Abstract：An abnormal operation was detected in a wind power gearbox after service for three years and six months. The failure of broken gear tooth was found on one of planetary wheels after disassembly. The fracture of planetary wheel was analyzed by macro and micro morphology, chemical composition, mechanical properties, metallographic structure, residual stress and grain distribution. The fracture reason of planetary gear tooth was discussed. The analysis results showed that the chemical composition of matrix and the carbon content of gear tooth surface could meet the requirements of corresponding standard specifications. The mechanical properties of planetary gear also met the requirements of the standard specification. Moreover, no abnormity was observed for the metallographic structure of gear teeth carburizing layer and planetary gear matrix. It indicated that the carburizing and heat treatment processes of planetary wheels both met the use requirements of materials. The aggregation of large size nonmetallic inclusions in the cracking source area of the gear was the main reason for the cracking initiating and extension.
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