Modeling and experimental verification of multiphase steel for components subjected to fatigue loading

Abstract

This research is done to acquire microstructure combinations that resist fatigue loading more than typical conventional microstructure (tempered martensite) and to study the effects of microstructure on fatigue properties of multiphase steels. A multiphase (polygonal ferrite and martensite) microstructure is developed. 2D and 3D square models with variation of ferrite fraction are used as local models and placed in front of the CT specimen in a global model. The ferrite shapes are from actual microstructure of multiphase material. It is found that the plastic zone size changes, as the ferrite fraction varies, and saturated at approximately 65% and 60% for 2D and 3D modelling, respectively. The influence of a ferrite areal fraction within a martensite matrix on fatigue crack propagation is studied. The variation of the areal fraction is achieved by means of intercritical thermal treatment, which specifically aims at optimizing the resistance to fatigue loading. The steels are annealed at different temperatures followed by water quenching and tempering process. Within the intercritical annealing temperature range, the areal fraction of ferrite increases with decreasing soaking temperature. Fatigue crack propagation tests are conducted according to ASTM E647-00 to obtain fatigue crack growth, FCG behaviour. It is found that the highest fatigue strength is achieved when the ferrite areal fraction is approximately 65%, which in this particular test, corresponds to 748 0C annealing temperature. It is concluded and is verified by computational modelling that appropriate thermal treatment can contribute to a significant improvement of fatigue properties and strength. The optimum ferrite fraction found from both computation and experiment is approximately 60% –65%.