Effect of initial forging temperature on mechanical properties and fatigue behavior of EA4T steel

An EA4T steel billet was forged at the different initial forging temperatures (1050, 1150, and 1250 °C). The microstructure and fatigue fracture of forged samples were observed by laser scanning confocal microscopy and scanning electron microscopy, respectively. The fatigue performance of samples forged at an initial forging temperature of 1250 °C was better owing to the presence of fine polygonal ferrite (PF). The difference in yield strength and ultimate tensile strength can be attributed to the average grain size of PF and acicular ferrite (AF) content, respectively. A relationship between the tensile strength and fatigue limit of EA4T steel is proposed. The mechanism of grain size and dislocation slip affecting the fatigue property was elucidated. Because the fine PF grains delayed the dislocations moving to the surface and inhibited the formation of intrusion and extrusion, premature cracks were not easily produced. This contributed to excellent fatigue behavior. Additionally, the pearlite colonies and fine PF delayed the crack growth rate and changed the crack propagation path. Studies on fatigue behavior provided a foundation for quality control and engineering application of EA4T steel.