Abstract
Creep strength of high Cr ferritic steel welds decreases than base metals due to Type-IV creep damages formed in the heat affected zone (HAZ) during long-term use at high temperatures. In the present study, creep tests of ASME Gr.91 and Gr.122 steel welded joints were interrupted at several time steps at 600°C. Microstructural changes and damage behaviors in the fine-grained HAZ of welds during creep were investigated by using SEM, TEM, and EBSD. KAM, grain boundary length and hardness of fine-grained HAZ decreased till 0.2 of life and saturated in the Gr.91 steel weld, whereas they decreased after 0.5 of life in the Gr.122 steel weld. It was found that the recovery of dislocation structures in the fine-grained HAZ was completed at early stage of life for the Gr.91 steel weld, whereas it occurred in the later stage of life for the Gr.122 steel weld. These differences of microstructural changes was considered to relate to the differences of Type-IV creep damage behavior; early initiation of creep voids at 0.2 of life in the Gr.91 steel weld and later damage evolution after 0.5 of life in the Gr.122 steel weld.
