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

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 T_FDBT (fatigue ductile-brittle transition temperature), lower temperature leads to increased flow stress, and then, the higher fatigue performance. When the temperature was lower than T_FDBT, the longitudinal tensile stress near the crack-tip increased and the location of the maximum normal stress (σ_yymax) was closer to the crack tip with the drop of temperature, resulting in σ_yy⩾σ_f 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 T_FDBT. Because the strain rate under impact load is higher, which results in higher yield stress and strain hardening exponent under impact load.