Abstract: The reasons for hair cracks appearance on LZ50 steel axle surface after machining were analyzed by means of metallographic examination, scanning electron microscope analysis and energy dispersive spectroscopy analysis. The results show that alumina inclusions exposed on surface of the axle steel when machining, which resulted in the formation of hair cracks. Based on the finding, the alumina inclusions in the steel could be reduced efficiently by protecting with argon sealing casting, so the magnetic powders inspection qualified rate of the axle steel was improved.