Fig. 5 TEM micrographs of the outer arc side in the X90 bend zone: (a)
bainite laths; (b) carbides; (c) dislocation packets and precipitation
on dislocations; (d) M/A constituent with angular.
The microstructure of the neutral axis position is composed of PF and
GB. In Fig. 6(a), GB is composed of laths with a size of 0.24
~1.04 μm. However, the length of bainite laths in GB is
shorter than that of LB laths, and the characteristic of the laths is
not as obvious as that in LB. M/A constituents are granular or equiaxed
morphology distributed between the laths. Due to the short heating time
during hot induction bending process, only a small amount of the
original precipitates would dissolve that would lead to austenite
composition non-uniformity and form the mix microstructure of GB and PF.
Microalloying elements such as Ti, Nb and V, forming the fine carbide
and carbonitride precipitates that could inhibit austenite grain growth
and refine grains effectively [10]. M/A constituents decomposed
after tempering at a high temperature, as shown in Fig. 6(d). In
addition, a large number of dislocation tangles, dislocation cells and
substructures still be observed in the position of the neutral axis to
remain the high strength, as shown in Fig. 6(c). However, the
dislocation density in PF is decreased compare with the outer arc side,
as shown in Fig.6 (b, c), and the toughness improved.