Abstract: During the operation of a traction motor bearing, a ball inside it peeled off. Macroscopic observation, scanning electron microscopy and energy spectrum analysis, chemical composition analysis, hardness test, metallographic examination were used to analyze the causes of ball peeling. The results show that the root cause of the peeling of the ball was the carbide aggregation and Cr element segregation in the material. This kind of defect caused the early fatigue damage of the material under the action of repeated concentrated contact stress. With the continuous operation of the bearing, the multi-source fatigue effect continued to accumulate, which eventually led to the peeling of the ball.