AbstractsEngineering

Ferritic Reactor Pressure Vessel (RPV) steels exhibit a ductile to brittle transition with decreasing temperature. Factors such as irradiation embrittlement cause the ductile to brittle transition curve of fracture toughness to shift to higher temperatures. Consequently, a need to improve the knowledge of the mechanism of cleavage microcrack nucleation and propagation in RPV steels is required to enable the prediction of fracture behaviour during service.This work focuses on characterising the microstructure at the initiation site in RPV steels. The fracture surfaces of post-tested A508 Grade 3 Compact-Tension samples were studied using scanning electron microscopy and it was found that cleavage fracture occurred from a single initiation site in each specimen and no initiating particle was observed. Several fractured samples were analysed and the locations of each initiation site was in good agreement with the peak stress ahead of the crack tip predicted by finite element analysis. An in-house surface matching technique using confocal microscopy was developed in this project and showed that local interference between the topographic data was detected close to the location of cleavage initiation. This finding is consistent with the theory that cleavage fracture is preceded by localised plasticity and showed promise in identifying the point of initiation. Furthermore, from the cross- sectional examination of these Compact-Tension samples, several secondary microcracks were revealed underneath the main critical crack path. Microstructural techniques, including electron backscatter diffraction, focused ion beam microhole drilling and nano-indentation were used to analyse these cracks in order to further understand the role of local microstructure and stress on the crack path and crack arrest. Specifically it was observed that these cracks were not always interconnected and that high-angled grain boundaries with a misorientation greater than 40° led to microcrack arrest whilst low angle grain boundaries did not affect crack propagation. These observations are discussed in terms of mechanistic insight into the early stages of cleavage fracture in ferritic steels.