On the fatigue ductile-brittle transition of EA4T steel for high-speed railway axle

Abstract

EA4T is a widely used high-speed railway axle steel with BCC crystal structure. It has the characteristics of ductile-brittle transition under single impact load, but whether it has the ductile-brittle transition under cyclic load has not been investigated. For this reason, the rotating bending fatigue tests (stress ratio of R = −1) for EA4T steel were carried out at RT, −40 °C, −80 °C and −120 °C, respectively. For all temperatures tested, except −120 °C, the fatigue strength increases with the drop of temperature. When the temperature is −120 °C, the fatigue strength at lower stress level was higher than that at −80 °C, whereas, the fatigue strength at higher stress level was lower than that at −80 °C. Based on the above, the mechanism of low temperature strengthening of fatigue performance was explained. When the temperature was higher than TFDBT (fatigue ductile-brittle transition temperature), lower temperature leads to increased flow stress, and then, the higher fatigue performance. When the temperature was lower than TFDBT, the longitudinal tensile stress near the crack-tip increased and the location of the maximum normal stress (σyymax) was closer to the crack tip with the drop of temperature, resulting in σyy⩾σf was satisfied at the characteristic distance, and then, cleavage fracture occurred. In addition, the FATT50 (Fracture Appearance Transition Temperature) of EA4T was −35.46 °C determined by impact test, which was higher than the TFDBT. Because the strain rate under impact load is higher, which results in higher yield stress and strain hardening exponent under impact load.