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Vacuum Heat Treatment Furnace

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Vacuum Oil Quenching Effects on Gear Distortion

Vacuum furnaces are used for quenching. Small parts with relatively large workpieces, quenching temperatures below 1000°C, and relatively high hardenability are intended to be quenched with oil. There are following requirements for vacuum quenching oil:
(1) The vapor pressure is low and not easy to volatilize. The vapor pressure is required to be lower than 10-2~10-4Pa to ensure that the vacuum quenching oil will not volatilize significantly when the vacuum degree is 10~10-1Pa.
(2) Less impurities and carbon residue, low acid value. After vacuum oil quenching, the brightness of the surface of the part should not be less than 70% of the standard sample.
(3) The critical pressure (that is, the lowest pressure with the same cooling capacity under atmospheric pressure) is low and the cooling performance is good. It is required that the workpiece can reach the same hardness value as salt bath quenching under normal pressure after cooling in vacuum quenching oil, and there must be no obvious carburized layer.
(4) Good thermal stability, that is, good anti-aging performance and long service life.
The hardness of ordinary quenching oil after quenching under normal pressure (0.1MPa) is 66HRC, and the sample can be completely quenched. However, as the pressure decreases during quenching, the hardness drops sharply, especially when the pressure drops below 5×104Pa. The workpiece can no longer be hardened. When mineral oil is quenched at normal pressure, the hardness of the sample only reaches 42HRC. When the pressure decreases, the hardness increases instead. When the pressure reaches 2.5×104Pa, the hardness after quenching can reach 63~65HRC. Quenching hardness tends to decrease at lower pressures. Vacuum quenching oil can be quenched to a high hardness above 63HRC in a wide and low range, which creates conditions for vacuum oil quenching of tool and die steel to achieve high hardness.
Under low pressure, the characteristic temperature of vacuum quenching oil is also higher. Therefore, vacuum quenching is also suitable for working under low pressure and has excellent quenching ability.
Vacuum quenching oil is prepared from paraffin lubricating oil fractions, solvent dewaxing, flux refining, clay treatment, vacuum distillation, vacuum degassing, and then adding refrigerants, brighteners and antioxidants.
Since the vacuum-heated workpiece has a good surface state, the same hardness value can be obtained by cooling the steel in vacuum quenching oil as in the conventional process. In principle, maintaining the liquid surface pressure at the critical pressure during vacuum quenching can achieve a cooling rate close to atmospheric pressure. In addition, increasing the air pressure can increase the evaporation and solidification temperature of the oil, thus avoiding volatilization losses and contamination of the equipment caused by the heating of the oil itself. In the process, the method of filling the cooling chamber with pure nitrogen to 40~73KPa (the effect on the characteristics is no longer obvious when it is higher than 67KPa) has been proved by practice. For some low-hardenability steels, if the air pressure is increased to below atmospheric pressure, , a higher cooling rate will be achieved. This is because the steam film has further become thinner, shortening the steam film stage with slow heat transfer, and pressurized oil quenching has further developed into oil quenching and gas cooling quenching, which improves the quenching effect of large and precision tools and molds, reduces deformation, and provides more possibility of choice.
The quality of vacuum quenching oil, such as acid value, residual carbon, moisture, ion content, etc., may cause serious coloring of the workpiece. Sometimes their impact on brightness is much greater than the impact of vacuum degree. During use, viscosity, flash point, etc. need to be analyzed regularly. Cooling performance and moisture. According to analysis, when the mass fraction of water in the vacuum quenching oil reaches 0.03%, it is enough to make the surface of the quenched parts dark. When the mass fraction of water reaches 0.3%, the cooling characteristics of the oil will change significantly, and quenching cracking is likely to occur. When the hydraulic pressure is reduced, the water-containing oil will boil, thereby seriously destroying the vacuum. In view of this, the new oil needs to be prepared before the first use, and the vacuum of the furnace should be maintained after each shutdown to prevent air and water dissolve again into the oil.
The operating temperature of vacuum quenching oil should be controlled between 40 and 80°C. When the oil temperature is too low, the viscosity is high and the cooling rate is low. The hardness of the workpiece after quenching is uneven and the surface is not bright. Before quenching in cold winter, the vacuum quenching oil needs to be preheated. Under vacuum conditions, if the oil temperature is too high, the oil will evaporate rapidly, causing pollution and accelerating the aging of the oil. In production, in order to quickly adjust and homogenize the oil temperature, a stirring device is permanently installed in the oil pool to enhance oil circulation and convection. Stirring must be in place. If the stirring of old oil is not strong enough, it is easy to cause soft spots and soft strips (soft lumps) on workpieces with large size and complex structure and long rods. If the stirring of oil is too strong, it will also cause the workpiece to produce soft spots. Big deformation. Controlling the start time of stirring after the workpiece is immersed in oil, adjusting the intensity of stirring, and implementing intermittent stirring can reduce deformation and soft spots.