Investigation Of Inclusions On Micro Fracture Mechanisms Of High Strengthened Steels

With the development of the world economy, the research of steels now is concentrated on how to make steels more space-saving, lighter, and multifunctional and in many fields steels are required to be high strengthened. That is to say in 21 century, high strengthened steel is a field o f much importance.But because of the impurity of raw materials and impropriety of melting process, more or less there are some inclusions in metal. For metal, even there is very limited amount of inclusions in it, its capacity will be affected greatly and especially its vital mechanical property—low cycle fatigue life—will be decreased greatly. Experiments shows that inclusions in the metal are the primary sites where fatigue cracks can initiate. So it is very important to do some work as how inclusions in metals affect the property of high strengthened steels.In this thesis, some fatigue experiments for specimens of MA250 and GE1014 alloys with different sizes and different types of inclusions in them were carried out using the servo-hydraulic fatigue testing machine with scanning electronic microscopy (SEM). The thesis concentrates on the micro fatigue crack growth behavior and fracture mechanism considering the effects of different sizes and different types of inclusions. It come to the conclusion hat under Low Cycled Fatigue range, the sizes of inclusions that can affect the property of high strengthened steels are 10-20um. Fatigue cracks often form at inclusions by one of three mechanisms: inclusion cracking, debonding of the interface between the inclusion and matrix, or cracking at lines of slip in the surrounding matrix. The initiation and propagation of micro-crack is greatly affected by micro-structure while the stable propagation of the crack is controlled by the load.Then through the observation of fracture with SEM, the differences between the surface failure process and interior failure process are well understood. This is very important to the understanding of the failure behavior of high strengthened steels. In this thesis, FEM is used to simulate the effects of inclusions on the initiation and propagation of micro-cracks and compared it with experimental results.