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].