Evaporation in a vacuum

Evaporation in a vacuum

When metal is heated in a vacuum, water in the furnace and nitrogen, oxygen, and carbon monoxide in the air, especially grease and other impurities coated on the workpiece, evaporate and escape at low temperatures. When the temperature rises to more than 800℃, decomposition gases of hydrogen, nitrogen, and oxide will be released from the surface of the workpiece to complete the degassing. Due to the thermal decomposition and formation of the phenomenon of evaporation dissipation eliminates harmful gas on the surface of the workpiece, and the oxide emissions, make the metal backlight, this is one aspect and the advantage of vacuum heat treatment, especially harmful gases exclude and bright heat treatment, etc, to improve the quality of the workpiece, this is another heat treatment method can not have both.
In addition, as the theoretical basis of the vacuum evaporation process, the evaporation of metal in a vacuum has been well applied. However, in the process of vacuum heat treatment, it is often found that parts are glued to each other, or between parts and frames. In the treatment of high chromium cold work die steel or chromium stainless steel, the surface is orange peel, very rough, and corrosion resistance is significantly reduced. These are the disadvantages of vacuum heat treatment-the evaporative properties of metals.
According to the phase equilibrium theory, the equilibrium pressure (vapor pressure) of the vapor acting on the metal surface is different at different temperatures. High temperature, high vapor pressure, and solid metal evaporation capacity is large; If the temperature is low, the vapor pressure is low, and if the temperature is certain, the vapor pressure is going to be certain. When the external pressure is less than the vapor pressure at this temperature, the metal evaporates (sublimation > phenomenon. Camphor in the air by sublimation from a solid into gas to evaporate. The lower the external pressure, the higher the vacuum, and the easier it is to evaporate. Similarly, metals with higher vapor pressure tend to evaporate more easily.

Therefore, due attention should be paid to evaporation in vacuum heat treatment. It is not comprehensive to think that good results can be achieved only by increasing the vacuum degree. Due attention must be paid to evaporation, depending on the type of work. That is, according to the vapor pressure and heating temperature of alloy elements in the metal material to be treated during heat treatment, the appropriate vacuum degree is selected to prevent the alloy elements from evaporating and escaping.
Such as the common alloy elements Mn, Ni, Co, and Cr in iron and steel, as well as as the main components of non-ferrous metals such as Zn, Pb, and Cu elements, heating in a vacuum is easy to produce vacuum evaporation, so that the workpiece adhesion between each other, as well as from the material frame caused obstacles. In addition, Cu and Ag-Mn alloys (as filler metals) were used for vacuum brazing of stainless steel. Mn was evaporated when heated below 0.0133Pa, and its composition changed significantly, resulting in a great decrease in the strength of the brazing site. In the case of vacuum annealing at 70 to 30 brass, the Zn is significantly evaporated, resulting in dezincification, so it is very difficult to obtain a bright surface.
However, many defects can be avoided if the choices are made well. Such as Cr12MoV cold stamping die steel, in the vacuum degree of 1.33pa, 1050℃ temperature after 90min, with X-ray microanalyzer determination of chromium in the distance from the surface of 150μm within the distribution, the results did not find the phenomenon of chromium removal. This is because, at 1.33Pa, the theoretical evaporation temperature of chromium is 1205℃. When at 1050℃, the corresponding vapor pressure is low, about 0.0133Pa, that is, the vapor pressure is lower than the external pressure, so there is no evaporation. The example shows that the evaporation of alloying elements can be prevented as long as the vacuum degree is appropriate.

It should be noted that some higher vapor pressure elements in the alloy, such as Mn, Cu, Al, etc., are usually dissolved or in the form of various compounds in a solid solution, in a vacuum, the heating volatilization method is not the same, but their tendency is the same, the volatile and generally speaking, the vapor pressure is lower than the pure metal vapor pressure.
In addition, when heating in a vacuum, it can also be considered according to the kind of metal materials, through high purity inert gas (namely, reverse inflation) to adjust the vacuum degree in the furnace, with low vacuum heating method to prevent the evaporation of alloy elements. Especially at 1200 degrees
When the temperature above is heated, Cr, Mn and so on all have high vapor pressure, are easy to evaporate, and need low vacuum heating more. Through the high purity, inert gas can not only adjust the vacuum degree but also due to the existence of inert gas, the formation of the convection cycle, is more conducive to uniform heating of metal materials.
Therefore, in vacuum heat treatment, the vapor pressure of metal is a problem that can not be ignored. For general alloy steel, firstly, the heating temperature of the heat treatment is less when it reaches 1200℃; secondly, generally, the cutting allowance of the workpiece after the heat treatment is greater than the thickness of the element evaporation layer, so the quality of the workpiece is not affected. However, for important and special steel grades, attention must be paid to the vapor pressure.

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