Analysis of the Crack Initiation at Non-Metallic Inclusions in High-Strength Steels

High-strength steels exhibit different fracture surface characteristics under cyclic load when failure is initiated by non-metallic inclusions depending on the lifespan. These can be attributed to different crack propagation mechanisms. These characteristics are correlated with the short and long crack propagation. As far as the short crack is concerned, the mechanism of the discontinous decohesion of carbides in the immediate vicinity of the inclusions and the merging of the resulting microcracks to form a macrocrack is discussed. This postulated mechanism is investigated by means of a combination of scanning electron microscope (SEM) analyses of the fracture surface and target preparation by means of focused ion beam (FIB) by a detailed resolution of the local microstructure around the crack-initiating inclusion and the crack path.