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Vacuum Heat Treatment Furnace

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How much do you know about vacuum heat treatment

Classification of vacuum heat treatment process

Vacuum heat treatment is the development of ordinary heat treatment technology. Its classification method is basically the same as ordinary heat treatment. Vacuum heat treatment can basically do what ordinary heat treatment can do. For example, vacuum annealing, vacuum quenching, vacuum tempering, vacuum carburizing, vacuum nitriding, vacuum carbonitriding, etc. The vacuum quenching medium can be oil quenching, gas quenching, water quenching, etc.

Classification of vacuum heat treatment equipment

There are many classification methods for vacuum heat treatment equipment (furnace), which are usually classified according to the following characteristics. According to the purpose, it can be divided into: vacuum annealing furnace, vacuum tempering furnace, vacuum carburizing furnace, vacuum quenching furnace, vacuum brazing furnace, vacuum sintering furnace, etc.

According to the vacuum degree, it can be divided into: low vacuum furnace (pressure between 1333 and 1.33×10^-1Pa), high vacuum furnace (pressure between 1.33×10^-1 and 1.33×10^-4Pa), and ultra-high vacuum furnace (pressure above 1.33×10^-4Pa).

According to the working temperature, it can be divided into: low temperature furnace (temperature≤700℃), medium temperature furnace (temperature 700～1000℃), high temperature furnace (temperature＞1000℃).

The role and characteristics of vacuum heat treatment

The protective role of vacuum

Vacuum heat treatment is a heat treatment carried out in a negative pressure atmosphere, and its heating process is called vacuum heating. The gas pressure state (i.e., working pressure or working vacuum degree) that needs to be maintained in this process is generally around 100～10-3Pa. According to gas analysis, the residual gas in the vacuum furnace at this pressure, such as water vapor, oxygen, carbon dioxide, and organic vapors such as grease, is already very small, which is not enough to cause oxidation, decarburization, and carbon increase of the treated metal material. During vacuum heating, since the partial pressure of oxygen in the atmosphere is lower than the decomposition pressure of the oxide on the surface of the heated metal, the oxidation is suppressed, so the brightness of the treated metal surface can be largely unchanged compared with the original surface. It can be seen that the vacuum not only protects the heating of the workpiece, but also keeps the original bright surface of the metal, so vacuum heat treatment also belongs to the category of bright heat treatment.

Surface purification effect

Before vacuum heat treatment, oxides, nitrides, hydrides and other substances are often attached to the metal surface. When heated in a vacuum, these compounds are reduced, decomposed or volatilized and disappear, so that the metal has a smooth surface. For example, a very thin oxide film formed on the surface of high-speed steel or stainless steel can be removed in a vacuum heat treatment furnace above about 1000°C. The vacuum provides conditions for the decomposition of oxides and can purify the metal surface.

Vacuum degassing effect

During heat treatment in a vacuum furnace, the gas in the metal workpiece is removed, thereby improving the performance of the workpiece. Compared with conventional heat treatment, the mechanical properties, especially plasticity and toughness of metal materials after vacuum heat treatment are significantly improved. The main reason is the degassing effect during vacuum heat treatment, which removes oxygen, hydrogen, nitrogen and other gases in the metal, which can significantly improve the fatigue and toughness indicators of the metal. For example, 30CrMnSi steel bolts with a strength of 1700-1800MPa will produce hydrogen embrittlement when the hydrogen content reaches 13×10^-6~17×10^-6. After adopting the vacuum quenching process, the hydrogen content of the material can be reduced from 8×10^-6 in the atmospheric isothermal quenching process (removing the black skin on the surface) to about 4.2×10^-6. The degassing effect of vacuum heating can reduce the oxygen content of titanium alloy containing 2% Fe, 2% Cr, and 2% Mo from 0.036% to 0.003%.

Vacuum degreasing effect

Various coolants and lubricants are often used in the mechanical processing of metal parts before heat treatment. These coolants and lubricants containing grease will inevitably be adsorbed on the surface of the parts. However, during vacuum heat treatment, the parts can be heat treated by simply cleaning and drying without special degreasing treatment. Because grease is a hydrocarbon compound with a high saturated vapor pressure, it will evaporate or decompose into gases such as water vapor, hydrogen and carbon dioxide when heated in a vacuum, and be pumped out by a vacuum pump. Therefore, a smooth metal surface without oxidation and corrosion can be obtained, which is impossible to achieve under normal pressure of gas.