Effect of Nb on the Microstructure and High-Cycle Fatigue Properties of the Coarse-Grained Heat-Affected Zone in Low-Carbon Microalloyed Steel

A comprehensive investigation was conducted into the microstructural evolution, high-cycle fatigue properties, and corresponding fatigue fracture mechanism of the simulated coarse-grained heat-affected zone (CGHAZ) in low-carbon microalloyed steel with different Nb contents. The results demonstrated that an increase in Nb content led to a higher density of both low-angle and high-angle grain boundaries (LAGBs and HAGBs), a reduction in the mean equivalent diameter (MED), and a refinement of the prior austenite grains (PAGs) in the CGHAZs. The crack initiation lifetimes accounted for over 97% of the total fatigue life in the CGHAZs, thereby establishing it as the dominant mechanism governing fatigue failure. The fatigue strength of the simulated CGHAZs exhibited a continuous increase from 212.6 MPa to 231.9 MPa as the Nb content was increased from 0.018 wt.% to 0.055 wt.%. The augmentation of Nb content has been demonstrated to be a successful strategy for enhancing the CGHAZ fatigue strength of low-carbon microalloyed steels.