Effect of Structural State of Metal in Weld Fusion Zone of Large-Diameter pipes on Fracture Mechanism and Results of Crack-Resistance (CTOD) Tests

We describe a study of fractures and metal structure in the fusion zone (FZ) and weld center (WC) for longitudinal welds and base metal (BM) in large-diameter pipe with strength class K60 (Grade X70) tested for crack resistance including a determination of the crack tip opening displacement (CTOD) on through-cut SENB samples (N-P per BS EN ISO 15653). Tests indicated that CTOD –20 (at temperature–20 °C) is generally more stable for BM and WC metal than for FZ metal. The effect of crack front displacement from the specified location on measured CTOD –20 values was studied for FZ samples. The CTOD –20 values for the FZ metal were found to depend on the failure mechanism caused by initiation of the fatigue crack, including maximum fatigue-crack length and maximum ductile-failure segment length. The structure of the FZ metal along the fatigue crack front was studied at various crack resistance levels using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The failure mechanism and resultant CTOD values for FZs in double-sided welds depend not only on the structural characteristics of the metal in the heat-affected zone (HAZ) (ratio of lath-like bainite to granular bainite, the martensitic-austenitic (MA) component and cementite morphology), but also on whether or not a fatigue crack is present in the section of pipe base metal where the deformation is largest. This differentiates the CTOD samples from the KC V samples. The fact that the BM area is located at the crack front has a substantial effect on the fracture mechanism in a FZ CTOD.