Abstract: During the production process of the Φ1 219 mm×27.5 mm X80M steel pipe,the outside perimeter of some pipes exceed the standard upper limit after the hydrostatic test.The reasons for the said nonconformance of the pipe are analyzed by means of checking the thickness of the steel plate,studying the chemical composition,tensile property,hardness and microstructure,etc.of the bulging pipe.As a result,it is identified that the blocky ferrite proportion in the microstructure of the base metal of the bulging pipe is higher than that of the other pipes.This microstructure difference leads to an obvious yield platform of the tensile curve of the steel plate,and poor deformation enhancement ability as well,which in turn causes bulging of the pipe as undergone the hydrostatic test.