Microstructural Evolution, Fracture Behavior, and Toughness Deterioration Mechanism of EH36 Steel Under Ultra‐High Heat Input Welding (600 kJ cm − 1 )

Ultra‐high heat input welding (600 kJ cm −1 ) is critical for efficient single‐pass joining of thick EH36 steel plates in marine/offshore engineering (e.g., polar ship hulls). However, it induces severe low‐temperature (−40 °C) toughness loss, with the underlying microstructure‐toughness relationship still unclear for 60 mm‐thick EH36. This study investigates 60 mm‐thick EH36 via Gleeble simulation, mechanical testing, and multiscale characterization (optical microscopy (OM), scanning electron microscope, electron probe microanalysis, X‐ray diffraction, electron backscatter diffraction). Results show the core exhibits an 85.28% impact energy reduction and complete cleavage fracture postsimulation, while the surface only has 52.13% toughness loss with residual dimples. Three synergistic factors cause core embrittlement: 1) grain coarsening; 2) C–Mn–Si segregation‐induced brittle phases; 3) reduced high‐angle grain boundary density. This work identifies the core as the critical toughness‐loss region and clarifies the embrittlement mechanism, providing guidance for optimizing ultra‐high heat input welding of thick EH36 steel in low‐temperature marine applications.