1. Control the heating and cooling rate
When the gear material is heated to 450°C, it changes from an elastomer to a plastic body, making it easy to produce plastic deformation.
At the same time, the residual stress will also disappear due to recrystallization when it is slightly higher than 450 °C. Therefore, during rapid heating, due to the temperature difference between the inside and the outside of the gear, the outside becomes a plastic zone when it reaches 450 °C, and there is residual stress in the area with a lower internal temperature. After cooling, this area will deform.
Since it is difficult to achieve uniform and slow heating during the actual heat treatment operation, partial deformation can be well reduced by preheating treatment before carburizing, such as maintaining at 480 °C for one hour.
Strictly controlling the temperature uniformity, the carburizing furnace is controlled within +/- 10 °C, and the furnace temperature uniformity test is carried out on the carburizing furnace every year to discover and solve hidden dangers in time.
2. Use a lower carburizing temperature
To meet the requirements of the heat treatment process, try to use a lower carburizing temperature, which can slow down the grain growth, reduce the thermal deformation of the gear due to its gravity, and reduce the high-temperature strength loss of the gear. And the plastic resistance is enhanced, and the comprehensive ability of the gear to resist stress deformation and quenching deformation is enhanced, thereby reducing deformation.
3. Shorten carburizing time
To meet the gear production process requirements, the carburizing time should be shortened as much as possible, and the high-temperature creep should be reduced. For example, using infiltration technology, setting a higher carbon potential in the vital infiltration stage, and carburizing under saturated carbon potential to increase the carburizing speed can generally improve production efficiency by 20%.
The low-pressure vacuum carburizing technology can also be used, and the carburizing speed will be significantly improved, dramatically shortening the time to obtain the same carburized layer.
4. Lower the quenching temperature
According to the adequate thickness of the gear, to meet the core hardness requirements, the quenching temperature should be selected as low as possible to reduce the degree of undercooling. Chamol adopts the interval of 810-830℃.
Heat treatment experiment for bevel gear with the material of 20CrMoH:
Using the same multi-purpose quenching furnace and the same quenching oil, the quenching temperature was reduced from 870°C to 810°C, the core hardness decreased by 5-6HRC, and the deformation was significantly reduced.
The gear is rapidly cooled from high temperature to the quenching oil temperature. At this time, the control of deformation is mainly to reduce the temperature evenly during cooling to avoid excessive temperature differences between the upper and lower parts of the gear. The thermal stress during the quenching process is reduced, and the martensite transformation process is slowed down. Tissue stress is significantly reduced.
Lowering the quenching temperature can reduce thermal stress and facilitate participation in austenite production, and the structural focus is relatively small.
5. Choose mild quenching to cool the medium
Cooling is the critical process of quenching. It is related to the quality of quenching and is also the link that is most prone to problems in the quenching process. The influence on the deformation during the cooling process after metal quenching is also essential.
To ensure hardness, try to use quenching oil with higher viscosity because this type of quenching oil cools more slowly.
Using graded quenching can significantly reduce the thermal and structural stress generated during metal quenching, which is an effective way to reduce some gears with complex shapes.
Using this quenching method, since the temperature of each part of the gear has tended to be uniform before the martensitic transformation, and the martensitic transformation is completed under slow cooling conditions, this not only reduces the thermal stress of quenching but also significantly reduces the structural stress. Effectively reduce or avoid gear quenching deformation.
6. Optimize the stirring and flow direction of the cooling medium
Reducing deformation requires accumulating experience in the actual operation process. For gears with different shapes and sizes, other quenching methods and different quenching parameters should be selected according to the product’s specific structure and weight so that the gear’s cooling speed is uniform everywhere.
Field experiments and measured deformation values found the optimum quenching parameters. Factors that control the cooling rate in oil quenching: oil inlet temperature, stirring speed, quenching holding time, and oil outlet temperature; if high-pressure gas quenching is used for quenching, it is recommended to choose high-pressure nitrogen below 20bar, with different flow rates and flow directions to change you. Cooling conditions.
7. Normalizing annealing to refine grains
For gears with complex shapes and high precision requirements, sometimes it is necessary to add a normalizing process to the forging material or add a normalizing process to remove stress or a pre-vacuum normalizing process before precision turning to make the material structure more uniform.
Sometimes, we heat treatment parts by induction hardening or other heat treatment methods, and each heat treatment method has different process requirements, no matter what questions you have about heat treatment, please don’t hesitate to contact us.
