High-Cycle Fatigue Behavior and Fracture Mechanism of Q370qD Bridge Steel Subjected to Different Stress Levels

In engineering practice, fatigue failure is the primary failure mode of bridge steel, which is a brittle fracture without obvious symptoms. This work characterizes the microstructure of Q370qD steel before and after fatigue testing using a series of characterization techniques, including optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and electron back-scatter diffraction (EBSD). The fatigue fracture mechanism of Q370qD steel at different stress levels is further investigated. Results indicate that the fatigue strength of the S-N curve (97.7% guarantee rate) at N=2×106 is 245 MPa for naturally linear fitting and 197 MPa for fixed-slope linear fitting. The initial microstructure is characterized by uniformly distributed Fe-C dendritic eutectic phases with the dendrite spacing of approximately 10.63 μm. The microstructure analysis near the fracture surface indicates that grain size and the percentage of low-angle grain boundaries significantly affect fatigue performance. The deformation band can drastically reduce the energy needed for crack initiation and provide a low-resistance path for crack propagation. Small-sized grains can hinder dislocation motion and delay crack initiation. In addition, crack initiation is dominant in high-cycle fatigue. Average kernel average misorientation (Ave. KAM) is a statistical representation of the overall dislocation structure and has a weak effect on fatigue performance. Consequently, the correlation between microfactors affecting fatigue performance can be expressed as deformation band > grain size > Ave. KAM. As the stress level increases, the {100} texture evolves from two pole density peaks, which deviate 45° from normal direction (ND) toward transverse direction (TD), to three pole density peaks extending along the loading direction. For the {111} texture, the pole density peaks parallel to ND gradually dismiss, and the other two pole density peaks that deviate 45° from ND to TD are retained. This study provides strong data support for the high-cycle fatigue behavior and fracture mechanism of bridge steel from the perspective of microstructure.