Why should heat treatment furnaces be vacuum sealed?

The importance of vacuum sealing
The chemical activity of the high vacuum atmosphere is extremely low. During vacuum heat treatment, the reactions that occur at the interface between the gas phase and the solid phase, such as oxidation, reduction, decarburization, carburization, etc., will not proceed to an influential level; the high vacuum atmosphere makes the gas The change in volume increase is very rapid, which can cause the metal or alloy to release dissolved gas or cause the metal oxide to decompose. It is precisely because of the characteristics of high vacuum atmosphere that in high vacuum atmosphere, because the partial pressure of oxygen is very low, oxidation will occur. is suppressed, so in order to achieve the purpose of no oxidation, the partial pressure of oxygen must be lower than the decomposition pressure of the oxide.
Degassing (degassing) action of vacuum. The degassing action of vacuum is as follows. Metal degassing can improve the plasticity and strength of metal. Under vacuum conditions, a certain amount of dissolved gases (hydrogen, oxygen, nitrogen, etc.) in the metal workpiece will overflow and degas from the metal surface, which is beneficial to improving the plasticity and strength of the workpiece. , the higher the temperature, the more violent the movement of molecules, which is more conducive to the diffusion of gas dissolved in the metal to the surface, increasing the degree of vacuum. The lower the pressure, the more conducive to the overflow of gas diffused on the metal surface.
During the smelting process of metal materials, the liquid metal must absorb H2, O2, N2, CO and other gases. Considering that the solubility of the metal to the above gases increases with the increase of temperature, when the liquid metal is cooled into a steel ingot, the gas will be absorbed in the metal. The solubility in the metal is reduced, but because the cooling rate is too fast, the gas cannot escape (release) completely, and remains inside the solid metal, generating metallurgical defects such as pores and white spots (formed by H2) or solid solution in the atomic and ionic state. Metal interior.
In addition, during the thermal processing of metals such as forging, heat treatment, pickling, and brazing, gases will inevitably be reabsorbed. At this time, the resistance, thermal conductivity, magnetic susceptibility, hardness, yield point, strength limit, and elongation of the metal will Mechanical and physical properties such as rate, area shrinkage, impact toughness, and fracture toughness are all affected. Therefore, the gas content of the raw materials in the metallurgical process must be controlled, and at the same time, efforts must be made to eliminate the gas absorbed during the thermal processing, or through improvements process to prevent gas absorption.
Reasons affecting vacuum degree
The diffusion speed of gas molecules in the solid phase often determines the speed of degassing. The reason why vacuum degassing can remove the gas inside the metal is that the gas in the metal can be removed under negative pressure conditions, so the vacuum degree in the furnace affects the vacuum. The speed and effect of degassing. Another factor that determines the degassing effect is the temperature in the furnace. The higher the temperature, the better the degassing effect. The third factor is time. The longer the degassing time, the better the degassing effect. Taking into account the influence of factors such as grain coarsening and metal phase change, the temperature cannot rise too high. For metal materials with phase change such as steel, vacuum degassing at a temperature near the phase change point has the best effect. , The reason is that the solubility of gases in metal materials decreases during phase change or the change in lattice facilitates the migration of gas atoms during phase change.

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