Influence of Microstructure on Low-Cycle and Extremely-Low-Cycle Fatigue Resistance of Low-Carbon Steels

Abstract

The goals of this study were to quantify and explain the effects of microstructure on the resistance of low-carbon steels to low-cycle fatigue (LCF) and to extremely low-cycle fatigue (ELCF). Three different microstructures (ferrite-pearlite, ferrite-martensite, and ferrite-bainite-martensite) were tested, and their fatigue properties were analyzed using the strainbased Coffin-Manson model and an energy-based model. According to the Coffin-Manson model, ferrite-pearlite showed the best ELCF resistance, whereas in the energy-based model that considers the effect of tensile strength ferrite-bainite-martensite revealed the highest ELCF resistance. At similar tensile strength, ferrite-bainite-martensite had longer ELCF life than ferrite-martensite; the difference may be a result of the smaller strain incompatibility between bainite and ferrite than between ferrite and martensite. In all three microstructures, cracks initiated at the surface and propagated into the interior; this result indicates that fracture mode was not altered during cyclic loading at high strain amplitudes. Ferrite-martensite microstructure developed many sub-cracks surrounding a main crack; they could facilitate propagation of a main crack, and thereby degrade fatigue life at high strain amplitudes. On the other hand, welds made of ferrite-bainite-martensite steels were also tested to investigate their fatigue resistance. Although the ultimate tensile properties of the welds were similar to pre-tested ferrite-martensite and ferrite-bainite-martensite, their fatigue lives were shorter than the others. The fatigue fractures of welds occurred at the softened heat affected zone (HAZ) of the steel. Finite element simulation of the fatigue frature showed that the stress concentration occured at the most softened HAZ even in LCF regime. Finally, their fatigue lives were predicted by combining fatigue parameters with hardness distribution at HAZ.