In order to get better understanding of the mechanism of cleavage fracture in the heat affected zone (HAZ) of X100 pipeline steel, secondary microcracks underneath the brittle fracture surface of a Charpy impacted sample with the notch located in the HAZ were characterized using electron backscattered diffraction. Since the coarse grained (CG) HAZ and intercritically reheated coarse grained (ICCG) HAZ are well accepted as the weakest region in the HAZ, the cleavage secondary microcracks in these two regions were observed respectively. Initiation and propagation of cleavage microcracks were discussed. The results show that the fracture behavior is obviously influenced by local microstructure. There are more secondary microcracks in the ICCGHAZ than in the CGHAZ which shows different probability for microcrack nucleation. Fracture mechanism changes from nucleation control in the CGHAZ to propagation control in the ICCGHAZ. The main reason for the increased possibility of secondary microcracks formation and the change in fracture mechanism is due to the formation of coarse necklacing martensite–austenite constituent in the ICCGHAZ. The results also show that high angle boundary, with the misorientation larger than \(45^{\,\circ }\) , is effective in deflecting or arresting brittle cracks, while low angle boundary ( \(15^{\,\circ }{-}45^{\,\circ }\) ) seems not. Most preferred crack planes are {100}, with decreasing probability of {110}, {112}, {123}.

The effects of prior austenite grain (PAG) refinement on the mechanical properties of bainitic/martensitic steels not only come from itself, but also have more complex effects by affecting the substructure formed by coherent transformation. In this study, the samples of a low-alloy steel were water quenched from different austenitizing temperatures and the bainitic/martensitic microstructures with different PAG sizes were obtained. Electron back-scattered diffraction was used to characterize the microstructure and different types of boundaries were identified and quantitatively analyzed. The tensile tests and series temperature Charpy impact tests of different heat treatment were also carried out and comprehensively analyzed with microstructure characterization works. The results show that the uniform refinement of prior austenite grain can increases the density of packet boundary and block boundary, which leads to microstructure refinement with higher density of high-angle grain boundaries with misorientation >45°. The contribution of this microstructure refinement to toughness is significant, but its contributions to strength and elongation are relatively limited. Compared to uniform refined PAG, if the PAGs are mixed crystal, the density of block boundary will be reduced, which leads to a lower density of the high-angle boundary with misorientation >45° and the positive effects of microstructure refinement on toughness improvement are weakened. The observation of fracture surface of impact specimens indicates that refining the PAG can delay the tendency of brittle fracture with the decrease in test temperature, and even in the case of brittle fracture, the cleavage facet of the fracture surface is relatively smaller. This result also verifies that PAG refinement can effectively improve toughness by inhibiting cleavage fracture.