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:
  1. 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.
  2. 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.
  3. 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.
  4. LB, angular M/A constituents and the highest KAM value cause the worst impact toughness compared with the other specimens.