Effect of microstructure and crystallographic orientation characteristics on low temperature toughness and fracture behavior of pipeline steels

In order to elucidate the relationship among microstructure, crystallographic orientation characteristics and fracture behavior of pipeline steels at low temperature, three pipeline steels with different microstructures were fabricated by varying the cooling rate. Different characteristics of the three microstructures, such as high angle grain boundaries(HAGBs), crystallographic orientation and brittle fracture characteristics were investigated. The study showed that HAGBs had a considerable possibility to make the cracks deviate greatly from the original direction and were a decisive factor in determining the features of fracture and impact toughness. The AF(acicular ferrite) microstructures of steels C2 and C3 had the stronger arrestability to cracks because of their high density of HAGBs, which was reflected from more tortuous crack propagation paths and smaller cleavage fracture units on the fracture surface. In addition, the maximum content of {001} cleavage planes parallel to the fracture surface in steel C1 also led to the highest DBTT (ductile–brittle transition temperature). Compared with steel C2, steel C3 had more {100} cleavage planes parallel to the surface of the V-notch, resulting in larger and more secondary cracks which could significantly alleviate the stress concentration at the tip of the primary crack during impact test. Steel C3 also exhibited higher intensities around the {332}<113> components than steel C2. Therefore, the impact absorbed energy of steel C3 was higher than that of steel C2. Moreover, a modified equation can be used to quantitatively predict the DBTT of PF(polygonal ferrite), however, it can be only used for an approximate prediction for AF due to its complex microstructure.