Technical characteristics of vacuum heat treatment-Degassing

Technical characteristics of vacuum heat treatment

        Gases dissolved into metal under normal pressure, such as hydrogen, oxygen, nitrogen, etc., diffuse from the inside of the metal to the surface and escape under negative pressure. This phenomenon is called degassing. According to the principle of thermodynamics, H2, O2. The solubility S of diatomic gases such as N2, in metals is proportional to the square root of its partial pressure p.

        As the partial pressure of the gas decreases under vacuum, the solubility of the gas in the metal will decrease, that is, the higher the vacuum, the better the degassing effect. The degassing process is:

            1.The gas in the metal diffuses to the surface;

            2.The gas escapes from the metal surface;

            3.The gas is continuously pumped out and eliminated.

        The concentration gradient formed on the surface of the workpiece when the surface gas escapes outward is the main reason why the gas is continuously discharged from the inside to the outside. According to the exponential relationship between the diffusion coefficient and the temperature, the temperature diffusion coefficient (D increases, so that the degassing speed increases. The diffusion coefficient is greatly affected by the temperature. Under the same vacuum degree, increasing the temperature can improve the degassing effect .

        When heating in vacuum, the degassing effect of hydrogen is the most obvious, because hydrogen easily diffuses in steel. Compared with hydrogen, oxygen and nitrogen are more difficult to diffuse in steel, and they can only be carried out at a temperature above 900°C.