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vacuum heat treatment introduction:

The metal heat treatment process of the workpiece is carried out in a vacuum chamber with a vacuum degree of 133 65 Recently, a series of new heat treatment processes, such as vacuum carburizing, have been developed by combining vacuum with controlled atmosphere. Due to its obvious advantages in treatment quality, energy saving, especially in environmental protection, vacuum heat treatment, which started in the 1950s, has rapidly developed into the mainstream of modern heat treatment process and equipment renewal; in industrial developed countries, vacuum heat treatment has accounted for 25% of the total metal heat treatment market by the end of 1980s. The biggest drawback of vacuum heat treatment is slow heating of workpiece, especially at low temperature (below 600 C). As a result, the total heating time is 6 times longer than that of the salt bath furnace and 1.5 times longer than that of the air and other gas medium furnaces, which is a major obstacle to reducing energy consumption and increasing productivity. In order to solve this problem, the so-called vacuum carrier gas heating method has been widely used, that is, after the vacuum degree is reached, the high purity nitrogen below 105Pa is filled, the natural convection of the gas is used to increase the heat conduction, and then the positive pressure (1-2)*105Pa inert gas is forced to circulate in the low temperature stage, further accelerating the temperature rise.
 Special effect
 The term "vacuum" in industrial sense refers to a controlled atmosphere in which the atmospheric atmosphere is pumped to a set low pressure and its chemical properties are controlled by the value of negative pressure. The vacuum reduction, no decarburization and no carburization, degassing and evaporation of metal materials during heat treatment are special effects that traditional heat treatment methods do not have.
 In vacuum reduction in diluted atmosphere, the equilibrium of oxidation and reduction of metals can be expressed as follows: M represents metallic elements; PO2 is the partial pressure of oxygen in vacuum; po2MO is the equilibrium decomposition (partial pressure of oxygen) of oxides, that is, the saturated vapor pressure of M0. When the vacuum degree satisfies po22MO, not only does M have no oxidation, but also the original oxide will decompose and oxygen will be extracted, which is called vacuum reduction effect. The vacuum reduction effect causes the workpiece surface to be in a highly clean and chemically active state.
 The dew point of vacuum atmosphere (1.33 Pa, 10-2 Torr) used in non-decarbonizing and non-carburizing industries is about 60 C, and the decarbonizing-carburizing reaction of steel materials is neutral. Therefore, vacuum heat treatment is superior to other heat treatments in maintaining the surface carbon content of steel materials, including controlling the carbon potential in protective atmosphere. Gases, such as H2 and H20, that remain in solid state and are inhaled during forging and rolling or other processes, diffuse from the interior to the surface and escape and are removed under sufficient negative pressure. The higher the vacuum degree or the higher the temperature, the better the degassing effect. Degassing is one of the main reasons why the strength and toughness of vacuum heat treated structural parts are higher than that of conventional heat treatment (air, salt bath, controlled atmosphere).
When the pressure of the vacuum atmosphere is lower than the vapor pressure of the metal, the metal will evaporate and be pumped away. When evaporation occurs, the surface of the workpiece will become rough and uneven, and bond with other workpieces and fixtures; alloy elements with high vapor pressure, such as chromium in steel, will be preferentially evaporated, resulting in the depletion of alloy elements on the surface of the workpiece. Fig. 2 is the relationship between the vapor pressure and temperature of various metals. Any element whose vapor pressure is higher than that of iron, such as copper, aluminum, chromium and manganese, is liable to form a depletion zone on the surface of alloy steel during vacuum heat treatment. In order to prevent evaporation, low vacuum and low temperature should be adopted as far as possible, or proper inert gases should be filled into the closed chamber after exhaust to raise furnace pressure.


 The most widely used technologies are vacuum annealing, vacuum quenching and vacuum carburizing.

Vacuum annealing:

Vacuum annealing workpiece

Vacuum annealing workpiece

High quality spring steel, tool steel, bearing steel wire, stainless steel products and titanium alloys can be annealed by vacuum. The lower the annealing temperature, the higher the vacuum degree is required. In order to prevent chromium evaporation and accelerate heat conduction, carrier gas heating (insulation) method is generally used, and attention should be paid to the stainless steel and titanium alloy is not suitable for nitrogen but should be used in argon.

Vacuum quenching:

Vacuum quenching furnace

Vacuum quenching furnace


Vacuum quenching vacuum quenching furnace can be divided into oil quenching and gas quenching according to cooling method. It can be divided into single chamber type and double chamber type according to working number. 904 mountain bats are all periodic working furnaces. The vacuum oil quenching furnace is double chamber, and the back room is equipped with an electric heating element, and an oil tank is arranged below the front chamber. After heating and heat preservation, the workpiece is moved into the anterior chamber. After closing the middle door, the inert gas is filled into the anterior chamber to about 2.66 *lO4-1.01 *105 Pa (200-760 mm mercury column), and oil is added. Oil quenching can easily cause deterioration of workpiece surface. Due to the high surface activity, significant thin layer carburization can occur under the action of a short high temperature oil film. In addition, the adhesion of carbon black and oil on the surface is not conducive to simplifying the heat treatment process. The recent development of vacuum quenching technology mainly lies in the development of gas cooled quenching furnace with excellent performance and single station. The two-chamber furnace can also be used for gas quenching (air-jet cooling in the front chamber), but the operation of the two-station makes it difficult to load the furnace in large quantities, and it is easy to cause deformation of the workpiece during high temperature movement or change the orientation of the workpiece to increase quenching deformation. The single station gas cooled quenching furnace is cooled by jet cooling in the heating chamber after heating and heat preservation. The cooling rate of gas cooling is not as fast as that of oil cooling, and is lower than that of conventional molten salt in isothermal and graded quenching. Therefore, increasing the pressure of the spray chamber, increasing the flow rate, and using helium and hydrogen as inert gases with smaller molar mass than nitrogen and argon are the mainstream of the development of vacuum quenching technology. In the late 1970s, the pressure of nitrogen spray cooling was increased from (1-2)*105Pa to (5-6)*105Pa, making the cooling capacity close to that of oil cooling under normal pressure. In the mid-1980s, super-high pressure gas quenching appeared. The cooling capacity of (10-20)*105Pa helium was equal to or slightly higher than that of oil quenching. It has been put into industrial use. At the beginning of 90s, the hydrogen capacity of 40 x 105Pa was close to the cooling capacity of water quenching, which is still in its infancy. Industrial developed countries have progressed to high pressure (5~6) x 10.

Vacuum carburizing:

After being heated to the carburizing temperature in vacuum and kept warm to purify and activate the surface, a thin carburizing rich gas (see controlled atmosphere heat treatment) is introduced to infiltrate under about 1330 Pa (10T0rr) negative pressure, and then the gas is stopped (depressurized) for diffusion. Quenching method can adopt gas cooling or oil cooling. The latter is transferred into the anterior chamber after austenitizing, filling nitrogen to atmospheric pressure and entering the oil. Vacuum carburizing temperature is generally higher than ordinary gas carburizing, usually 920 ~ 1040 C infiltration and diffusion can be divided into two stages as shown in Figure 3, or pulse ventilation, gas stopping, multi-stage infiltration and diffusion, the effect is better. Because of the high temperature, especially the clean and active surface, vacuum carburizing layer is formed faster than ordinary gas, liquid and solid carburizing. If the carburizing layer is required to be 1 mm, it only takes 5 hours at 927 C and 1 hour at 1033 C.

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