Vol.:(0123456789) 1 3 Metals and Materials International https://doi.org/10.1007/s12540-020-00819-1 Infuence of Microstructure on Low‑Cycle and Extremely‑Low‑Cycle Fatigue Resistance of Low‑Carbon Steels Kyungmin Noh 1,2  · Seyed Amir Arsalan Shams 1  · Wooyeol Kim 1  · Jae Nam Kim 1  · Chong Soo Lee 1 Received: 25 June 2020 / Accepted: 1 July 2020 © The Korean Institute of Metals and Materials 2020 Abstract The goals of this study were to quantify and explain the efects of microstructure on the resistance of low-carbon steels to low-cycle fatigue and to extremely low-cycle fatigue (ELCF). Three diferent microstructures (ferrite–pearlite, ferrite–mar- tensite, and ferrite–bainite–martensite) were tested, and their fatigue properties were analyzed using the strain-based Cof- fn–Manson model and an energy-based model. According to the Cofn–Manson model, ferrite–pearlite showed the best ELCF resistance, whereas in the energy-based model that considers the efect of tensile strength ferrite–bainite–martensite revealed the highest ELCF resistance. At similar tensile strength, ferrite–bainite–martensite had longer ELCF life than fer- rite–martensite; the diference 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. Keywords Extremely low-cycle fatigue · Microstructure · Low carbon steel · Bainite · Martensite 1 Introduction Fatigue failure in steels is typically classifed depending on the number of cycles to failure. Ductile fracture that occurs in fewer than 10 4 to 10 5 cycles is called low cycle-fatigue (LCF) failure, and that occurs in fewer than 100 cycles is typically categorized as extremely-low cycle-fatigue (ELCF) failure [1]. ELCF can occur when a material is cyclically loaded with a large amount of strain amplitude. For exam- ple, a coiled tube (CT) commonly used for oil- or gas-well drilling experiences a bending strain of about 2%–3% during each operation, and shows extremely short fatigue life [2–4]. To predict fatigue life in the LCF and ELCF regimes, the Cofn–Manson (C-M) equation [5, 6] is usually applied. However, many experimental data of ELCF deviate significantly from the predictions of the C-M equation [7–11], especially at the high plastic strain amplitudes ( Δ p 2 ). The deviation has been attributed to a transition of fracture mode from the surface (that usually occurs in LCF loading condition) to the interior of the specimen [7] or to unsta- ble loading conditions [11]. A cumulative fatigue damage model [8] considered the observation that damage due to ductility exhaustion dominates the fatigue damage at large level Δ p 2 . The strain-life model has been further modifed by combining an exponential function with the power law that is used in the C-M relation [9], by partitioning the damage into ductile and cyclic damage components [10], or by using models of the growth and coalescence of plastic voids [12]. However, the efects of microstructural variables on ELCF life have rarely been investigated. Most of the studies are on the LCF or fatigue crack growth of low-carbon steels com- posed of ferrite/pearlite or ferrite/martensite phases [13–19]. In order to achieve cost reduction and higher performance in applications such as CT unit that is usually made of low-car- bon steels, it is desirable to use steels with higher strength and longer ELCF life. Fatigue life is greatly afected by a steel’s microstructure, so the microstructure of low-carbon steel must be optimized to exhibit excellent ELCF life. Typically, * Chong Soo Lee cslee@postech.ac.kr 1 Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 37673, Republic of Korea 2 Technical Research Laboratories, POSCO, Gwangyang 57807, Republic of Korea