Large gear and gear shaft are important parts of reducer. In the process of carburizing and quenching, the deformation of gear is objective, and the deformation of gear shaft is larger. In the actual production, at present, it mainly depends on increasing the final machining allowance and finishing after quenching to eliminate the deformation. This will lead to the different depth of the penetration layer after the tooth surface cutting, especially the uneven distribution of the residual stress on the tooth surface, which will affect the manufacturing accuracy of the gear, reduce the bearing capacity and service life, and improve the cost of finishing. Therefore, in order to master the deformation law and minimize the deformation of carburizing and quenching, we have carried out in-depth analysis and research.
According to the measurement and analysis of carburizing and quenching deformation of large gears in actual production, the deformation of outer diameter, common normal length andhelix angle are the biggest factors affecting the quality and service life of gears, and the deformation is related to each other.
1.1 deformation law of large gear
(1) After carburizing and quenching, the outer diameter (the outer diameter of the tooth top circle) shows an obvious expansion trend, and the upper and lower parts are uneven, showing a cone shape.
If the carbon concentration is out of control, there will be a lot of retained austenite after quenching, and the outer diameter will shrink, which is less.
(2) The deformation of the addendum circle diameter is related to the outer diameter of the gear.
When the ratio of surface area to volume is large, the external expansion of gears increases with the increase of diameter.
(3) The outside diameter deformation of gear is related to the ratio of its surface area and volume.
The larger the ratio is, the more the outer diameter expands, the smaller the ratio is, and the smaller the outer diameter expands or may contract.
(4) The deformation of large gear during carburizing and quenching is closely related to the way of card during quenching.
If a single gear is deformed in korean drum shape, the expansion at both ends is large, and the expansion in the middle is small; if it is hung on the fixture, the upper and lower ends are tapered, as shown in Figure 1.
1.2 deformation law of large gear shaft
(1) After carburizing and quenching of large gear shaft, the diameter of tooth top circle is obviously reduced, but it can expand and shrink in each part of the same shaft.
(2) The carburizing and quenching deformation form of a gear shaft is that the outer diameter of both ends of the gear part slightly expands, while the middle section shrinks, as shown in Fig. 2.
When large gear and gear shaft are carburized and quenched, the cooling speed, structure and hardness of each part are analyzed and compared. It can be found that the cooling speed of the upper, middle and lower parts is different, and the cooling speed of the surface, transition area and center is different. The main reason for the deformation of gears is that the organization changes unevenly.
Control measures to reduce deformation or improve deformation conditions
(1) Carburizing should be as uniform as possible to avoid and reduce the uneven structure and stress caused by uneven carburizing.
(2) Choose the appropriate quenchant, control the oil temperature (> 100 ℃), control the final cooling time and temperature, and cool rapidly at high temperature, and slowly at low temperature martensitic transformation area.
(3) Cold and hot processing coordination, pre deformation compensation, i.e. adjust the machining allowance, leave less allowance for expansion, and leave more allowance for contraction. For example, before carburizing, the spiral angle of large helical gear is deliberately deviated from a certain angle (in the opposite direction of deformation), and the spiral angle of shaft type is always increased after carburizing, so when milling teeth, the angle is intentionally reduced, and the quenching deformation is just restored to the required angle. Generally, for the large gear shaft with deep carburizing and quenching, the spiral angle of the full length of the tooth width should deviate by 1mm, as shown in Fig. 3.
Improve the cooling uniformity of the upper, middle and lower parts of the gear, and make the structural transformation tend to be synchronous. It can spray oil in the middle section of the gear shaft to accelerate the cooling, and it can also improve the uneven cooling speed of the end section and the middle section of the gear by heating the upper and lower end faces of the gear.
Through the production practice and theoretical analysis in recent years, we have preliminarily mastered the deformation law 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 heat treatment process and practical operation, as well as the complex deformation after carburizing and quenching of type gears and gear shafts, further exploration and research are still needed.