Strain localization–driven deformation and damage evolution during low-cycle fatigue of flux-cored arc welded 460 MPa-class H-section steel

In this study, the microstructural characteristics and low-cycle fatigue (LCF) behavior of a 460 MPa-grade offshore H-section steel and its flux-cored arc welded (FCAW) joint were systematically investigated. The base metal consisted of a ferrite–pearlite microstructure with fine Nb–Ti–V MC-type precipitates, whereas the welded joint exhibited acicular ferrite–dominated weld metal and bainitic transformation products across the heat-affected zone. Hardness and tensile testing confirmed a typical overmatching condition, and all tensile and fatigue fractures occurred within the base metal, indicating sound weld integrity. Under strain-controlled LCF loading, the as-rolled steel showed typical cyclic softening followed by a clear saturation stage. In contrast, the welded joint exhibited continuous cyclic softening without saturation, accompanied by a slight reduction in fatigue life. Kernel average misorientation (KAM) analysis revealed relatively homogeneous strain distribution in the as-rolled steel, whereas the welded joint exhibited pronounced strain localization within the BM. This localization appeared as inclined deformation bands aligned with high-Schmid-factor slip systems, as confirmed by EBSD-based crystallographic analysis. Transmission electron microscope (TEM) observations further demonstrated that the welded joint developed planar slip-band structures with dislocation pile-up, exhibiting superficial resemblance to early-stage persistent slip band (PSB) formation. These PSB-like features serve as preferential sites for crack initiation and account for the absence of a saturation stage and the reduced fatigue life in the welded joint. Overall, this study provides mechanistic insight into strain partitioning, crystallographically governed deformation localization in the adjacent base metal, and fatigue damage evolution in FCAW joint of H-section steel under room-temperature LCF conditions.