Analysis and improvement measures of quenching cracks in powder metallurgy motorcycle gear high-frequency induction heating power supply

The motorcycle clutch is made of iron-based powder metallurgy. The workpiece is characterized by large radial size, small thickness and high position accuracy requirements for the central 7 holes. The gears are quenched using high-frequency induction heating power. The workpieces are quenched and cooled along with the quenching fixture. After quenching, the gears are tempered at 200°C %above.

The occurrence of cracks in the clutch gear during quenching and heating is related to the structural characteristics of the workpiece. According to the structure and world requirements of this gear, one hole with a diameter of 37mm and six holes with a diameter of 28mm are evenly distributed in the center. The structure of the gear causes uneven thickness of the gear. The narrowest point between the holes is only 3.5mm, which leads to the high-frequency induction heating process of the workpiece. Larger tissue stress and thermal stress appear between the heated parts regardless of medium heating. The thin-walled part between the holes is the weakest part of the gear. After cracks appear in the workpiece, fracture failure often occurs earlier here. On the other hand, there are gaps in powder metallurgy products. High-frequency induction heating is due to uneven heating. Stress concentration has formed in the angular gaps and cracks and Cui's cracks have propagated and cracked. At the same time, due to the uneven thickness and large radial size of the powder metallurgy workpiece, the powder is prone to not being filled in the thin wall. The uneven density of the workpiece is due to the different thermal conductivity of each part during high-frequency induction heating, which is the structure of the workpiece. The difference in stress and thermal stress in various parts increases, causing cracks and fracture failures in weak areas where the stress is concentrated.

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

(1) The vibration powder loading method is used to improve the density distribution so that the gears are evenly distributed throughout, preventing underfilling and reducing the occurrence of quenching and heating cracks.

(2) Slow down the heating speed and reduce the risk of cracks.

(3) Design and manufacture special fixtures for high-frequency induction heating and quenching of gears to reduce and eliminate high-frequency induction heating cracks during heat treatment. The special quenching fixture has improved the original quenching fixture. The outer end ring of the supporting plate is used as the supporting surface. There is no balance pressure block on the gear to reduce stress and the tendency of deformation and cracking. When the workpiece is heated, cooling water flows through the middle of the gear to alleviate the problem. Heat can reduce fractures in thin walls caused by uneven tissue stress and thermal stress or stress concentration caused by high-frequency induction heating.

After adopting the above-mentioned improved process measures in production, the cracks disappeared when the gear was quenched and heated by high-frequency induction heating power supply, and the workpiece qualification rate reached 95%, which met the technical requirements and production requirements.