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.
Peng et al. (2026) studied this question.