Defects of J55 Casing Fusion Zone:
After ERW casing, it was found that the welded pipe in heat affected zone of fusion line was not qualified. Metallographic analysis, scanning electron microscopy and technological research were carried out. The results show that J55 pipe is in fusion line. There is a deep crack defect at the edge of the plate in the heat affected zone. Because of the small diameter of the pipeline and the insufficiency of repairing quantity in the pipeline manufacturing process, the crack defects of the users are obviously enlarged. Cracks on the edge of hot-rolled slab originate from subcutaneous bubbles at the corner and side of casting slab.