Microscopic investigation of cleavage initiation in modified A508B pressure vessel steel

A microscopic study on ductile–brittle crack growth in a modified version of the A508B pressure vessel steel has been performed. Small SEN(B)-specimens tested at different temperatures in and above the transition region have been thoroughly examined with a scanning electron microscope. Focus was directed towards: amount of ductile crack growth prior to cleavage, distance from the crack front to cleavage initiation sites, and type of defect that caused the cleavage initiation. The results show, among other things, that cleavage facets are present in specimens tested at all temperatures, even on the upper shelf where no global failure by cleavage was observed. These preliminary results give an indication that the ability of the matrix material to arrest and sustain small cleavage cracks can be crucial in explaining why ferritic steels show a transition behaviour.