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Vacuum Oil Quench Heat Treating Services

Vacuum oil quenching heat treatment service

Vacuum heat treatment technology has a series of advantages such as no oxidation, no decarburization, degassing, degreasing, good surface quality, small deformation, excellent comprehensive mechanical properties of heat-treated parts, long service life, and energy saving. It has achieved rapid development in recent years, and is currently widely used in aerospace industry, tooling, precision parts, oil pump nozzles and other mechanical product parts with high product quality requirements.

Vacuum oil quenching technical requirements

Vacuum oil quenching is the most common way in vacuum quenching treatment. The quality of vacuum quenching and cooling directly determines the structure and performance of the workpiece after quenching. As shown in Figure 1, during the vacuum quenching cooling process, the heated workpiece has to go through three stages of vapor film (I), boiling cooling (II) and convective cooling (III). In the initial steam film cooling stage, if the steam film time is too long, the characteristic temperature (TA) is low, which will cause insufficient or uneven hardness of the workpiece; in the boiling cooling stage, if the cooling rate is too slow The cooling speed will cause the cooling curve to pass through the C curve to form a non-martensitic structure, which not only affects the quenching hardness, but also may form a defect area of mechanical properties after tempering, which will cause premature failure of the workpiece and affect the service life of the workpiece ; In the convection stage, the workpiece begins to undergo martensitic transformation. If the cooling rate is too fast at this stage, it will lead to excessive structural transformation stress, causing deformation and cracking. Therefore, the ideal quenching medium cooling curve should be shown in Figure 2, the steam film stage is short, the maximum cooling rate is greater than the critical transformation cooling rate of the workpiece, and has a suitable low-temperature cooling rate.

In vacuum oil quenching, the quenching medium generally adopts vacuum oil instead of ordinary quenching oil. This is because: ①The saturated vapor pressure of ordinary quenching oil is high, and it is easy to volatilize under vacuum conditions, even in a boiling state, which changes the physical properties of the oil. The capacity is reduced, and the vacuum oil has a low saturated vapor pressure and is not easy to evaporate, which can reduce the pollution of vacuum equipment; ②Under normal atmospheric pressure, the quenching cooling capacity of ordinary quenching oil may be slightly higher than that of vacuum quenching oil, but with the vacuum As the pressure decreases, the quenching capacity of ordinary quenching oil decreases significantly with the decrease of pressure, while vacuum quenching oil can maintain a cooling capacity close to atmospheric pressure in a large pressure range (below atmospheric pressure). Vacuum oil quenching is generally carried out under negative pressure, so special vacuum quenching oil should be selected as the quenching medium during vacuum quenching to ensure that the quenching and cooling capacity can approach or reach normal pressure under negative pressure.

Vacuum quenching oil should have the following characteristics:

1) Low saturated vapor pressure, less evaporation under low pressure, strong anti-gasification ability, not easy to volatilize, easy to vacuum, and will not pollute the vacuum system;

2) Critical pressure (the minimum air pressure with the same quenching ability as atmospheric pressure is low, the cooling ability does not change much with the reduction of air pressure, and there is still a certain cooling speed under vacuum;

3) After quenching, the surface brightness of the workpiece is high;

4) Good chemical stability, long service life, less impurities and carbon residue, and low acid value.

Vacuum oil quenching process

When using vacuum oil for the first time or adding new vacuum oil, the new vacuum oil should be degassed, because the vacuum quenching oil will contain gas during long-term storage and transportation, and the purpose of degassing is to remove the gas in the vacuum oil , The vacuum after degassing can be used normally.

In a vacuum state or below the critical pressure of the vacuum oil, the cooling capacity of the vacuum quenching oil will drop significantly. For some steels with poor hardenability or larger workpieces, the purpose of quenching may not be achieved. It is necessary to fill the quenching chamber with high-purity neutral or inert protective gas to make the quenching chamber reach a certain pressure, so as to achieve the purpose of quenching. Theoretically speaking, only the filling pressure needs to reach the critical pressure during quenching to obtain the quenching effect close to the standard atmospheric pressure. If the inflation pressure is increased, the evaporation and condensation temperature of the oil can be increased, the volatilization loss caused by the instantaneous temperature rise of the oil itself can be avoided, and the pollution of the heating chamber and the cooling chamber by oil vapor can be reduced. It is recommended to use a higher inflation pressure during vacuum quenching. Generally, the recommended inflation pressure is around 5×104 Pa.