Fracture Separation Reason in Drop Weight Tearing Test for Nb/V X70 Pipeline Steel

The microstructure of crack tip and undeformed zone near the fracture was observed by optical microscope to analyze the fracture separation reason in drop weight tearing test (DWTT) for Nb/V X70 pipeline steel. It was found that the fracture separation was initiated from the side of band structure. Grain size and area percentage of pearlite were measured by picture analysis software. And the relationship of fracture separation and microstructure was found. The results showed that the central segregation band structure and pearlite band structure induced fracture separation. The band structure was disappear and grain size was thinning by TMCP technology optimizing, especially by accelerate cooling. Finally, the separation was eliminated.