Tensile properties, fracture toughness, charpy impact value and rotating bending fatigue strength were obtained for ductile, vermicular and gray cast irons with ferritic matrix specially melted for this investigation. Detailed SEM observations were conducted on fracture surfaces of these failed specimens.
A step like pattern at the crack initiation area and ductile striation at the crack propagation area were characteristically observed on the fatigue fracture surface of ductile or vermicular cast iron. On the contrary, a step like pattern was not observed on either fractured surface of crack initiation and propagation areas of gray cast iron.
Dimple and river patterns were predominant on tensile and charpy impact fracture surface, respectively. However, a step like pattern was not found on the both fracture surfaces. Since the step like pattern could be identified by SEM observations with a relatively lower magnification and was not observed on tensile or charpy impact fracture surface, it is considered that the step like pattern might be a useful key pattern to identity fatigue fracture in the service failure of machine components made of ferritic cast iron. The stretched zone could be clearly observed on the fracture surface of ductile cast iron but it became harder to be identified as graphite nodularity decreased, and the stretched zone could not be identified on the fracture surfaces of gray cast iron.
