The Effect of High Heat Input on the Microstructure and Impact Toughness of EH36 Steel Welded Joints

Ultra-high heat input welding offers high efficiency for large-scale offshore engineering, but excessive heat input can degrade low-temperature toughness. This study investigates the microstructural evolution and impact toughness of EH36 ship steel under high heat inputs (300–500 kJ/cm) using Gleeble-3500 thermal simulation, Charpy impact tests, and multi-scale characterization (OM, SEM, EBSD). Results show that impact toughness peaks at 400 kJ/cm, with surface and core energies reaching 343.33 J and 215.18 J, respectively. The optimal toughness is attributed to the formation of acicular ferrite and a high fraction of high-angle grain boundaries (up to 48.7%), which effectively inhibit crack propagation. These findings provide a practical basis for selecting heat input to balance welding efficiency and mechanical performance in marine steel fabrication.