2017 Volume 57 Issue 4 Pages 746-754
In order to establish a basis to predict fracture toughness of ferrite-pearlite steel, a two-stage investigation was performed. A 3D observation was first performed to characterize the relationship between micro-cracks and microstructures using a serial sectioning technique. Two types of micrographs before and after nital etching were taken to obtain 3D images of micro-cracks and microstructures, respectively. As a result of superposition of both the 3D images, it is found that a micro-crack initiated at ferrite-pearlite boundary and then propagated into pearlite grain. It is also found under the ellipse approximation of the micro-cracks that the distribution of major axis of micro-crack was close to that of the ferrite grain diameter and the distribution of miner axis was an intermediate between those of ferrite grain diameter and pearlite band thickness. Based on the results, we then proposed a 3D model on micro-crack with an oblate spheroid assumption of pearlite grain to quantitatively relate microstructures of ferrite-pearlite steel to the fracture stress. It was shown that the fracture stresses corrected by the proposed 3D model were 20–30% less than those evaluated by the conventional 2D model. It is expected to be caused by the reason that the accuracy of the conventional 2D model was less than that of the proposed 3D model based on the detailed 3D observation because the conventional model is based on the simplified assumption of 2D micro-crack in the 3D microstructure. Therefore, we can give a valuable basis to develop an accurate fracture toughness prediction model based on the detailed 3D observation.