Fig. 4 TEM micrographs of API X90 pipeline steel: (a) dislocation substructure in QPF; (b) bainite laths; (c) acicular ferrite plates; (d) M/A constituent.
After hot induction bending, the microstructure of the outer arc side and the neutral axis of X90 bend under TEM are shown in Fig. 5 and Fig. 6, respectively. A high volume fraction of LB laths with misorientation were formed at the prior austenite grain boundaries, the laths width is about 0.53~1.34 μm. Moreover, some fine PF with the size of 0.31~0.65 μm are observed in the outer arc side, as shown in Fig.5(a). Undecomposed carbides play an important role in strength improving, as shown in Fig. 5(b). A large number of dislocation cells and substructures can be observed in the PF grains, as shown in Fig. 5(c). Because of the effect of tensile stress on the outer arc side, a large number of dislocations formed in the deformed grains increasing the nucleation site of LB that could refine the grains. During the bending process, dislocations sliped to the grain boundaries and intertwined together to form dislocation cells or substructures eventually. Therefore, high density dislocations in the quenched microstructure, only amount of dislocations were eliminated after the high temperature tempering. The laths began to be widen and degenerated. The size of bainite laths in the outer arc side is 0.53~1.34 μm, as shown in Fig. 5(a). It is wider than that of the X90 parent pipe. The bainite microstructure still maintain the lath morphology due to the presence of carbide-forming elements such as Mo, Nb and Ti. Therefore, ensuring the high yield strength of 750 MPa, which is the highest in the bend zone. Dislocation cells and grain boundaries of laths could hinder the crack propagation and improve toughness. In general, the ability to prevent crack propagation is closely related to the size of the lath. The smaller lath, the better toughness. According to the TEM observation, two types of M/A constituents in the outer arc side: one is long strip distributed between bainite laths, and the other is blocky distributed between PFs or between ferrite and bainite boundaries. As shown in Fig. 5(d), M/A constituent with sharp morphology, it is prone to stress concentration when M/A constituent is large in size and with angular, crack initiation and propagation that would detrimental to toughness [25].