Microscopic examination of crack growth in a pressure vessel steel

A fairly systematic microscopic study on ductile and ductile-brittle crack growth in the A508B pressure vessel steel has been performed. The main method of investigation was to subject fracture mechanics specimens to predetermined load levels and then cutting the specimens perpendicular to the plane of the crack. The area in front of the crack was examined in a S.E.M. The object of these examinations was to determine if newly encountered computational results could be correlated to crack extension characteristics and to study whether the mechanism of ductile growth was of the void growth type or of the fast shear mechanism. Both the original material and a specially heat treated piece were investigated. The heat treatment was performed in order to raise the transition temperature to provide a more convenient testing situation. Charpy V tests and fracture toughness determination were performed for the specially heat treated material to obtain the temperature dependence of the toughness. The heat treatment was successful in elevating the transition temperature. It was found that the mechanism of the ductile crack growth is of the localized shear type. The distribution of particles that nucleates voids which contribute to the ductile fracture, is at least bimodal and contains particles of vastly different sizes, each type contributing to the fracture at different stages of the growth. The number of larger voids that interact with the crack-tip during growth is on a two dimensional cut one to three.