Fatigue crack growth behaviors in hot-rolled low carbon steels: A comparison between ferrite-pearlite and ferrite-bainite microstructures

The roles of microstructure types in fatigue crack growth behaviors in ferrite-pearlite steel and ferrite-bainite steel were investigated. The ferrite-bainite dual-phase microstructure was obtained by intermediate heat treatment, conducted on ferrite-pearlite hot-rolled low carbon steel. This paper presents the results from investigation using constant stress-controlled fatigue tests with in-situ scanning electron microscopy (SEM), fatigue crack growth (FCG) rate tests, and fatigue fractography analysis. Microscopy images arrested by in-situ SEM showed that the fatigue crack propagation in F-P steel could become unstable more ealier compared with that in F-B steel. The fatigue cracks in ferrite-pearlite were more tortuous and could propagate more freely than that in ferrite-bainite microstructures. However, frequent crack branching were observed in ferrite-bainite steel and it indicated that the second hard bainite phase effectively retarded the crack propagation. The variation of FCG rate (da/dN) with stress intensity factor range (Delta K) for F-P and F-B steels was discussed within the Paris region. It was shown that FCG rate of F-P steel was higher than that of F-B steel. Moreover, the fatigue fracture surface analysis proved that grain boundaries could also play a role in the resistance of crack propagation. (C) 2012 Elsevier B.V. All rights reserved.