13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Figure 2. Creep rupture times of the base metal, simulated f-HAZ and welded joints of the Gr.122 steel. For the Gr.122 steel welds, Type-IV failure did not occur at 550 ˚C within this test stress. It was observed after 5,000 h at 600 ˚C. The creep rupture times of the simulated f-HAZ were nearly the same as the base metal at 550 ˚C; they decreased than the base metal after 5,000 h at 600 ˚C. The differences in creep rupture times between base metal and simulated f-HAZ were more than one order at all test conditions for the Gr.91 steel, whereas they appeared at lower stresses than 140 MPa at 600 ˚C for the Gr.122 steel. At these test conditions, the simulated f-HAZ of the Gr.122 steel revealed void- type inter-granular failure and its creep ductility decreased. 3.2. Evolution of Type-IV creep damage Creep rupture times of the L-welded joints were 8,853 h at 600 ˚C and 90 MPa for the Gr.91 steel and 16,340 h at 600 ˚C and 100 MPa for the Gr.122 steel; they also showed the Type-IV failure and their rupture times were slightly longer than the S-welded joints as shown in Figs. 1 and 2. We have conducted the creep interruption tests using the L-welded joints for both steels at these test conditions and investigated the processes of Type-IV damage and fracture. Figure 3 shows the binary images of creep voids and cracks observed in HAZ of the central cross-section of the L-welded joints of the Gr.91 steel creep-interrupted at 600 ˚C and 90 MPa. It was found that a small number of creep voids formed at about 0.2 of creep rupture life, and the number of voids increased with time, and then coalesced to form a crack at 0.8 of life. Creep voids and cracks were mostly observed in the area about 20% below the plate surfaces. The area fraction of creep voids in the HAZ of the Gr.91 and Gr.122 steel welds are plotted against the life ratio (t/tr) in Fig. 4. In this figure, the area fraction of creep voids in the area about 20% below the plate surfaces where it shows the highest value is plotted. It was found that the Type-IV damage behavior and amounts of creep voids were considerably different for both steel welds. In the Gr.91 steel weld, Type-IV creep voids formed at the early stage 0.2 of creep life and increased gradually with elapse of time. On the other hand, in the Gr.122 steel weld, only a small amount of creep voids formed at 0.5 of creep life and slightly increased till 0.9 of life. Type-IV crack was not observed till 0.92 of life; it was observed in the specimen interrupted at 0.98 of life as shown in Fig. 4. The formation and growth of creep voids were suppressed during steady-state condition;
RkJQdWJsaXNoZXIy MjM0NDE=