Abstract: The properties and causes of the crack of 12Cr2Ni4A elastic mounting case were studied by observing the maro and micro morphology of the faulty part. The properties and cause of the crack of the failure part of 12Cr2Ni4A was analyzed. The results show that the character of the crack is grinding stress crack, which occurs before phosphating treatment and does not propagate during engine testing. The cyanidation in the crack area is protected by integral copper plating, which makes complex compressive stress distribute at the cyanidation and non-cyanidation boundary. The subsequent chamfering and polishing processes reduce the compressive stress or generate tensile stress in the area. The process route lacks acid corrosion inspection and stress elimination measures, which eventally leads to cracks.