Abstract: After heat treatment, magnetic particle testing was perofrmed on GCr15 bearing steel blade ring, it was found that the magnetic particle aggregation along the axial direction occurred on its outer surface. The surface returned to normal after wiping the magnetic powder. Using methods such as chemical composition analysis, macro observation, metallographic inspection and energy spectrum analysis, the reasons for the accumulation of magnetic particles on the outer surface of blade ring were analyzed. The results showed that the blade ring was quenched with multiple stacks and clamps. The quenching medium was easy to remain in the gap between blade rings and was difficult to clean. The chlorine ions of quenching medium induced corrosion, and then pitting pits were formed at a distance of 8. 9 μm from the outer surface of the blade ring, magnetic particle aggregation was caused by pitting corrosion.