EBSD characterization of secondary microcracks in the heat affected zone of a X100 pipeline steel weld joint

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 inter-critically 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 degrees, is effective in deflecting or arresting brittle cracks, while low angle boundary (15 degrees-45 degrees) seems not. Most preferred crack planes are {100}, with decreasing probability of {110}, {112}, {123}.