Vacuum heat treatment is an ideal bright heat treatment technology without oxidation and decarburization. However, during the vacuum heat treatment process, problems such as the evaporation of metal elements, the depletion of alloy elements on the surface of parts, the decrease in corrosion resistance, and the improvement of surface brightness of parts are inevitable.
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 back-charging 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.
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 1.33~13.3Pa 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 degree of vacuum (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 conduction capacity 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.
