1. Defect content
The automobile rear axle gear is a hyperbolic bevel gear with complex shape. The deformation is inevitable in the process of carburizing heat treatment, and the gear deformation has a direct impact on the three performance indexes of gear strength, accuracy and noise. Moreover, in use, the driving and driven gears are required to have good contact, stable operation and easy installation, Therefore, controlling gear heat treatment deformation is a key project for many heat treatment manufacturers. Figure shows the deformation requirements of the main and driven gears of the rear axle of Dongfeng five ton vehicle. The flatness of the driven wheel and the roundness of the center hole shall be controlled, and the coaxiality of the driving wheel shall be controlled.
2. Defect causes
In the process of heat treatment, carburizing is heated to high temperature and quenching is cooled to low temperature. The thermal stress and structural stress produced in this process are the main causes of gear deformation; Uneven temperature and atmosphere in the furnace will cause different degrees of warpage and deformation of the gear; Unreasonable clamping method, deformation of quenching fixture and non-standard manual operation will also cause deformation to a certain extent.
3. Preventive measures
(1) heat treatment deformation is inevitable, but there are rules to follow. The selection of reasonable heating and cooling process, especially the selection of cooling process, will reduce the deformation to a great extent. Therefore, the appropriate quenching temperature, quenching oil temperature and oil flow rate should be selected, and the thermal stress and structural stress produced during heat treatment should be minimized under the condition of ensuring normal performance. Practice has proved that higher quenching medium temperature and appropriate stirring can help to make the cooling conditions of different parts of the gear and between gears in the same furnace tend to be consistent, and can greatly reduce the deformation.
(2) the clamping method shall be selected reasonably. The driven wheels of some manufacturers shall be installed parallel to the quenching table to solve the out of round deformation or the hanging method of vertical oil feeding shall be used to solve the plane deflection, but it is best to use the press for quenching, and the flatness and inner hole roundness shall be controlled within the allowable deformation range through the action of die. Special fixture is designed for quenching of driving wheel, and axial quenching mode is adopted to avoid warpage and deformation caused by self weight.
(3) the selection of quenching medium is also very helpful to control the deformation of gear. In the past, the cooling characteristics were mainly improved by changing the oil temperature of quenching oil. At present, some scholars have made some progress in using PAG water-soluble medium for gear quenching. The minimum heat treatment deformation is achieved by adjusting the concentration of PAG medium. However, its application in manufacturers is limited due to management, operation and maintenance.