulting in the depletion of alloy elements on the surface of the parts and a decrease in corrosion resistance. Stainless steel contains a large amount of Cr, Ni, AL, Mn, Ti and other alloying elements, and its vapor pressure is relatively high.