J55 oil casing in the fusion zone of defect analysis of the causes of the ERW oil casing, pipe in the heat affected zone of the weld line occurred inspection unqualified phenomenon. The of metallography, scanning electron microscope (SEM) and process investigation analysis. The results show that: J55 welding pipe in the fusion line heat affected zone plate edge there are cracks in a certain depth, users of tube due to the smaller diameter and cutting edge weight shortage caused by the crack defects are obvious extensions, hot-rolled plate edge of crack source in continuous casting slab angle and lateral subcutaneous bubble.