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Heat treatment of involute spline parts realized by vacuum oil quenching technology

With the continuous expansion of the automobile market, the processing requirements for various types of parts are also getting higher and higher. Vacuum heat treatment has the advantages of high quality, low energy consumption, no pollution (or less pollution), etc. It can effectively prevent defects in gear parts, improve surface roughness, and reduce friction factor. Vacuum heating is carried out in the form of radiation, the heating is slow, the furnace temperature is uniform, the product deformation is small, and it also has a dehydrogenation effect, which is conducive to improving the processing quality. At present, vacuum heat treatment technology has been widely used in the processing of auto parts. With its excellent performance, it has become the preferred method for heat treatment of high-precision and high-strength parts.

Taking a complex involute internal spline automobile part as an example, the advantages and main points of vacuum heat treatment are explained:

The key structural parts of a certain vehicle are shown in the figure, and require higher hardness after final heat treatment, and at the same time, the shape and position tolerances of the involute splines are also higher. Due to the high hardness (37-44HRC), conventional machining methods such as drilling and milling cannot be used after final quenching, and it is required to be processed in place before final quenching, and only a small amount of grinding is allowed. For the parts with the reserved grinding amount, the ordinary box-type resistance furnace is used to heat in the air, and after the lifting and final quenching are completed with the cooperation of the crane, the spline part will be seriously deformed. Due to the complexity of the shape of the involute spline, it is impossible to achieve the accuracy requirements of the size specified in the drawing by grinding after final quenching. The traditional processing flow is as follows: first, the spline parts of the part are processed with smooth holes, the rough blank is heated and finally quenched in a box-type resistance furnace, and then the involute splines are processed at one time by the slow wire cutting method. Due to the long time-consuming of wire cutting, this processing method has low production efficiency, which seriously affects the production progress.

In order to improve production efficiency, a new processing scheme of vacuum heat treatment is adopted. That is, the part of the involute spline is first broached and formed with a special broach, and then the fine blank is heated and finally quenched in a vacuum oil quenching furnace. Then use a special broach to finely pull it to meet the precision requirements of the drawing. After the batch production implements this processing plan, the production efficiency is increased by 20 times, the product quality is stable, and the production cost is reduced by 700,000 yuan per year. The following is a detailed introduction to the control points of using the double-chamber vacuum oil quenching furnace to realize the micro-deformation quenching of the involute spline inner hole parts.

Key points of double-chamber vacuum oil quenching control:

1.Control of pre-heat treatment

In order to reduce the deformation during final quenching, the forging blank is normalized and tempered after the parts are forged to homogenize the structure, refine the grains, reduce the stress, and reduce the final heat treatment deformation.

2.Control of vacuum heating furnace loading method

The workpieces are laid flat in layers, and the spline holes of the upper workpiece and the lower workpiece are aligned to facilitate uniform heat penetration and uniform cooling. Control of vacuum heating process

3.Control of the preheating process

In order to eliminate the temperature difference between the surface and the core of the part and reduce the thermal stress to reduce the quenching deformation, two isothermal sections are set up when the part is quenched and heated, preheated once at 600 ℃, and kept for several hours.

4.Control of heating speed

In the process of heating the part to the quenching temperature, there are two heating stages. If the heating speed is too fast, the temperature gradient from the surface to the core of the part is too large, which will increase the thermal stress and increase the quenching deformation. To avoid this, the heating rate should be controlled. According to the general requirements for the heating of structural steel and the heating characteristics of the internal heating vacuum furnace, the heating time of the low temperature section is set to a longer time, and the heating time of the higher temperature section is set to a shorter time.

5.Control of vacuum degree

According to the characteristics of low-carbon alloy structural steel, the vacuum degree in the vacuum heating process is selected to be 1-10Pa. The purpose is to prevent the surface of the parts from being evaporated and depleted due to the high vacuum, and to prevent the surface of the parts from being oxidized and depleted due to the low vacuum. carbon.

Quenching and cooling attention points:

1.Smooth transfer

Check and ensure that the transmission mechanism in the vacuum hot oil quenching furnace is stable and reliable, the hot gate is flexibly opened and closed, and the operating speed of the transmission mechanism is adjusted to control the transfer time between heating and cooling, and prevent excessive attenuation of the quenching temperature of the parts.

2.Cooling speed

Select the vacuum quenching oil with low saturated vapor pressure that matches the vacuum degree, and regularly check and adjust the cooling rate curve and aging condition of the oil. The quenching process also needs to introduce an appropriate amount of N2 gas to ensure the stability and effectiveness of the cooling effect.

3.Stir

In order to reduce the quenching deformation as much as possible, and at the same time ensure the quenching and cooling effect, the stirring is started manually in different periods. 20 minutes before the transfer of the parts from the heating chamber to the cooling chamber, turn on the oil and stir, and then turn off after stirring for a period of time. After the oil surface has calmed down, quench and cool the workpiece. After cooling, turn on the oil and stir. After stirring for a period of time, the quenching and cooling are completed. Improve the uniformity of oil temperature and increase the quenching intensity of vacuum oil.

4.Time

In order to reduce the quenching deformation of the parts, it should be ensured that the cooling time is sufficient, and the parts can be released after they are completely cooled.

Selection of vacuum oil quenching equipment: VOGQ series vacuum heat treatment furnaces produced by SIMUWU are high-quality products for vacuum heat treatment of auto parts. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.