Fracture toughness behavior of the coarse-grained heat-affected zone in API 5L X80 steel welded joints produced by manual and robotic welding

This study investigates the influence of manual shielded metal arc welding (SMAW, referred to as M1) and robotic flux-cored arc welding (FCAW, referred to as R1) on the fracture toughness of welded joints in API 5L X80 steel. Single-edge notched bend (SE(B)) specimens were prepared according to ASTM E1820 and tested using the crack tip opening displacement (CTOD) method to quantify fracture resistance. The coarse-grained heat-affected zone (CGHAZ) exhibited significant reductions in fracture toughness. The CTOD of the R1 process is approximately 22% higher than that of the SMAW process, indicating superior fracture resistance. The more pronounced degradation observed in manual welds was attributed to higher heat input during the root pass, promoting excessive grain growth and the formation of brittle microstructures, including Widmanstätten ferrite and coarse polygonal ferrite. In contrast, the robotic process produced a more homogeneous bainitic-ferritic microstructure and reduced hardness variability due to better thermal control and faster cooling rates. These findings emphasize the critical role of welding thermal cycles in controlling HAZ microstructural evolution and demonstrate the superior performance of automated welding in preserving fracture resistance for high-performance pipeline applications.