In the whole manufacturing process, such as design, material, forging, machining, heat treatment, grinding and so on, the distortion of gear and gear shaft of reducer is not caused by carburizing and quenching, so the distortion control is a systematic collaborative process. This paper only discusses the control and correction of carburizing and quenching distortion.
1. Preliminary heat treatment
Before carburizing and quenching, normalizing or tempering should be carried out to improve the structure and reduce the distortion. In general, the gear and gear shaft with larger size should be treated by quenching and tempering. On the one hand, it is conducive to cutting, and on the other hand, the microstructure after quenching and tempering is more uniform and reasonable. The smaller size gear and gear shaft should be normalized.
2. Carburizing temperature control
Carburizing temperature range is very wide. In order to reduce distortion, try to use a lower carburizing temperature. For the shallow layer, the carburizing or carbonitriding at 860-880 ℃ can be used to reduce the distortion and control the depth of the layer. For the deeper requirements of carburizing layer, considering the long carburizing time and equipment utilization, the carburizing temperature should be higher. Generally, the carburizing temperature is 900-920 ℃, the carburizing distortion above 920 ℃ will increase obviously, and the service life of tooling will be shortened obviously, so it is not suitable to use. If the carburizing layer is required to be deep, but it is easy to be distorted and requires high distortion, a lower carburizing temperature should also be used. Carburizing temperature and distortion are a pair of contradictions, which should be used flexibly and considered comprehensively.
3. Tooling design and charging method
Improper design and use of tooling and incorrect charging mode are the main causes of distortion and the key process to reduce the distortion of gear and gear shaft. In order to grasp this process, different tools should be designed according to the shape, size and distortion of the gear shaft. Tooling design should be scientific and reasonable, easy to use, durable. During loading, different tooling and mounting methods shall be selected according to the shape, size and distortion requirements of gear and gear shaft, and attention shall be paid to whether the flow of medium from top to bottom is uniform and unobstructed. Gears and gear shafts shall be vertically hung on the tooling, with equal distance, not close together, and clearance shall be reserved to avoid uneven cooling and warping, bending or ellipse. If the large gear cannot be hung vertically and needs to be placed in parallel, it shall be placed on the flat tooling which can make the medium flow smoothly. The gear with large inner diameter can easily become ellipse in vertical hanging, and should also be placed on special tooling horizontally. The distorted tooling shall be reworked and leveled in time.
4. Heating mode
In general, it can be directly heated to carburizing temperature after entering the furnace. However, the gears and shafts with long and thin parts and great thickness difference should be heated slowly or preheated at 600-700 ℃, which is beneficial to reduce distortion.
5. Quenching temperature
The higher the quenching temperature, the greater the distortion tendency. In order to reduce distortion, the quenching temperature should be reduced as much as possible. After carburizing, direct quenching is generally adopted. The secondary heating quenching is not only distortion and high cost, but also rarely used now. The lower the quenching temperature, the smaller the distortion. According to Ar3 transformation point of various materials, the appropriate quenching temperature can be selected to ensure the hardness of the core. The quenching temperature should not be too low, otherwise too much ferrite will appear in the core and the required core hardness will not be achieved. It is suitable to choose 800 ~ 830 ℃ quenching temperature.
6. Cooling mode
Oil cooling is adopted after carburizing. What kind of cold speed oil should be used should be considered comprehensively according to the size, shape and technical requirements of gear and gear shaft. If the size is large, the shape is simple, and it is not easy to be distorted, use fast oil; otherwise, use slow oil. The higher the oil temperature is, the smaller the distortion is. In the range of oil temperature, try to use high oil temperature to reduce the distortion. Choosing appropriate stirring speed is beneficial to reduce distortion. The faster the stirring speed, the greater the distortion. As long as the technical requirements can be guaranteed, lower mixing speed shall be adopted as far as possible. After oil cooling, wash and temper again when it is cooled to room temperature in air to reduce retained austenite and avoid grinding distortion and cracks.
