Application Of Partial Pressure In Vacuum Heat Treatment

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Heat treatment under vacuum conditions is called vacuum heat treatment. Vacuum heat treatment is an ideal bright heat treatment technology without oxidation and decarburization. However, in the process of vacuum heat treatment, the evaporation of metal elements, the depletion of alloy elements on the surface of the parts, the decline of corrosion resistance and the improvement of the surface brightness of the parts are inevitable.

1.Evaporation of metal elements under vacuum heat treatment

The metal elements in pure metals and alloys will evaporate at a certain temperature and vacuum degree. It has a non-negligible harmful effect on the quality of the vacuum heat treatment of the parts and the pollution of the vacuum heating chamber. According to the phase equilibrium theory, at different temperatures, the equilibrium pressure (vapor pressure) of metal evaporation on the metal surface is different. As the temperature increases, the vapor pressure also increases. When the external pressure is less than the vapor pressure of the metal element at this temperature, the metal element will evaporate (sublime). The smaller the external pressure, that is, the higher the vacuum degree, the easier it is for metal elements to evaporate. Elements with higher vapor pressures evaporate more easily than elements with lower vapor pressures under the same conditions. In alloys (including parts materials, structural materials of vacuum heating chambers and fixture materials) or the surface of iron wires used to bind parts, if they contain 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 adhere to the solid metal. Contaminate the metal surface, causing mutual bonding between parts or between parts and the material basket during cooling, and in severe cases, a short circuit between the heating element and the furnace body. When the evaporation is serious, the alloy elements on the surface of the parts are depleted and the performance is affected, and the surface is rough and the surface brightness of the parts is affected. Therefore, when heating in vacuum, the choice of vacuum degree is a very important issue, which must be given enough attention.

2.Low pressure gas protection

It 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 (also known as low pressure gas shielding method) can not only prevent the evaporation of alloying elements, but also obtain a bright surface. Another benefit of this method is the increased convective heat transfer, which is more conducive to uniform heating of the part. 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, not only will it not reduce the evaporation rate of alloy elements, but it will increase it, resulting in the depletion of alloy elements on the surface of the parts and the decline in corrosion resistance. Stainless steel contains a large amount of alloying elements such as Cr, Ni, AL, Mn, and Ti, and its vapor pressure is relatively high. Vacuum solution treatment is required to be carried out under a relatively high degree of vacuum (1.33 × 10 -2 ～ 1.33 × 10 -3 Pa ); in order to prevent the evaporation of alloy elements, vacuum partial pressure treatment should be used, that is, the vacuum of the heating chamber should be evacuated first. to 1.33 × 10 -2 ～ 1.33 × 10 -3 Pa , and then refill with high-purity neutral or inert gas to keep the inflation pressure at 133 ～ 13.3Pa or higher. High-purity neutral or inert gas should always be recharged into the heating chamber. One is to make the gas fully protect and increase the pressure to reduce the evaporation of metal elements; the other is to increase the convective heat transfer effect of the gas, which is more conducive to the uniform heating of the parts. A higher vacuum degree (1.33 × 10 -2 to 1.33 × 10 -3 Pa ) can be used for vacuum aging. 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 is difficult to ensure the surface brightness of the parts.

3.Selection of back gas

The choice of back gas by vacuum protective atmosphere heat treatment method. When partial pressure heating is used in vacuum heat treatment, high-purity neutral or inert gas should be selected as the backfill gas. Hydrogen, helium, nitrogen, argon and other gases are common gases. The cooling speed of the four gases is hydrogen, helium, and argon. Nitrogen, Argon. If the cooling rate of air is 1, the ratios of the cooling rates of hydrogen, helium, nitrogen, argon and air are 7, 6, 0.99, and 0.7, respectively. Hydrogen has the greatest thermal conductivity and the greatest cooling rate at any pressure, and can be used in vacuum furnaces where graphite is used as a heating and insulating element. However, for steels with high carbon content, slight decarburization may be caused in the high temperature stage of cooling (above 1050°C); for high-strength steels, 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 should be discharged in time and filled with nitrogen before the furnace door can be opened, otherwise, there is a danger of explosion. Helium has the highest price. Since it also has a certain cooling capacity under low pressure, it is generally used under the inflation pressure below 1 × 10 4 Pa. Nitrogen is the most commonly used gas and the cheapest. For forced circulation cooling at a pressure slightly below atmospheric pressure, the cooling intensity value can be increased by about 20 times. In the range of 200 ~ 1200 ℃, nitrogen is neutral to general steel, and has a certain activity to titanium alloy, stainless steel, superalloy, etc. If vacuum heating and cooling are carried out under nitrogen partial pressure, Cr 2 N will be formed on the surface of the parts. It leads to the deterioration of the surface properties, that is, the tensile strength increases, the reduction of the area decreases, and the corrosion resistance will also be affected to a certain extent. Therefore, nitrogen should not be used. Argon is more common and cheaper than helium, but more expensive than nitrogen. Argon is required for vacuum heating and cooling of stainless steel, superalloys and titanium alloys where nitrogen is not suitable.

4.Selection of vacuum degree

The surface brightness of parts after vacuum heat treatment is closely related to factors such as vacuum degree and heating temperature during heating. At a certain temperature, the surface brightness of the parts after vacuum heat treatment increases with the increase of vacuum degree.

Editor: Frank Lee

Copyright: SIMUWU Vacuum Furnace