Investigating the fracture toughness of weld in S355 KT-40 offshore jacket leg using scanning electron microscopy and nanoscale modelling

Welds are critical in cyclically loaded offshore jackets. This structure is utilized in wind energy farms and oil and gas processing. The study evaluates welds by welding experiments utilizing S355 KT-40 72 mm thick welded by flux-cored arc welding with gas-shielding (FCAW-GS). The investigation focuses on the grain-coarsened heat-affected zone (GCHAZ) of the weld within the jacket’s leg, utilizing optical microscopy (OM), scanning electron microscopy (SEM) combined with electron backscatter diffraction (EBSD), and nanoscale simulations and modelling through both analytical and numerical methods. OM and SEM techniques provide data regarding microstructural phases. EBSD yields information regarding phase fractions, crystal structures, and lattice characteristics. Alpha (α)-iron body-centered cubic, constitutes 93% of the primary phase, whereas gamma (γ)-iron face-centered cubic, accounts for 0.16%. Analytical and numerical modelling utilize the second derivative of energy with respect to volume (d2E/dV2) through quadratic equations, exponential functions, and finite difference techniques, which are essential for determining the bulk modulus. The effective fracture toughness of the weld in the GCHAZ region is determined based on stress intensity factors, resulting in values of 117 and 118 MPa.√m, respectively. The modelling of fracture toughness presented in this study proved beneficial as a supplementary tool for physical fracture toughness testing.