Abstract: Electroslag welding involves high heat input, which may cause hardening of the weld metal (WM) and heat-affected zone (HAZ). Additionally, the electroslag weld is inaccessible, making it difficult for quality control. In this paper, electroslag welded joints made of Q355B steel were designed and fabricated. Metallographic analysis revealed that the electroslag welding has wide fusion lines and HAZs. Welding defects were found in the electroslag weld. Tensile tests were conducted on smooth round bar specimens of the WM and HAZs of the electroslag welding. Mechanical properties and constitutive relationships of the materials were obtained. Notched round bar specimens were fabricated for the WM and HAZs of the electroslag welding and tested under monotonic tension. Parameters of the void growth model were calibrated for the materials based on test results and finite element simulations of the tests. The resistance of the WM and HAZs of the electroslag welding to ductile fracture was evaluated.