Induction heat treatment has been widely used in the automotive, construction machinery, and other industries due to its advantages of fast heating speed, high efficiency, energy saving, and easy automation. Induction annealing is a common branch of the process, and this article focuses on the annealing process of the external thread of the active spiral bevel gear.
The active spiral bevel gear is assembled with the flange yoke. The flange yoke receives the power transmitted from the transfer case and transmits the power to the matching active spiral bevel gear through the spline connection. The active spiral bevel gear needs to withstand a torque of 2000 Nm.
Original Technical Requirements and Process of the Product
The raw material of the active spiral bevel gear is 20CrMoH steel in accordance with GB/T 5216 – 2014 “Structural Steels with Specified Hardenability Bands”. It requires carburizing and quenching treatment. The effective hardened layer depth at the pitch circle of the tooth part is required to reach 1.0 – 1.4 mm (550HV), the surface hardness is 58 – 63HRC, and the core hardness is 30 – 40HRC (evaluated at the intersection of the tooth root circle and the tooth center axis). The hardness of the external thread shall not exceed 45HRC. In addition, a strong shot peening treatment is finally performed in accordance with JB/T 10174 – 2000 “Quality Inspection Method for Strengthening Shot Peening of Steel and Iron Parts”. The surface coverage rate shall not be less than 200%, and the shot peening strength is evaluated as the arc height value of the Type A Almen test strip shall not be less than 0.44 mm, and the residual compressive stress index at the tooth root of the finished product shall not be less than 896.5 MPa.
During the new product trial production period of the sub-supplier, the external thread area was subjected to anti-seepage treatment before heat treatment to reduce the hardness of the external thread. During the road test, an abnormal fracture occurred in the R-angle area at the transition between the external thread and the external spline connection before the specified driving distance was reached. After the dissection and analysis of the failed part, it was found that the hardness of the local position of the external thread exceeded 48HRC. Due to the many factors involved in the anti-seepage link, such as the correct use of the anti-seepage coating and the human factor in the “man, machine, material, method, and environment”, and considering that the R-angle at the transition between the external thread and the external spline connection is an area where stress is prone to concentrate, after technical communication and obtaining the review and approval of the product customer, the process of reducing the hardness of the external thread was changed and optimized, from the heat treatment after the anti-seepage of the external thread to the direct heat treatment without anti-seepage, and then the induction annealing treatment was implemented on the external thread.
Problems in the Induction Annealing Process of the Product
The induction heat treatment equipment owned by the sub-supplier is the WZP60 fully solid-state induction heating equipment, with an input voltage of (380 ± 38) V, an oscillation frequency of 10 – 30 kHz, a water pressure of 0.15 – 0.3 MPa, and a water temperature of 10 – 45 °C. The debugging state of the induction operation is shown in Figure 1, and the induction coil is shown in Figure 2.
During the initial trial production of induction annealing, there were two quality problems: one was that the external thread area requiring induction annealing was manually sent into the induction coil for heating and annealing, resulting in poor stability of the hardness index after annealing; the other was that the induction heating parameters were not controlled properly, resulting in some workpieces not effectively reducing the hardness after induction annealing, but even increasing the hardness. Therefore, our factory and the sub-supplier jointly improved the operation process.
Optimization Measures for the Induction Annealing Process of the Product
First, a limit tooling was added to the simple induction positioning tooling, that is, a welded limit bolt was used to ensure the effective positioning of the extension position of the external thread, as shown in Figure 3. The active spiral bevel gear was placed in the tooling, its back end face was close to the limit bolt of the tooling, the center of the active spiral bevel gear must be aligned with the center of the inductor, and the threaded end of the active spiral bevel gear was placed in the position inside the induction coil (see Figure 4), and the threaded end was flush with the induction coil. The external spline tooth area needed to extend about 8 mm into the induction coil.
Second, the process parameters were optimized and adjusted to ensure that the working voltage was 380 V, the working current was 12 – 30 A, the total heating time was controlled at 11 s, and the heating frequency was determined to be 29 kHz. First, the heating test was carried out with the test material head to check and verify the process parameters and hardness test (see Figure 5), and then the formal workpiece was used to start the pedal switch to start heating, and the active spiral bevel gear needed to be manually rotated slowly to ensure the uniformity and consistency of heating. After heating, the active spiral bevel gear was taken out for air cooling. After completely cooling to room temperature, the thread end of the active spiral bevel gear was aligned with the center of the shaft for wire cutting, and the cutting length and thickness were 55 mm and 15 mm, respectively, as shown in Figure 6. The cut sample is shown in Figure 7. The wire-cut sample was sent to the metallographic inspection room for hardness testing.
After that, the P – 2T metallographic sample polishing machine was used for polishing and cleaning. After the sample was dried and blown dry, it was placed flat on the inspection platform of the microhardness tester, and the position and focal length were adjusted for testing. It was required to perform hardness testing at 3 points at 5 mm, 15 mm, and 25 mm of the thread and 0.1 mm away from the root of the thread tooth, and at 1 point at the R-angle, that is, 0.1 mm away from the end, and at 3 points at 5 mm and 15 mm of the external spline and 0.1 mm away from the root of the external spline tooth, as shown in Figures 8 and 9.
After the first piece inspection of each shift, one more piece was randomly inspected during the process and the hardness value was recorded. The results are shown in Tables 1 and 2.
Confirmation and Solidification of the Technical Indicators of the Product Induction Annealing
According to the above optimization results, after technical communication with the customer and obtaining approval, the hardness technical requirements of the active spiral bevel gear were adjusted as follows: the hardness at the external thread was 28 – 42HRC, the evaluation area of the external spline was 5 – 15 mm away from the end face, and its hardness was 42 – 56HRC, while the 15 – 25 mm was the transition area for reference only, as shown in Figure 10.
After the induction annealing process, the entire length of 29 mm of the external thread at M22 × 1.56g was inspected and the thread was processed. The die was inserted into the thread of the active spiral bevel gear and the perpendicularity to the axis of the bevel gear was noted. The die was rotated evenly, and after the thread processing, the thread ring gauge was used to detect the thread to ensure the qualified parts were transferred.
After the above measures were solidified, the abnormal fracture at the R-angle at the transition between the external thread and the external spline connection no longer occurred, and the road test passed. Currently, mass production has been realized.
Conclusion
For the induction annealing heat treatment of similar parts, our company also made the following suggestions to the sub-supplier:
- This process must fix the operating personnel to ensure the qualified and stable quality of the product.
- It should be noted that when the ambient temperature is < 10 °C, the red-hot active spiral bevel gear needs to be placed in the asbestos insulation box for slow cooling to prevent cracks due to too fast cooling speed.
- The upgrade of the induction equipment needs to be considered. Currently, the induction equipment has high production efficiency and can realize the preventive control of the heating temperature, alarm, automatic separation of high and low temperature products, and other functions. The real-time online monitoring of the operating status and parameters of the power supply (such as quenching energy, voltage, current, power, frequency, time, and temperature) can truly be realized. More optimized equipment is needed to ensure the quality stability of large quantities of products.
The above suggestions have been adopted and solidified in the operation documents of the sub-supplier. After implementation, the product quality is stable, and no mass quality problems have occurred again.
In summary, the optimization of the induction annealing process for the external thread of the active spiral bevel gear has effectively solved the problems of abnormal fracture and hardness exceeding the standard. By adding a limit tooling and adjusting the process parameters, the stability and consistency of the hardness of the product have been improved. At the same time, the confirmation and solidification of the technical indicators, as well as the suggestions for the sub-supplier, have ensured the quality and reliability of the product in mass production. This case provides a reference for the improvement of the induction annealing process of similar products.
Here is a summary table of the key points:
| Key Points | Details |
|---|---|
| Product Requirements | Active spiral bevel gear made of 20CrMoH steel, requiring carburizing and quenching treatment. Hardness requirements: tooth part surface hardness 58 – 63HRC, core hardness 30 – 40HRC, external thread hardness not exceeding 45HRC. Final shot peening treatment with specific surface coverage and shot peening strength requirements. |
| Original Process and Problems | Initially, external thread area was subjected to anti-seepage treatment before heat treatment, but abnormal fracture occurred at the R-angle due to local hardness exceeding 48HRC. The process was changed to direct heat treatment without anti-seepage followed by induction annealing. Problems during induction annealing included poor hardness stability and hardness increase. |
| Optimization Measures | Added a limit tooling to ensure accurate positioning of the external thread. Adjusted process parameters: working voltage 380 V, working current 12 – 30 A, total heating time 11 s, heating frequency 29 kHz. Manual rotation of the gear for uniform heating. Cut and test samples for hardness at specific locations. |
| Technical Indicators Confirmation | Adjusted hardness technical requirements: external thread hardness 28 – 42HRC, external spline hardness 42 – 56HRC at 5 – 15 mm from the end face. Inspected and processed the external thread after annealing. |
| Conclusion and Suggestions | Abnormal fracture no longer occurred after optimization, and mass production was realized. Suggestions for the sub-supplier include fixing operators, slow cooling at low temperatures, and considering equipment upgrade for quality stability. |
Further Discussion on the Improvement of Induction Annealing Process
In addition to the measures mentioned above, further considerations can be made to enhance the induction annealing process and ensure the quality of the active spiral bevel gear.
One aspect to focus on is the material properties of the gear. Understanding the characteristics of 20CrMoH steel and its response to induction annealing is crucial. This includes studying the phase transformations that occur during the annealing process and how they affect the hardness and microstructure of the material. For example, the heating and cooling rates can have a significant impact on the formation of different phases, which in turn can influence the mechanical properties of the gear.
Another important factor is the design of the induction coil. The shape and size of the coil can affect the distribution of the magnetic field and the heating pattern on the gear. Optimizing the coil design can ensure more uniform heating of the external thread and improve the efficiency of the annealing process. Additionally, exploring different types of induction coils or using multiple coils in combination may provide better results.
The control of the annealing atmosphere is also worth considering. The presence of certain gases or the level of vacuum can affect the oxidation and decarburization of the surface during annealing. Proper atmosphere control can help maintain the surface quality of the gear and ensure the accuracy of the hardness measurements.
Furthermore, implementing real-time monitoring and feedback systems during the induction annealing process can be beneficial. Using sensors to measure temperature, hardness, and other parameters in real-time can allow for immediate adjustments to the process parameters if any deviations are detected. This can help prevent the production of defective parts and ensure the consistency of the product quality.
In terms of quality control, it is essential to establish a comprehensive inspection plan. Not only should the hardness be tested at specific locations, but other properties such as the microstructure, residual stress, and surface integrity should also be evaluated. This can provide a more complete picture of the quality of the gear and identify any potential issues that may not be apparent from hardness testing alone.
Comparison with Other Annealing Methods
To gain a better understanding of the effectiveness of the induction annealing process, it can be compared with other annealing methods commonly used in the industry. For instance, furnace annealing is a traditional method that involves heating the entire part in a furnace to a specific temperature and then slowly cooling it. While it can provide uniform heating, it may be less energy-efficient and take longer processing times compared to induction annealing.
Another method is laser annealing, which uses a laser beam to heat the specific areas of the part. This method offers precise control over the heating zone but may be more expensive and suitable for certain applications only.
By comparing the advantages and disadvantages of these methods, the suitability of induction annealing for the active spiral bevel gear can be further evaluated. Factors such as production volume, cost, and quality requirements need to be taken into account when choosing the most appropriate annealing method.
Case Studies and Examples
Looking at case studies from other companies or industries that have implemented similar improvements in their induction annealing processes can provide valuable insights. Sharing these examples can help illustrate the potential benefits and challenges that may be encountered during the optimization process.
For instance, a case study could show how a company successfully reduced the hardness variation in their gears by implementing advanced control systems for the induction annealing process. Another example might demonstrate how careful selection of the induction coil and process parameters led to improved efficiency and product quality.
These case studies can serve as references and inspiration for further improvements in the induction annealing process of the active spiral bevel gear.
Future Trends and Developments
As technology continues to advance, there are likely to be new developments in the field of induction annealing. For example, the use of artificial intelligence and machine learning algorithms to optimize the process parameters based on real-time data could become more common. This could lead to more precise control over the annealing process and further improvements in product quality.
Additionally, the development of new materials or coatings that can enhance the performance of the gears during induction annealing may also emerge. These advancements could open up new possibilities for improving the efficiency and reliability of the manufacturing process.
In conclusion, the improvement of the induction annealing process for the active spiral bevel gear is an ongoing effort that requires continuous optimization and innovation. By considering factors such as material properties, coil design, atmosphere control, quality control, and comparing with other methods, as well as staying updated on future trends, the quality and performance of the gears can be further enhanced, meeting the increasingly demanding requirements of the industry.
Here is a summary table of the additional points discussed:
| Key Points | Details |
|---|---|
| Material Properties | Study the characteristics of 20CrMoH steel and its response to induction annealing, including phase transformations and their impact on hardness and microstructure. |
| Induction Coil Design | Optimize the shape and size of the induction coil for more uniform heating and improved efficiency. Consider using multiple coils or different types of coils. |
| Annealing Atmosphere | Control the atmosphere during annealing to prevent oxidation and decarburization and maintain surface quality. |
| Real-time Monitoring and Feedback | Implement systems to measure temperature, hardness, and other parameters in real-time for immediate process adjustments. |
| Quality Control | Establish a comprehensive inspection plan to evaluate properties such as microstructure, residual stress, and surface integrity. |
| Comparison with Other Methods | Compare induction annealing with furnace annealing and laser annealing in terms of advantages, disadvantages, and suitability for the gear. |
| Case Studies | Share examples from other companies to illustrate the benefits and challenges of process improvement. |
| Future Trends | Look forward to developments such as the use of artificial intelligence and new materials for further optimization. |