With the medium frequency induction heating machine, you no longer have to worry about cracks and defects in the guide plate.

The guide plate is the main part of the small-pitch externally guided tooth chain. The characteristics of the meshing between the guide plate and the sprocket are small impact, low noise and high working reliability, so it is widely used in mechanical transmission and timing chain transmission. The technical requirements are that the guide plate hardness is 46-49HRC, the structure is fine and uniform tempered troostite, the workpiece must not have cracks and deformations, and the surface must be free of scratches, decarburization and other defects. The guide plate material is 40Mn steel, and the heat treatment process is heating at 850°C, methanol cracking protection, heating and quenching, then nitrate tempering and water cooling. During production, it was discovered that a few guide plates cracked, causing workpieces to be scrapped. After analyzing the causes of the defects, it was found that changing the tempering process to induction tempering using a medium frequency heating machine can effectively avoid the occurrence of such defects.

The inspection found that most of the cracks in the guide plate were located in the holes and connected to the hole walls, and there were also a few cracks in the workpiece between the two holes. The main crack is a thick line that penetrates the wall and is slightly curved, with a width of about 0.16-0.22mm. The secondary crack is herringbone-shaped and adjacent to the main crack, does not penetrate the wall, and has a width of 0.094mm. Fracture analysis and observation showed that the dark gray fracture was a crystalline brittle fracture, the fracture was flat, and a brown-yellow rust pit was found on the opposite surface of the fracture. No signs of plastic deformation were found on the fracture surface. The hardness test found that the hardness of the workpiece was relatively high, with a hardness of 45.5-51.2HRC, and 58.5% were greater than 49HRC. The hardness distribution of the same part was very uneven, and defects were obvious. Metallographic analysis found that the cracks were relatively large, with no decarburization layer and obvious non-metallic inclusions around the cracks. The cracks extended from the surface along the austenite grain boundaries, and the cracks became thinner inward, branches appeared, and the number of cracks increased.

Based on the above analysis, the following process improvement measures are proposed:

(1) Improve the tempering process to obtain the required strength and hardness, minimize residual stress, and reduce the hydrogen content in the steel. The tempering process is changed to use a medium frequency induction heating machine, the temperature is 340°C, the hardness of the workpiece after tempering is 46-48HRC, and the hardness uniformity is ≤2HRC.

(2) Improve the processing accuracy and matching accuracy of workpieces, improve the assembly process, and prevent interference assembly and other bad methods.

After adopting the above process improvement measures, cracks have never been found in the guide plate, the quality of the workpiece meets the technical requirements, and the production runs smoothly. What's even better is that this process is suitable for large-scale production and can greatly improve workers' production efficiency.