Effect of monotonic mechanical properties on the low-cycle fatigue lifetime of carbide-free bainitic steels

Abstract

The influence of monotonic mechanical properties on the low-cycle fatigue (LCF) lifetime of carbide-free bainitic steel is systematically investigated. The tensile mechanical properties, LCF, and microstructure of carbide-free bainitic steel with strength grades from 1200 MPa to 1700 MPa are tested. The results show that an increase in dislocation density and a decrease in bainitic ferrite lath size lead to an increase in tensile strength, and a decrease in retained austenite volume fraction leads to a decrease in elongation. Due to the mechanical stability of retained austenite, the influence of microstructure parameters on fatigue lifetime is not proportional. The increase of dislocation density and the decrease of bainitic ferrite lath size lead to a much larger increase of fatigue lifetime than the decrease of retained austenite volume fraction. Using a damage-hysteresis energy method, the fatigue lifetime could be modeled to show its dependency on the tensile strength and elongation. Increasing tensile strength and elongation is beneficial to increasing fatigue lifetime. The increase of fatigue lifetime caused by the increase of strength is larger than the decrease of fatigue lifetime caused by the decrease of elongation. The mathematical analysis of fatigue lifetime in terms of tensile strength, elongation and static toughness can effectively predict the variation of the LCF lifetime for different grades of bainitic steels.