Fractographic observations of cleavage fracture initiation in a bainitic A508 steel

Abstract

Cleavage initiation in CVN (Charpy V-notch) and CT (compact tension) specimens made of a bainitic pressure vessel steel was investigated, in the ductile-to-brittle transition region. The fracture surfaces were examined with a scanning electron microscope (SEM), in order to measure the coordinates of the cleavage initiation sites, and to identify the fractographic and/or microstructural features suspected to have triggered cleavage. Cleavage initiation was found to be promoted by the same microstructural features for both CVN and CT specimens. However, the types of cleavage initiation sites differ over the range of temperature investigated. At low temperatures, small spherical and large elongated inclusions are equally important for cleavage initiation. As the temperature increases, the role of the large MnS inclusions also increases. In 80% of the specimens tested, over the temperature range investigated, manganese sulfides play a major role in cleavage initiation.