13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Figure 6. Changes in the micro hardness and KAM in the fine-grained HAZ during creep for the Gr.91 and Gr.122 steel welds measured by EBSD. 4. Conclusions In the present paper, creep strength, Type-IV damage evolution and microstructural change during creep in the Gr.91 and Gr.122 steel welds were investigated quantitatively using the large-scale welded joint specimens. The results are summarized as follows: (1) In the Gr.91 steel weld, the Type-IV failure occurred after 10,000h at 550 ˚C and 1,000h at 600 ˚C. In the Gr.122 steel weld, Type-IV failure did not occur at 550 ˚C; it occurred after 5,000 h at 600 ˚C. (2) Type-IV creep voids in the Gr.91 steel weld formed at the early stage 0.2 of creep life, increased with elapse of time, and then coalesced to form a macro crack after 0.8 of life. (3) In the Gr.122 steel weld, a small number of Type-IV creep voids formed at 0.5 of life, increased slightly until 0.9 of life, and then rapid crack growth occurred after that. The area fraction of creep voids in the Gr.122 steel weld was much smaller than that in the Gr.91 steel weld. (4) In the Gr.91 steel weld, the recovery of dislocation structures of fine-grained HAZ occurred at early stage 0.2 of life, and was followed by early initiation and evolution of Type-IV creep voids. In the Gr.122 steel weld, the recovery of dislocation structures occurred after recrystallization of fine-grained HAZ, and then damage evolution occurred at later stage of life. (5) Form the above experimental results, for the Gr.91 steel weld; it is considered that ultrasonic nondestructive testing etc. is available for the residual life assessment because creep voids and cracks increase gradually inside the plate thickness. Hardness measurement and microstructural observation of fine-grained HAZ are not available because their changes saturate in the early stage of life. (6) For the Gr.122 steel weld, ultrasonic testing is considered to be difficult to detect Type-IV creep damages because the amount of voids are small and crack grows rapidly after 0.9 of life. Evaluation of hardness and dislocation structures (KAM) are available for the residual life assessment because they change largely after 0.5 of life.
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