| The brittle fracture behaviour for a range of microstructures in a low-carbon, microalloyed pipeline steel is investigated. Each microstructure consists of conventional bainite and features a different prior austenite grain structure (PAGS) and Charpy impact toughness.; The microstructures were characterized using conventional optical and scanning electron microscopy. Quantitative data, including the PAGS and bainitic packet size was determined. In addition, electron back-scatter diffraction (EBSD) orientation mapping (OM) was performed in order to measure mean OM domain sizes and identify the frequency distribution of boundary angles.; Charpy impact testing, conducted over a temperature range of 25°C to -196°C, was performed on test samples in order to determine the ductile-to-brittle transition temperature (DBTT) for each microstructure. Cleavage facet measurements, using scanning electron microscopy, were performed on samples tested at -196°C and representing brittle cleavage fracture.; Correlations show that as the OM domain size decreases, impact toughness improves. Additionally, impact toughness improves as a result of an increase in the percentage of high-angle boundaries. OM domain measurements have been used to predict the DBTT for the steel studied in this thesis. |
