Fig. 11. Local misorientation distribution maps showing strain
distribution of X90 bend: (a) inner arc side; (b) outer arc side; (c)
the neutral axis position. The high angle grain boundaries are
delineated in black lines.
Fig. 12 is a quantitative analysis of the strain distribution of the bcc
matrix in the specimens. It can be seen from Fig. 12 that the local
misorientation curve of the outer arc side is shifted right relative to
the inner arc side and the neutral axis, and the local misorientation
curve of the inner arc side is shifted right relative to the neutral
axis. The average local misorientation of the inner arc side, the outer
arc side and the neutral axis is 0.57 °, 0.62 °, 0.48 °, respectively,
which is consistent with the variation of the blue and yellow color
areas in Fig. 12. According to the statistics of the average local
misorientation, it can be confirmed that the outer arc side of bend
suffered the largest strain, followed by the inner arc side, and finally
the neutral axis position.
Fig.12 Change of local misorientation in the X90 bend
Conclusions
In the present study, effect of hot induction bending on the
microstructure evolution in thick-walled X90 pipeline steel was studied.
The major conclusions are derived as follows:
- The
strength of X90 bend decreased 30 ~ 80 MPa compared to
the X90 parent pipe, and the Charpy impact absorbed energy of X90 bend
increased 20 J except for the outer arc side, which is the lowest, 153
J.
- The microstructure of X90 pipeline steel is consist of GB, AF, QPF and
with a small amount of M/A constituents. After hot induction bending,
the outer arc side in the X90 bend zone composed of a large number of
LB and PF, and the inner arc side consist of GB, LB and PF, hardly
observed AF in the bend zone. The position of the neutral axis is
composed of GB and PF.
- The width of bainite laths is about 0.2 ~ 0.3 μm
increased to 0.35~1.34 μm in the outer arc side, and
the width of laths is about 0.24~1.04 μm. The main
component of the precipitate is NbC, with a small amount of TiC,
possibly (Ti, Nb) C.
- LB, angular M/A constituents and the highest KAM value cause the worst
impact toughness compared with the other specimens.