Influence of non-metallic inclusions on the fracture-toughness properties on the longitudinal welding of an API 5L steel pipeline

In previous works, it was demonstrated that the average fracture-toughness values were higher in the circumferential-longitudinal direction (CL) than in the circumferential-radial direction (CR) on the longitudinal submerged arc welding (SAW) in API 5L pipeline steel. Nevertheless, such differences in fracture-toughness values were attributed to the density distribution of microstructural phases and porosity, but the effect of non-metallic inclusions on this fracture property was not considered; therefore, the present work analyzed the effect of non-metallic inclusions on the fracture-toughness values by fractographic and electron diffraction scattered analyses. The results showed the presence of titanium (Ti), calcium (Ca), and aluminum (Al) constituents in the non-metallic inclusions for the CR direction of the pipeline. These elements tended to form compounds that promoted brittlefracture by cleavage, decreasing the toughness properties of the weld. For the CL direction analysis, the elements presented on the non-metallic inclusions were manganese (Mn), silicon (Si), and sulfur (S), which promoted the nucleation and coalescence of cavities, all of which were typical for ductile fracture. Calculations on the K_IC showed that the non-metallic inclusions with elements of Ti, Ca, and Al contributed to lower K_IC values on the CR direction under analysis.