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.