Induction heat treatment has the advantages of low energy consumption, no emissions, no pollution, low cost, and is suitable for mass production, etc., and it is used more and more widely. However, for some large and irregular thin-walled parts, induction hardening has a large deformation problem. My company produces a spline connection dish after induction hardening exists large deformation, and has certain taper deformation, deformation of the follow-up to the finishing process to repair, this not only reduces the production efficiency, increased the cost of production, due to the surface hardening layer of process at the same time, reduces the surface compressive stress, the service life of the product. In this paper, by improving the process of induction heat treatment, the problems of large deformation and taper after quenching have been successfully solved, and the processing after quenching has been canceled.
1. Connection plate and induction heat treatment overview
The connecting plate is made of 40CrMn, the hardened layer depth is 4.0-6.0mm, and the surface hardness is 52-60HRC. The structure of the connecting plate is shown in Figure 1. The main technological process of the connecting disc is as follows: blanking → forging → normalizing → roughing → tempering → fine-tuning → gear shaper → rolling outer teeth → induction hardening → tempering → hard gear shaper.
According to the workpiece material, structure, relevant technical requirements, equipment, etc., determine the sensor structure and process of the connecting plate. The sensor structure is shown in Figure 2. The inductor body is made of the pure copper plate after processing by bending and welding, the inner side is covered with a magnetic conducting body made of silicon steel sheet, and the lower end is installed with a quenching spray device. The connection plate adopts the direct heating and quenching method. After the heating is completed, the sensor’s sprinkler is lifted to the original heating position to spray water for cooling. Due to the large size of the connecting plate, the power of the existing equipment is low, and the large heating area requires a long heating time, there is a large deformation situation.
The test items and results of the workpiece after induction quenching and tempering are shown in Table 1. The hardness meets the requirements, and the change of M value is about 0.10mm and the roundness is about 0.10mm. The workpiece appears in different degrees of elliptic deformation, and there is a certain taper of about 0.05mm. In order to solve the deformation problem, a hard gear shaping process was subsequently adopted to repair the deformation. After the repair, the roundness was controlled at about 0.01mm, and the tooth shape and tooth direction accuracy could reach level 8. The taper deformation is mainly manifested as flared mouth, with small upper and large lower sizes, as shown in FIG. 3.
Table 1 Test data of connection plate after induction quenching and tempering
Connection plate after cutting induction hardening, induction hardening of the distribution of the hardening layer as shown in figure 4, can be seen from the distribution of the hardening layer left deeper hardening layer, the right end of hardening layer shallow (block after cutting horizontally placed vertically, corresponding connection plate of the upper left side, right side corresponding to the bottom), and the taper deformation distribution of the corresponding test results. For this part, spline mainly plays a connecting role, which requires high roundness, but not high accuracy of tooth shape. If the size and roundness can meet the requirements after quenching, it is not necessary to carry out finishing.
2. Cause analysis and improvement measures
Connection plate of induction quenching deformation caused big problem is mainly due to the workpiece structure: spline workpiece in asymmetric structure, spoke at the lower end of the spline, induction heating heat spline spline nowadays end points to spokes conduction heat dissipation is more, causing reaches shallow heating of quenching temperature, quenching after hardening layer shallow, less martensite microstructure transformation, circumferential direction small shrinkage, on the other hand wheel has a certain supporting function, can prevent the bottom of the spline contraction deformation;And upper spline is in outstanding condition, is not attached directly to the spokes, unable to transfer heat directly to the spokes, and heat conduction of heat loss is less, to achieve the depth of the quenching temperature is relatively lower, after quench hardening layer is deeper, a large quantity of martensite microstructure transformation, spoke at the same time support small, circumferential direction shrinkage deformation (see figure 5), resulting in large deformation and the formation of taper.On the other hand, limited by the small power of the equipment, it is impossible to transfer a large power, so it is necessary to extend the heating time to reach the required quenching temperature, and the longer heating time will increase the deformation.At the same time, the sensor heating and the water jet cooling adopt a split structure. After the heating is completed, the sprinkler needs to be moved to the heating position for the water jet cooling. In this way, the moving process takes a period of time, which increases the heat conduction at the lower end and increases the deformation.
In view of the above reasons, our company decided to adopt the following improvement measures through discussion:
(1) Direct injection inductor is designed, which can realize direct water injection quenching after heating, shorten the interval time between heating and quenching, and help reduce deformation. The structure of direct injection inductor is shown in Figure 6.
(2) Improve the technological process, add induction heating preheating and uniform temperature processes.Preheating process of spline parts for a period of preheating (about 500 ℃, the heating temperature control does not affect the spokes of the mass organization), the mean temperature can transfer heat to the spokes in, make the spokes has a certain temperature, reduce the spokes and spline in the difference in temperature, reduce the thermal stress, and can also reduce the subsequent heating spline to heat transfer on both ends of the spokes, help to increase the bottom layer, improve the deformation situation.
3. Effect confirmation
Use the new design of the sensor and take increasing preheat temperature process production a batch of artifacts, test results are shown in table 2, meet the requirements, the surface hardness of quenching M value change quantity control within 0.02 mm, roundness control over 0.02 mm, the taper control within 0.01 mm, roundness and taper are controlled in a reasonable range, meet the requirements, can be installed after the induction heat treatment, do not need to follow-up for hard gear shaper repair deformation, tooth gear to the test results to achieve more than 9, also meet the requirements.
Table 2 Test data of connection plate after induction quenching and tempering
The distribution of hardened layer after cutting the workpiece is shown in Figure 7.It can be seen from Figure 7 that the hardening layer at both ends is more uniform, greatly improving the distribution of the hardening layer compared with the original process. The detection result of the hardening layer depth is 3.5mm, the hardened area structure is tempered Martensite, and the grain size is 9, as shown in Figure 8A. The structure of the spokes near the spline was tempered Soxhlet with a grain size of 8 grades, as shown in FIG. 8b. The increase of preheating and uniform temperature did not affect the metallographic structure there. Through the above analysis, it can be seen that the improved measurement results of size, metallographic structure, and hardened layer distribution meet the design requirements.
By improving the induction quenching deformation of large-sized irregular thin-walled parts, ZHENGZHOU KETCHAN ELECTRONIC CO.,LTD has improved the product induction heat treatment quality, reduced the deformation, canceled the subsequent finishing process, reduced the production cost, and improved the production efficiency, thus providing a reference for the production of similar parts
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