Cracks in the gearbox worm require improvement in processing technology

Adddate：2014/4/1　Views：1825

Many planetary gear reducers use different worm gear devices. For many worm gear manufacturers, they often encounter many problems during use, among which the worm gear has cracks or even a tendency to break during use, which poses a great threat to safety production. Worm gear process requirements: The hardness of the worm gear teeth should be greater than 45HRC, and the hardness of the core should not exceed 30HRC. The original heat treatment process uses 20Cr steel that has been carburized and then quenched in a salt furnace. Metallographic analysis revealed that the surface hardness of the worm meets the process requirements, but the core hardness is relatively high. It is evident that carbon during carburizing has diffused into the core, resulting in a high hardness and insufficient toughness. The increase in internal stress is the root cause of cracking and fracture tendency; And we believe that the two-step heat treatment of 20Cr carburizing followed by salt furnace quenching has a relatively high production cost and large deformation. After quenching, the large deformation often requires an additional straightening process, which further increases the production cycle and cost.
The first heat treatment process: salt furnace heating temperature of 840-850 ℃, insulation for 10 minutes, water cooling, and low-temperature tempering. Metallographic analysis and hardness testing: The hardness of the root of the worm gear teeth is above 50HRC, and the depth of the hardened layer is about 2.5mm. Magnetic particle inspection and metallographic analysis found that most of the worms have small cracks, which are located at the root of the teeth, with an average length of 4-5mm. This is because the diameter of the worm gear root is exactly within the dangerous size range of 45 steel conventional quenching.
The second heat treatment process: salt furnace sub heating at 780-790 ℃, holding for 10 minutes, water cooling, and low-temperature tempering. Metallographic analysis and hardness testing: The hardness of the worm gear teeth is above 45HRC, and the depth of the quenched layer is about 2mm. Microhardness testing found that the hardness gradient distribution is reasonable. Magnetic particle inspection and metallographic analysis did not reveal any quenching cracks in the worm gear.
The third heat treatment process: salt furnace heating temperature of 840-850 ℃, zero insulation for 1-2 minutes, water quenching and oil cooling, low-temperature tempering. Metallographic analysis and hardness testing: The hardness of the worm gear teeth is above 45HRC, and the depth of the hardened layer is about 2mm. Microhardness testing found that the hardness gradient distribution is reasonable. Magnetic particle inspection and metallographic analysis also did not detect quenching cracks.