How to reduce quenching deformation and cracking in gear processing
2017-03-01
How to reduce quenching deformation and cracking in gear processing
The purpose of heat treatment of gear processed products is to improve the hardness and wear resistance of the gears, thereby increasing the service life of the gears, but improper quenching can also cause cracks and deformations of the gear processed products, so how to reduce quenching cracks and deformations?
First of all, it is necessary to select the gear material correctly and design the tooth shape reasonably. For gears with complex shapes, it is better to choose gear steels with higher hardenability so that they can be cooled in a slow-cooling quenching medium to reduce internal stress. For gears with higher precision requirements, bottom deformed steel can be selected, and graded or austempered can be used. When designing gears, try to minimize the disparity in section thickness and avoid thin edges and sharp corners; make the transition as smooth as possible at the junction of gear thicknesses. For large gears, a separate inlay structure should be used to create conditions that can still be repaired by machining after heat treatment. . Secondly, the preparation heat treatment and casting must be carried out correctly. Gear blanks should be subjected to appropriate preliminary heat treatments, such as normalizing, annealing, and quenching and tempering treatments, to obtain a satisfactory structure to meet the requirements of mechanical processing and final heat treatment. When casting gear blanks, it is necessary to minimize casting defects. These defects are likely to cause cracking and deformation of the gear during quenching. After that, a reasonable heat treatment process must be used. When heating, try to be evenly heated to reduce the thermal stress during heating; for large gears, preheating should be used. Choose the right quenching temperature, and generally try to choose the lower limit temperature of the quenching. The quenching medium and cooling method should be selected correctly. Under the premise of meeting the performance requirements, a milder quenching medium should be selected, or methods such as graded quenching and austempering should be used.
After the large gear is processed and quenched, there are many small areas on the gear that have insufficient hardness. These areas are called soft cushions. The soft spot is often the center of gear wear or fatigue fracture, which reduces the service life of the gear. Therefore, no soft spot is allowed on the finished gear. The main reasons for soft spots are:
1. The metallographic structure of the gear is not uniform, such as carbide segregation, band structure or bulk ferrite in the steel. In this case, forging or appropriate preliminary heat treatment should be carried out before quenching to make the structure as uniform as possible.
2. After partial decarburization or carburization of the gear surface, the surface carbon concentration is uneven, and the low-carbon zone becomes a soft spot after quenching.
3. Insufficient cooling capacity of the quenching medium, such as oil or soap mixed with impurities in the water, will easily cause soft spots on the gear.
4. Improper operation, such as the surface of the workpiece is dirty, uncleaned oxide scale or dirt; the workpiece is immersed in the quenching medium, and the movement is not sufficient, etc., soft spots will be formed.
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