13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- f-HAZ in the Gr.91 steel weld, the grain boundary length decreased till 0.2 of life and saturated after that. On the other hand, for the f-HAZ in the Gr.122 steel weld, the grain boundary length increased till 0.5 of life, and then decreased after that. The decrease of grain boundary length occurs due to the recovery of microstructures during creep. Figure 6 shows the changes in the micro-hardness and KAM of fine-grained HAZ during creep for both steel welds. Here, average value of KAM is plotted excluding the points whose misorientation is larger than 5˚ in order to investigate the changes of dislocation structures. For the f-HAZ in the Gr.91 steel weld, hardness and KAM decreased till 0.2 of life and saturated to constant value, whereas they did not change till 0.5 of life and then decreased after that for the Gr.122 steel weld. The decrease of KAM and hardness occurs due to the recovery of dislocation structures. Because the grain boundary length increased without changing KAM and hardness till 0.5 of life, dynamic recrystallization was considered to be occurred in the fine-grained HAZ of the Gr.122 steel. These differences of microstructural changes are considered to relate to the differences of creep damage behavior between two steels. In the Gr.91 steel weld, the recovery of dislocation structures of f-HAZ occurs at early stage of life, and then early initiation and evolution of Type-IV creep voids occur. In the Gr.122 steel weld, the recovery of dislocation structures occurs after the recrystallization of f-HAZ, and then damage evolution occurs at later stage of life. From these experimental results, we consider about the methods for the residual life assessment of weld components as follows. For the Gr.91 steel weld, ultrasonic nondestructive testing is available for residual life assessment because creep voids and cracks increase gradually inside the plate thickness. Hardness measurement and microstructural observation are not available because their changes saturate in the early stage of life. Local necking on the specimen surface in HAZ is also available because the creep ductility is high for the Gr.91 steel. For the Gr.122 steel weld, ultrasonic testing may 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 because they change largely in the latter half of life. Local necking on the specimen surface was scarcely observed for the Gr.122 steel weld with low creep ductility. Figure 5. Changes in the grain boundary length in the fine-grained HAZ during creep for the Gr.91 and Gr.122 steel welds measured by EBSD.
RkJQdWJsaXNoZXIy MjM0NDE=