Effect of Quenching Temperature on Tensile Strength and Fatigue Behavior of an EA4T Steel

In this study, forged EA4T steel specimens are subjected to three quenching temperatures of 800, 860, and 900 °C. Microhardness tester, laser scanning confocal microscopy, and scanning electron microscopy (SEM) are employed to measure microhardness, metallographic structure, and fatigue fracture characteristics, respectively. Heat treatment is found to be necessary to improve mechanical properties and fatigue behavior of initial forgings, and the effects of microstructure and defect are significant. Hence, considering variations of grain types and roughness between the initial forgings and heat-treated specimens, the fatigue property with defects are modified for roughness-affected axles by an improved KT diagram. The results indicate that the samples quenching at 860 °C achieved better fatigue performance due to the presence of fine polygonal ferrite (PF), bainite, as well as martensite. The fatigue life prediction error for the roughness-affected axle is between 16 and 18%. All of the fracture mechanism, grain size, and dislocation slip with different quenching temperatures affecting the fatigue performance are analyzed. Because the volume fraction of the martensite and bainite is significantly higher reaching 57.48% and more refined grains are obtained, the dislocation slip to the surface is delayed at the quenching temperature of 860 °C. Thus, the fatigue behavior is superior. Moreover, the yield strength increased by 22% compared to that of the initial forgings. Studies on the effect of the heat treatment process on fatigue behavior of EA4T steel contribute with a practical reference to the engineering application of high-speed rail axles.