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Deformation and Control of Carburizing and Quenching for Large Gears and Gear Shafts

Large gears and gear shafts are important components in the reducer. In the process of carburizing and quenching, the deformation of the gears exists objectively, and the deformation of the gear shafts is larger. In actual production, it mainly depends on increasing the final machining allowance, vacuum quenching and then finishing to eliminate deformation. This will lead to different depths of the seepage layer after the tooth surface is cut, especially the residual stress distribution of the tooth surface is not uniform, which affects the manufacturing accuracy of the gear and reduces the bearing capacity and service life, and the finishing also increases the cost. Therefore, in order to grasp the deformation law and minimize the deformation of carburizing and quenching, we have carried out in-depth analysis and research.

1 Deformation law of large gears and gear shafts

According to the measurement and analysis of carburizing and quenching deformation of large gears in actual production, the biggest influence on gear quality and life is the deformation of outer diameter, common normal length and helical gear helix angle, and the deformations are interrelated.

1.1 Deformation law of large gears

(1 After carburizing and quenching, the outer diameter (outer diameter of the tip circle) shows a significant expansion trend, and the top and bottom are uneven and conical. If the carbon concentration is out of control and is too high, there will be a large amount of residual austenite after vacuum quenching, and the outer The diameter shows a shrinking trend, and this phenomenon is rare.

(2 The amount of deformation of the tip circle diameter is related to the outer diameter of the gear. For a gear with a larger ratio of surface area to volume, the amount of outer diameter expansion increases with the increase of the diameter.

(3) The outer diameter deformation of the gear is related to the ratio of its surface area and volume. The larger the ratio, the more the outer diameter expands, and the smaller the ratio, the smaller the outer diameter expands or may shrink.

(4 The deformation of large gear carburizing and quenching is closely related to the way of vacuum quenching. If a single gear is deformed in the shape of a Korean drum, the expansion at both ends is large, and the expansion in the middle is small; if it is overlapped and mounted on the fixture, the upper and lower Both ends are tapered and deformed, as shown in Figure 1.

1.2 Deformation law of large gear shafts

(1) After carburizing and quenching of large gear shafts, the diameter of the addendum circle shows an obvious trend of shrinking, but various parts on the same shaft can expand and contract.

(2) The form of carburizing and quenching deformation of a gear shaft is that the outer diameter of both ends of the tooth portion expands slightly, while the middle section shrinks, as shown in Figure 2.

2 Deformation Analysis

When large gears and gear shafts are carburized and quenched, the cooling rate, structure and hardness of each part are analyzed and compared, and it can be found that the cooling rate of the upper, middle and lower parts is different, and the cooling rate of the surface, transition area and core is different. Tissue transformation has unequal time, and these are the main reasons for gear deformation.

3 Control measures to reduce deformation or improve deformation conditions

(1 Carburizing should be as uniform as possible to avoid and reduce uneven structure and uneven stress caused by uneven carburization.

(2) Select the appropriate quenching agent, control the oil temperature (≥100 °C), control the final cooling time and the final cooling temperature, rapidly cool at high temperature, and cool slowly at low temperature martensitic transformation area.

(3 Cooperate with hot and cold processing, perform pre-deformation compensation, that is, adjust the machining allowance, leave some allowance for expansion, and leave more allowance for shrinkage. For example, the helix angle of large helical gears is deformed, and the helix angle is intentionally changed before carburizing. Deviating from a certain angle (in the opposite direction of deformation), the helix angle of the shaft always increases after carburizing, then a certain angle is intentionally reduced when milling teeth, and the quenching deformation is just restored to the required angle. For the gear shaft, the helix angle of the tooth width and the full length should be deviated by 1mm, as shown in Figure 3.

Improve the uniformity of cooling in the upper, middle and lower parts of the gear, so that the tissue transformation tends to be synchronized. Measures such as spraying oil in the middle of the gear shaft to accelerate cooling, and heating rings on the upper and lower end faces of the gear can improve the uneven cooling speed of the end face and the middle section, which can achieve obvious results.

4 Conclusion

After several years of production practice and theoretical analysis, we have initially mastered the deformation laws and control measures of carburizing and quenching of large gears and gear shafts, and applied these control measures to actual production, and achieved good results. Due to the limitation of equipment capacity, the implementation of the heat treatment process and the actual operation, and the complicated deformation of the gear and gear shaft after carburizing and quenching, further exploration and research are still needed.

Selection of heat treatment equipment: RVC series vacuum carburizing furnaces and RVGQ vacuum gas quenching furnaces produced by SIMUWU are high-quality products for vacuum heat treatment of tools and molds. Good temperature control accuracy and uniformity ensure the effective progress of the vacuum heat treatment process.