Induction hardening of Luoyang East Axis Bearing has the advantages of small deformation of workpiece, high efficiency, energy saving and environmental protection, and easy automation, which has become a common surface heat treatment technology for bearing manufacturers. With the development of industrial technology, higher requirements are put forward for the bearing capacity and quality of gears. However, in the existing induction quenching technology, there is a problem that the tooth roots at both ends of the gear are not hardened, and bending fatigue cracks are easy to occur in the service process. Especially for heavy-duty bearings, in the case of unbalanced load, the unhardened gear teeth at both ends are prone to cracking failure. Therefore, realizing the hardening of bearings in the whole tooth width range can greatly improve our induction quenching technology level, enhance the bearing capacity and quality of gears, and generate significant economic benefits. By optimizing the induction quenching process, this paper solves the problem that the tooth root of the gear end face is not hardened but the tooth surface is overheated and melted, and realizes full tooth width hardening, which is applied to mass production. I. technical requirements for full tooth width hardening. Induction hardening with full tooth width: that is, the effective hardening layer is distributed within the full tooth width range of the gear teeth, and the layer depth and structure of the two end faces of the gear are required to be close to the middle part of the tooth width (in line with the drawing standard requirements). Considering the technical difficulties of induction heating process, the requirements for full tooth width hardening in domestic and foreign standards are not clear and loose at present, and the specific requirements are subject to the customer.
Among them, JB/T 9171-1999 "Gear Flame and Induction Quenching Technology and Quality Control" standard stipulates that within the range of 150mm tooth width, the distribution range of effective hardening layer is 80% of the tooth width, and no judgment is made within the range of 10% of the tooth width from both ends. Iso 6336-5: 2003 "Calculation of Bearing Capacity of Spur Gear Bearings and Helical Gears Part 5: Strength and Quality of Materials" requires that the depth of hardened layer should be covered within the range of full tooth width, but the specific requirements for the depth of layers at both ends are not clearly stated. Others, such as AGMA and DIN standards, have relatively loose requirements for full tooth width hardening. Within the range from the end face to one-time modulus or 1/8 tooth width, the layer depth is not required to be judged. Second, the bearing process status analysis. The problems of induction quenching process include: the temperature of eddy current concentrated heating at sharp corners is high, while the inner corners are not easy to heat. Therefore, in the process of induction heating, the gear is prone to overheat and burn at the sharp corners of the end tooth surface, while the root of the inner corner is not hardened or the hardened layer is not deep enough, as shown in Figure 1. Because the domestic and foreign standards have loose requirements on the hardened layer depth of the end face, the method of reducing heating power is usually adopted to prevent the bearing gear from appearing the obvious appearance quality defect of burning and melting of the end face, while ignoring the quality hidden danger that the hardened layer depth of the end face root is too shallow or not hardened. The bearing capacity and quality of gears are reduced, and there is a risk of early failure. To solve this problem, the induction quenching process was optimized by designing a new inductor structure and adjusting process parameters.
Fig. 1 Burning and melting of bearing end face tooth surface and deep distribution of hardened layer of tooth root. III. Optimization of Bearing Process. (1) The sensor structure is optimized. The existing sensor with profiling structure is scanned along the tooth profile, and the tooth surface and tooth root position are heated at the same time in the heating process. Under the influence of induction heating effect, the tooth root position of both end faces of the gear is not heated enough, resulting in unhardened or insufficient hardened layer depth, while the temperature of the pitch circle position of the tooth surface of the end face is too high, resulting in overheating and burning. Therefore, the technical difficulties are deeply analyzed, and by optimizing the structure of upper and lower guide plates and silicon steel sheets of the inductor, the disadvantage of simultaneous heating of tooth profile in the existing inductor structure can be solved. The upper guide plate and the lower guide plate of the inductor with optimized structure are in the structure of arc-removed oblique upward/downward triangle, and the added conductor part enhances the heating effect of tooth root. At the same time, the special oblique upward/downward triangle structure realizes the asynchronous heating of the tooth root and the tooth surface, so that the inductor can only heat the tooth root at the end face, thus avoiding the excessive temperature of the tooth surface at the end face, as shown in Figure 2. The problem of overheating and melting at the pitch circle position of the tooth surface of the end face can be avoided while the depth of the hardened layer of the tooth root of the bearing end face is increased. Fig. 2 Schematic structure of inductor.
(2) Optimization of bearing process parameters The main influencing factors of induction quenching and full tooth width hardening process include coupling gap, heating power, time and heating position, etc. By screening the influencing factors, focusing on analyzing the influencing factors such as preheating power, preheating time, heating power and heating position, the optimal combination of parameter factors can be obtained. On the basis of adopting the inductor with the above-mentioned new structure (used for Mn14 wind power inner ring gear), the problem that the tooth root of the end face is not hardened and the end face is overheated and melted is solved by optimizing the process parameters of induction quenching end face. Partial factor DOE test design is carried out on four influencing factors, namely preheating power X1, preheating time X2, heating power X3 and heating position X4. the output variables are the minimum hardened layer depth Y1 of the end tooth root and the grain size Y2 of the pitch circle position of the tooth surface. the test parameters and results are shown in table 1. Table 1 DOE test design parameters and results. According to the corresponding analysis of the test data, the relationship between the depth Y1 of hardened layer of tooth root and the pitch circle grain size Y2 and preheating power X1, preheating time X2, heating power X3 and heating position X4 is obtained by fitting.
The response optimizer is used to predict the optimal parameters, and the optimal process parameters are obtained as the center point, that is, the parameters numbered 4. The depth of hardened layer of tooth root is obviously increased, the grain size of tooth surface meets the requirements, and there is no overheating and melting. The repeatability of the central point parameters optimized by DOE test is verified, and the repeatability is very good, as shown in Table 2. Table 2 Repeatability test results of excellent parameters. IV.
Popularization and application. By designing a new structure inductor and optimizing the DOE test of induction quenching process parameters, the asynchronous heating of the root and tooth surface of the turntable bearing is realized, and the problem that the root of the gear end face is not hardened and the tooth surface is overheated and melted is solved. The whole tooth width hardening is realized, and the bearing capacity and quality of gears are improved. At present, this process has been used in batch production of Mn14 to Mn20 large modulus internal gear rings, covering 1.5 ~ 4 MW wind power gear boxes, and has been fully promoted and supplied to customers at home and abroad such as GE in batches.
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