Control of uniform cooling of gear blank by isothermal normalizing

In order to improve the machinability of gears for transmission and stabilize the final heat treatment deformation, a company purchased a continuous push rod isothermal normalizing production line in 2010 as the processing equipment for gear blank pretreatment. After one year’s debugging and production operation, we believe that the difficulty of push rod isothermal normalizing production line lies in how to ensure the uniformity of gear blank temperature in the isothermal normalizing and rapid cooling stage, because only when the temperature is uniform, the hardness and structure will be uniform. This paper discusses the factors that may affect the uniformity of gear blank cooling. Please criticize and correct the deficiencies.

  1. Charging mode

Incorrect charging mode is the main reason affecting the temperature uniformity of gear blank. Incorrect charging method can easily cause the external gear blank of the basket to cool faster than the internal gear blank, resulting in great differences in hardness and structure between the internal and external gear blanks. The key of charging mode is to ensure smooth ventilation between gear blanks. It is recommended to place according to the wind direction of the equipment, and leave a gap between blanks as far as possible to avoid changing the direction due to blocked wind direction. For example, if the equipment adopts up-down periodic air cooling, it is not suitable to use the flat placement method shown in Figure 1, because at this time, the outermost gear blank contacts large air volume and fast wind speed, resulting in large internal and external temperature difference. In this case, it can only be placed in a single layer, but the production efficiency will be greatly affected.

Compared with the two charging methods in Fig. 2 and Fig. 3, it is found that there are more effective heat dissipation areas formed by placing the extension basket in the width direction (see Fig. 3), and the single row loading amount of gear blank is less, which is more conducive to the ventilation and heat dissipation of gear blank. At this time, the internal and external temperature difference of gear blank is the smallest (about 40 ℃), the metallographic organization is qualified, and the production efficiency is not reduced.

As another example, the double gear is suggested to be placed horizontally as shown in Figure 4. Because of the large size difference of each part of the duplex gear, the two gear surfaces shall be placed along the wind direction to avoid the formation of granular bainite structure at the intermediate connection part due to rapid cooling.

  1. Cooling speed and air volume

For isothermal normalizing, the cooling rate should not be too fast or too slow. Too fast cooling will cause large temperature difference inside and outside the workpiece, and the surface may be lower than the critical transformation temperature of bainite to form granular bainite structure. Too slow cooling will lead to the formation of coarser pearlite structure. At present, the vast majority of isothermal normalizing equipment adopts the air cooling mode of strong cooling + slow cooling, and has the selection of air volume. Through strong cooling, the gear blank can be quickly cooled to about 700 ℃, and then through slow cooling, the gear blank or different parts of the same gear blank can be evenly reduced to the isothermal temperature. For gear blank, its outer diameter, inner diameter and thickness should be comprehensively considered, and the strong cooling time, slow cooling time and air volume should be reasonably allocated.

  1. Ambient temperature

There are two main cooling methods for isothermal normalizing equipment: one is circulating cooling through the hot air heated by the equipment’s own heater or isothermal furnace, and the other is direct cooling by extracting air from the environment by exhaust fan. Since the latter is easily affected by ambient temperature and the air temperature changes with the weather, technicians should reasonably adjust the cooling time and air volume according to seasonal changes. For example, if the ambient temperature is high in summer and the heat taken away per unit time is less, it is necessary to appropriately extend the cooling time or increase the air volume.

  1. Heating temperature

In fact, the influence of heating temperature is also the influence of cooling speed. Reducing the heating temperature is equivalent to reducing the temperature difference in the cooling stage. At the same cooling speed, it is more conducive to ensure the uniformity of gear blank temperature. However, many factors need to be considered in the selection of heating temperature. For example, the isothermal normalizing heating temperature of our company is usually higher than the carburizing temperature, that is, the carburizing temperature is 930 ℃, and the heating temperature is 940 ~ 960 ℃. One reason for choosing this temperature is to determine the grain growth tendency of this batch of materials at this temperature through normalizing metallographic structure inspection, especially grain size inspection, So as to ensure the stability of carburizing process, which is an effective process monitoring. Therefore, it is recommended that each manufacturer should choose the heating temperature according to the actual situation of their own materials.

  1. Conclusion

After one year of commissioning and production operation, we have preliminarily mastered the influencing factors of uniform cooling of push rod isothermal normalizing line, and accumulated some experience to ensure uniform cooling of gear blank. However, due to the influence of equipment, tooling and materials, there are still many problems that have not been solved. We will continue to work hard to further explore and study the isothermal normalizing cooling law, and strive to minimize the impact.

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