FATIGUE FRACTURE ASSESSMENT OF HIGH CARBON STEEL COMPONENTS
Fractographic assessment of fatigue fractures may be difficult if they occur in metallic components characterized by low ductility complex microstructures. In these cases, reconciliation of known fatigue rupture mechanisms with fractographic appearance of fatigue-fractured surfaces is challenging. Special techniques assisted by theory development may be necessary. Fatigue failures in pearlitic steels, such as rail steels, are difficult to analyze and interpret, due to the metallographic microstructure that shows alternate lamellae of ductile ferrite and brittle cementite. Moreover, the task is challenging since their fracture surfaces at room temperature have ductile features, more characteristic of high temperature ruptures. In fact, a brittle to ductile Charpy-V transition curve for rail steels (≈ 0.7% C)indicates that brittle rupture predominates at the lower shelf, which extends up to 40°C ca. Fully ductile upper shelf fracture is reached only at 220°C. Features of their fatigue surfaces are clearly not brittle, but rather ductile, although not fully ductile. Thus, striations, that usually characterize ductile fatigue failures, are not clearly visible because they cannot appear on the brittle carbides, but only on the ductile ferrite lamellae. The examined fatigue fracture surfaces show striations on top of broken, previously necked, ferrite lamellae only at very high magnification.