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.