After ultrasonic cleaning, the fracture surface of the fractured gear can clearly see the crack source area and the reverse crack source on the top sub surface of the rack tooth, showing radiation pattern, occupying a large area of crack growth area. The crack propagation zone shows the traces left by the crack front propagation, and the radiation pattern takes up a large area, which indicates that the crack propagates rapidly and the gear crack has the characteristics of brittle fracture. In addition, on the opposite side of the crack source, there is a trace left by the impact of hard objects, which proves that the gear is crushed or collided first and then fractured.
In order to facilitate analysis, wire cutting method is used to take tensile, impact and metallographic samples. The sampling position and corresponding samples, metallographic observation of grinding surface, and surface micro Vickers hardness are measured on metallographic samples. The chemical composition sample is drilled at the center of the gear.
The key to prevent similar accidents is to improve the carburizing, quenching and tempering quality of planetary gear of pitch reducer. It is suggested to select the carburizing process of secondary quenching method and adjust the technical parameters of carburizing layer depth properly. In order to make tempering more sufficient, uniform and effective, timely tempering should be carried out after carburizing and quenching. The tempering medium is oil or salt bath. The residual austenite at the top of rack after carburizing and quenching is an unstable structure. When the stress is large, it will induce martensitic transformation, cause volume expansion, destroy the original dimensional accuracy and increase the risk of cracking. The cryogenic treatment process should be increased to reduce the content of retained austenite in carburized and quenched layer.