Fig. 9 Grain boundaries distribution of X90 bend, where black and red lines indicate the high misorientation angle boundaries (θ≥15°) and low misorientation angle boundaries (2°≤θ≤15°), respectively: (a) the inner arc side; (b) the outer arc side; (c) the neutral axis position.
Literatures [27, 31] reported that HAGBs can effectively contribute to higher cleavage microcrack resistance, the crack would be distorted when the tip encountered the HAGBs during the fracture process. A large number of HAGBs would consume a lot of energy when the crack torsion. On the contrary, the LAGBs have a less contribution on the crack torsional effect. During the bending process, the inner and outer arc side of the bend suffered stress effect. The relative frequency of HAGBs greater than 50° on the inner arc side and the outer arc side is higher than the neutral axis position, as shown in Fig.10. The dislocations are transferred by the way of dislocation accumulation from the sub-grain boundaries to the grain boundaries in the deformed grains. The high density dislocations and sub-grain boundaries in the grains of the outer arc side are regarded as LAGBs, and the outer arc side have the highest strength because of the strengthening of dislocations and sub-grain boundaries is accompanied by fine grain strengthening [9]. According to the statistics of the results of EBSD, the relative frequency of LAGBs is 82%, 79%, and 83% in the inner arc side, outer arc side and the neutral axis, respectively. The relative frequency of HAGBs is 18%, 21%, and 17%, respectively. The bend zone was subjected the double effect of heat treatment and deformation, and the bainite lath is accompanied by sub-grains (Fig. 5(a, b)). From the statistics data, the magnitude of LAGBs in the outer arc side is the lowest of the bend. In the grain boundaries theory, the outer arc side have the highest impact toughness. However, it is contrary to the actual toughness properties that the outer arc side has the lowest toughest, is 153 J. Actually, a large number of LB with bainite laths in the outer arc side and it is believed that a crack could easily propagate along the bainite sheaves. In addition, some M/A constituents are sharp morphology existed in the outer arc side, it is detrimental to toughness.
Fig.10 The misorientations distribution of the X90 bend.
3.5Local strain distribution by EBSD
Kernel average misorientation (KAM) mapping approach can be used for visualizing of plastic deformation. Local misorientation was used to evaluate small local strain gradients in the material. Fig.11 shows the local misorientation distribution maps of X90 bend. The black lines represent grain boundaries. The local misorientation distribution maps represent the average misorientation between the given point and adjacent position in the same grain (usually less than 5 °), mainly used to assess the small local strain gradient in the material [33, 34]. Because of the formation of sub-grains with a displacive transformation in LB, and the misorientation gradient is significant, as shown in Fig.11(a, b). It is a very effective method to define the stress concentration through 0° ~ 5° misorientation. As shown in Fig.11, the misorientation of less than 1° is indicated by blue, 1° ~ 2° is green, 2° ~ 3° is yellow, 3° ~ 4° is orange, and 4° ~ 5° is red. The results show that most of the area of the specimens is blue, that is, within the 0° and 1° local misorientation, corresponding to the ferrite region. The area represented by the yellow color is characterized by a strip-like distribution, as shown in Fig. 11(a, b), which is the result of the dislocation accumulation at the boundaries of the bainite laths. Zaefferer [35] indicated that volume expansion during bainite transformation process, resulting in shear stress and dislocation accumulation. It is also observed in Fig. 11(c) that since the dislocation is around the hard phase, the orange region appears at the boundaries of the equilibrium ferrite. During the deformation process, some micropores nucleate in the deep color zone firstly. In Fig. 11(b), the outer arc side of the bend have appeared a red area, which is a large local misorientation difference of 5° that also verifies the fact that the outer arc side has the lowest toughness.