Microstructure and Effect of Orientation on Toughness of X65 Steel Pipe Electric Resistance Seam Weld

Abstract This study investigates the microstructure and toughness of a commercial electric resistance welded (ERW) X65 pipe by characterizing both the base metal (BM) and the bond line (BL) or fusion line. The analysis aimed to correlate these microstructural features with the material toughness. To evaluate how crack orientation affects toughness, Charpy and square-sectioned B×B single-edge-notched bend (SEB) specimens were tested in both the Charpy lower-shelf (brittle cleavage) and upper-shelf (ductile) regions. The findings revealed that the ERW weld lacked evidence of proper post-weld heat treatment (PWHT) or may not have undergone any PWHT, contributing to poor toughness. The microstructure displayed inhomogeneous microstructure, large irregular ferrite grains, a high density of preferential cleavage planes parallel to the fracture plane, and the presence of hard phases and inclusions. These factors likely accounted for the material’s low toughness. Fractographic analysis indicated that cleavage fracture initiated at inclusions and hard phases present at the BL. In one case a flaw contributed significantly to fracture initiation. The study also showed that crack orientation had minimal influence on Charpy absorbed energy (CVN) and fracture toughness (J-integral). Toughness in through-thickness-notched (TTN) specimens was either comparable to or slightly lower than in surface-notched (SN) specimens. This supports the use of TTN Charpy specimens for conservative qualification of welds.