Vacuum evaporation

    When heating metal in a vacuum, at low temperatures, the moisture in the furnace and the nitrogen, oxygen and carbon monoxide in the air, especially the grease and other impurities coated on the workpiece, will evaporate and escape. When the temperature rises above 800°C, the decomposition gas of hydrogen, nitrogen and oxides will be released from the surface of the workpiece to complete the degassing function. The evaporation and dissipation phenomenon formed by this thermal decomposition eliminates harmful gases on the surface of the workpiece, dissipates oxides, and makes the metal bright again. This is another aspect of the advantages of vacuum heat treatment, especially for harmful gases. Elimination and bright heat treatment, etc., are conducive to improving the quality of the workpiece, which is not available in other heat treatment methods.

    In addition, this evaporation and dissipation of metal in vacuum has been well applied as the theoretical basis of vacuum evaporation process. However, during vacuum heat treatment, it is often found that the parts are bonded to each other, or between the parts and the material frame. When processing high chromium cold work die steel or chromium stainless steel, the surface is orange peel, very rough, and the corrosion resistance is significantly reduced. These are the shortcomings of vacuum heat treatment-the evaporation characteristics of metals.

    According to the phase equilibrium theory, at different temperatures, the equilibrium pressure (vapor pressure) of the vapor acting on the metal surface is different. The higher the temperature, the higher the vapor pressure, the greater the amount of solid metal evaporation; the lower the temperature, the lower the vapor pressure, and if the temperature is constant, the vapor pressure will have a certain value. When the external pressure is lower than the vapor pressure at this temperature, the metal will evaporate (sublimation). Just like camphor volatilizes from a solid to a gas due to sublimation in the air. The smaller the external pressure, the vacuum degree. The higher the value, the easier it is to evaporate. Similarly, the higher the vapor pressure of the metal, the easier it is to evaporate.

    Therefore, in vacuum heat treatment, due attention should be paid to the evaporation problem. The view that good results can be achieved as long as the vacuum is increased is not comprehensive. It is necessary to pay full attention to the problem of evaporation according to the type of workpiece. That is, according to the vapor pressure and heating temperature of the alloy elements in the metal material to be treated, the appropriate vacuum degree is selected to prevent the alloy elements from evaporating and escaping.

    For example, the commonly used alloying elements Mn, Ni, Co, and Cr in steel, as well as elements such as Zn, Pb, and Cu, which are the main components of non-ferrous metals, are easily heated in vacuum to produce vacuum evaporation, which makes the workpieces adhere to each other, and Obstacles when taken out of the material frame. In addition, vacuum brazing of stainless steel with Cu and Ag-Mn alloy (as a brazing filler metal), and heating at 0.0133 Pa or less, Mn is evaporated, its composition changes significantly, and the strength of the brazed part is greatly reduced. When vacuum annealing is performed in the case of 70-30 brass, Zn is significantly evaporated and dezincification occurs. Therefore, it is very difficult to obtain a bright surface.

    However, if you choose properly, many defects can be avoided. For example, Cr12MoV cold-punching die steel, at a vacuum of 1.33 Pa and a temperature of 1050°C for 90 minutes, the X-ray microanalyzer was used to measure the distribution of chromium within a range of 150 μm from the surface, and no dechroming was found. This is because, at 1.33 Pa, the theoretical evaporation temperature of chromium is 1205°C. At 1050°C, the corresponding vapor pressure is low, about 0.0133 Pa, that is, the vapor pressure is lower than the external pressure, so no evaporation occurs. This example shows that as long as the vacuum degree is selected appropriately, the alloying elements can be prevented from evaporating.

    It should be pointed out that some elements with higher vapor pressure in the alloy, such as Mn, Cu, Al, etc., are usually dissolved in solid solution or in the form of various compounds, and they volatilize in different ways when heated in a vacuum. But its volatilization tendency is the same, and generally speaking, its vapor pressure is lower than that of pure metal.

    In addition, when heating in a vacuum, you can also consider introducing high-purity inert gas (ie, reverse inflation) to adjust the vacuum in the furnace according to the type of metal material, and use low-vacuum heating to prevent the evaporation of alloy elements. . Especially at 1200°C

    When heating at the above temperature, Cr, Mn, etc. have a higher vapor pressure, which is easy to evaporate, and low vacuum heating is required. The introduction of high-purity inert gas can not only adjust the vacuum degree, but also form a convection cycle due to the presence of the inert gas, which is more conducive to uniform heating of metal materials.

    Therefore, during vacuum heat treatment, the vapor pressure of metal is a problem that cannot be ignored. For general alloy steel, first, the heat treatment heating temperature is less when the heating temperature reaches 1200℃, and second, in general, the cutting allowance of the workpiece after heat treatment is greater than the thickness of the element evaporation layer, so it affects the quality of the workpiece Not big. However, for important and special steel grades, we must pay attention to the issue of vapor pressure.

(Huiputech)