Effect of heat input on microstructure evolution and fracture toughness of interlayer heat affected zone in ultra-high strength steel

This study investigates the effect of welding heat input on microstructural evolution and fracture toughness in the interlayer heat-affected zone (IHAZ) of ultra-high strength steel (UHSS). The IHAZ microstructure consists of acicular ferrite, bainite, and pearlite, with increasing heat input leading to reduced pearlite volume fraction and enlarged colonies spacing. Crystallographic analysis reveals that pearlite (ferrite and cementite lamellae) forms via single Bain group orientation through sympathetic nucleation and cooperative growth, while adjacent interlocking acicular ferrite exhibits multiple Bain groups, promoting high-angle grain boundaries (HAGBs). Crack initiation occurs preferentially at pearlite-ferrite interfaces due to plastic deformation incompatibility. Fracture toughness tests demonstrate that higher heat input (1.8 kJ/mm) significantly improves crack-tip opening displacement (CTOD) values (0.325 mm) compared to lower heat input (0.8 kJ/mm, 0.122 mm), attributed to reduced pearlite content and optimized crystallographic characteristics. These results provide critical insights for optimizing welding parameters to advance mechanical performance in UHSS welds.