Cause of Fracture on 18CrNiMo7-6A Steel Reducer Output Shaft

The reducer output shaft of a company fractured during operation. The causes of fracture on output shaft of 18CrNiMo7-6A steel were analyzed by means of chemical composition analysis, fracture analysis, metallographic examination, hardness test and tensile property test. The results show that the whole heat treatment process of the shaft was improper, the microstructure was not uniform, and thick feathery bainite formed in local area, which reduced the tensile properties of the shaft. The surface induction hardening process of the shaft was improper, and the hardening layer was too thin, which was far less than the standard.In addition, the surface of the shaft was not tempered sufficiently after quenching, which led to the instability of strengthening layer structure and large residual stress. Under the action of operating load, multiple fatigue sources generated on the stress transfer point of keyway and the surface of the shaft, and the cracks gradually propagated inward, eventually leading to the early fatigue fracture of the shaft.