Designing steel microstructure based on fracture mechanics approach

Abstract

Conventionally steel components subjected to high stress fatigue are heat treated by means of quenching and tempering. This method is popular due to both its convenience to implement and the fact that tempered martensite is good for both LCF (low cycle fatigue) and HCF (high cycle fatigue) areas. In LCF area, most of the life to failure is spent in crack initiation in un-cracked bodies. Under HCF loading conditions (where stress levels are far below the elastic limit), the life to failure is spent not only in crack initiation in un-cracked bodies, but also in early stages of fatigue crack propagation in cracked bodies. The combination of the LCF and HCF usage requires the component to have both high ultimate tensile strength and excellent fracture toughness. Controlling the microstructure is one of the methods to increase the toughness to maintain the tensile properties. In this research, a multiphase (polygonal ferrite and martensite) microstructure was developed. The idea of the multiphase is based on the fracture mechanics approach. Experimental and computational methods were used to solve the problem. The multiphase microstructure was then compared with the microstructure resulting from conventional quenching and tempering processes. It was concluded that the multiphase microstructure is suitable for applications where the combination of LCF and HCF is required.