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Vacuum Oil Quenching for steel hardening heat treatment

Oil quenching is proposed for small parts with relatively large workpieces, quenching temperatures below 1000°C, and relatively high hardenability. The requirements for vacuum quenching oil are as follows:

(1) Low vapor pressure, 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 under the condition of vacuum degree of 10~10^-1Pa.

(2) Less impurities and carbon residue, low acid value. After vacuum oil quenching, the surface brightness of the parts should not be lower 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 after the workpiece is cooled in vacuum quenching oil, it can reach the same hardness value as salt bath quenching under normal pressure, and there must be no obvious carburized layer.

(4) Good thermal stability, that is, good anti-aging performance and long service life.

Figure 1 shows the change curve of hardness of T8 steel after vacuum heating and quenching in vacuum quenching oil, common quenching oil and mineral oil. The hardness of ordinary quenching oil is 66HRC after quenching under normal pressure (0.1MPa), and the sample can be completely hardened. However, as the pressure decreases during quenching, the hardness drops sharply, especially when the pressure drops below 5×10⁴Pa. It is no longer possible to harden the workpiece. Mineral oil quenching at normal pressure only makes the hardness of the sample reach 42HRC, and the hardness increases after the pressure drops. When it 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 low and wide area, which creates conditions for the vacuum oil quenching of tool and die steel to seek high hardness.

The cooling rate is expressed by the time the vapor film exists and the time required for the sample to cool from 800°C to 400°C (°C/s). From the perspective of the existence time of the steam film, the vacuum quenching oil is the shortest below 104Pa, and the cooling time from 800°C→400°C is compared. Although the time of various oils in vacuum is longer than that under normal pressure, vacuum quenching oil is faster than mineral Oil cooling time should be short. At about 2x 10⁴Pa, the cooling time of vacuum quenching oil is the shortest.

Under low pressure, the characteristic temperature of vacuum quenching oil is also high. 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 fraction, solvent dewaxing, flux refining, clay treatment, vacuum distillation, vacuum degassing, and then adding refrigerant, brightener and antioxidant. Cooling curves under different vacuum degrees were measured with φ8mmx 24mm silver rods.

Since the vacuum-heated workpiece has a good surface state, the steel can be cooled in vacuum quenching oil to obtain the same hardness value as the conventional process. In principle, the cooling rate close to atmospheric pressure can be obtained by maintaining the liquid surface pressure at the critical pressure during vacuum quenching. In addition, increasing the air pressure can increase the evaporation and solidification temperature of the oil, so that the volatilization loss and equipment pollution caused by the heating of the oil itself can be avoided. In the process, the method of filling the cooling chamber with pure nitrogen to 40~73KPa (the influence on the characteristics is not obvious when it is higher than 67KPa) has been proved. On, a higher cooling rate will be obtained. This is due to the further thinning of the steam film, which shortens the steam film stage with slow heat transfer, and the pressurized oil quenching is further developed into oil quenching and air cooling quenching, which improves the quenching effect of large and precision tools and molds and reduces deformation. possibility of choice.

In order to meet the needs of quenching and cooling capacity of larger workpieces, the amount of vacuum quenching oil should be large enough. In the design, it is calculated according to the thermal balance of the heat released from the oil-introduced workpieces, trays, fixtures, etc. from the oil-in temperature to the oil pool temperature, and then add a certain amount of safe oil. Considering the expansion and boiling of oil caused by stirring and local intense heating, it is generally better to take the ratio of workpiece weight to oil weight as 1:10~1:15.

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 influence on brightness is far greater than that of vacuum degree. During use, regular analysis of viscosity, flash point, Cooling performance and moisture. According to the analysis, when the mass fraction of water in the vacuum quenching oil reaches 0.03%, it is enough to make the surface of the quenching parts gray. 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, which seriously damages the vacuum. In view of this, the new oil needs to be adjusted before the first use, and the vacuum of the furnace should be kept after each shutdown to prevent air leakage. and water redissolved into the oil.

The use temperature of vacuum quenching oil should be controlled at 40-80°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 the oil temperature and make the oil temperature uniform, the oil pool is equipped with a stirring device to strengthen the circulation and convection of the oil. The stirring must be in place. If the stirring of the old oil is not strong enough, soft spots and soft bands (soft blocks) will easily appear on workpieces with large size and complex structure and long rods; big deformation. Controlling the starting time of stirring after the workpiece is oiled, adjusting the intensity of stirring, and realizing intermittent stirring can reduce deformation and soft spots.

Vacuum oil quenching furnace selection: The double-chamber vacuum oil-gas quenching furnace produced by SIMUWU has an octagonal furnace with good temperature uniformity; the heating element adopts unique non-ceramic support technology. It can be used for five years without any insulation problems. The built-in heat exchanger and forced air cooling make the jet level uniform, and finally make the cooling speed extremely fast; the horizontal double-chamber oil quenching furnace is a horizontal double-chamber structure, compact and reasonable, beautiful and generous.