Study of macroscopic fracture surface characteristics of spiral welded API X65 gas transportation pipeline steel

Fractography of drop weight tear test (DWTT) specimens has received great attention by researchers in recent years due to the complex fracture surface of this test specimen. In this research, macroscopic characteristics of fracture surface of spiral seam weld in API X65 pipeline steel are investigated for the first time using chevron-notched DWTT specimensTest specimens were machined from an actual steel pipe of API X65 grade with an outside diameter of 1219mm and wall thickness of 14.3mm. Then chevron notch of 5.1, 10 and 15mm depth was placed in the center of each specimen and test samples were fractured under dynamic loading of 7m/s. Fractography of the fracture surface of test specimen with 5.1mm notch depth (as typical of test samples) showed that cleavage flat fracture initiated from the notch root (where stress intensity factor was high). Cleavage fracture changed immediately to ductile shear fracture, deviated to one side of specimen and grew extensively in heat affected zone, and finally terminated in base metal. Delaminations were observed in shear fracture area almost parallel to crack growth direction. After that, shear lips and inverse fracture appeared in hammer impacted area. By calculating the percent shear area from standard formulations, it was found that test specimen had above 95% shear area, and ductile fracture was the dominant fracture mode implying the fitness of tested steel for application in high-pressure gas transportation pipelines.