Causes of gear cracking
The pitch drive system is placed in a high altitude, and the planetary gear of pitch reducer is in a closed environment, and there will be no foreign matter entering to cause impact and collision on the gear. The crack source is located under the top of the gear. The microstructure is coarse acicular martensite + retained austenite, martensite grade 7 and retained austenite grade 6 ~ 7. This kind of structure has high hardness and brittleness. Once the impact or collision of heavy load occurs, it is likely to collapse and crack. According to the metallographic inspection standard for carburized gear of automobile (QC / t262-1999) or the metallographic inspection standard for piston pin of internal combustion engine part II (JB / t8118.2-2011), the microstructure does not meet the requirements. At the same time, the manufacturer may consider the 20-year service conditions and consciously increase the carburized layer depth. As can be seen from Fig. 4, the carburized layer depth reaches about 1.6 mm, which is about 0.4-0.6 mm deeper than the requirements of traditional carburizing technology. Moreover, metallographic examination shows that the gear is not fully tempered after carburizing and quenching, resulting in additional quenching internal stress. In the final analysis, the main cause of the gear cracking is the quality defects of carburizing and quenching heat treatment process, so the same batch of products have the same use risk.
Suggestions for improvement
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.