Mutual strengthening by fine MX precipitation and solute segregation on dislocations during complex low-cycle fatigue to simulate the seismic/fire situation in bainitic H-section steel

Abstract

In this study, a low carbon bainitic H-section steel alloyed finely with Mo, V, Ti, and Nb was developed for applications requiring both seismic and fire resistance. To assess its structural integrity following earthquake and fire event, a combination of room-temperature low-cycle fatigue (LCF) testing up to 10 cycles (to simulate earthquake conditions) and thermal exposure at 600 °C for 2 h (to simulate post-earthquake fire scenario) was conducted. It was interesting that the specimens, which underwent the aforementioned complex LCF testing combined with thermal exposure, exhibited a fatigue life comparable to that observed after a single LCF test. Fine MX precipitates additionally precipitated during thermal exposure. Furthermore, atom probe tomography results indicated that the uniform formation of nano-sized (C,V,Mo,Nb)-rich clusters were discovered after thermal exposure, and their segregation along dislocation cores was found, forming Cottrell atmosphere. These strong hindering/dragging effects of excess solute atoms on dislocations facilitated the occurrence of serrated flow on the hysteresis loop during the 11th cycle immediately after thermal exposure. These results suggest that the mutual strengthening resulting from fine additional MX precipitation, solute clustering, and segregation on dislocations primarily contributes to the suppression of cyclic softening, ultimately leading to an excellent complex LCF resistance. © 2025 Elsevier Ltd