The causes and practical effects of macrostructure “bubbles” in WB36 thick wall seamless steel pipe are analyzed and studied by means of scanning electron microscope

magnetic particle and ultrasonic flaw detection and performance test. The results show that the main reasons for the formation of macrostructure bubbles are poor deoxidation

high pouring temperature

and damp of ladle and refractory materials etc

the bubbles are short linear dark stripes at the macro

and circular or elliptical holes in the micro

it is mainly distributed near the inner wall to 1/2 wall thickness of the steel pipe corresponding to ingot head near hot top. Slight bubbles cannot be detected by magnetic particle

penetration and ultrasonic flaw detection

which has little impact on the mechanical properties of the steel pipe at room temperature. With controlling casting temperature of WB36 steel below 1590℃

holding time of VD refining at 67 Pa should not be less than 15 min

and moisture content of heating agent and thermal insulation material less than 0. 1%

the generation of macrostructure bubble defects can be effectively prevented.