13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- The simulated fine-grained HAZ (f-HAZ) specimens were produced using a weld simulator (a Gleeble testing machine) by rapid heating to a peak temperature of 900 ˚C for the Gr.91 steel and 950 ˚C for the Gr.122 steel, respectively. Creep tests of the base metal and simulated f-HAZ were conducted using smooth bar specimens and those of the welded joints were conducted using smooth plate type specimens of 17.5 × 5 mm2 in cross-section and 100 mm in gauge length (S-welded joint) at 550, 600 and 650 ˚C [7]. Creep interruption tests were conducted using a large-scale thick plate type specimen (L-welded joint) of 21 × 21 mm2 in cross-section and 100 mm in gauge length for the Gr.91 steel weld and of 24.5 × 24.5 mm2 in cross-section and 120 mm in gauge length for the Gr.122 steel weld, which included the full original plate thickness close to the structural components [7, 8]. Creep tests were conducted using several L-welded joint specimens at 600 ˚C and interrupted at several time steps. Because creep voids formed inside the specimen, we cut the specimens in the center of width after interruption of creep tests, and observed the creep voids in HAZ using SEM or laser-microscope. The area and number of creep voids were measured using the image processing software. Changes of microstructures in fine-grained HAZ were investigated by measuring KAM (Kernel average misorientation) and grain boundary length using EBSD (Electron backscatter diffraction). 3. Results and discussion 3.1. Creep strength of welded joints Figure 1 and 2 show the creep test results for the base metal, simulated f-HAZ and welded joints of the Gr.91 steel and Gr.122 steel, respectively. The failure locations of the welded joint specimens are indicated with the subscripts attached to the plots, where BM means failure in base metal and HAZ means Type-IV failure in the fine-grained HAZ. In the Gr.91 steel weld, the Type-IV failure occurred after 10,000h at 550 ˚C and 1,000h at 600 ˚C. The differences in creep rupture times between welded joint and base metal tended to widen with decreasing stress. The creep rupture times of the simulated f-HAZ were more than one order shorter than those of the base metal for the same stresses at all temperatures. The creep rupture times of the welded joint approached those of the simulated f-HAZ for low stresses at all temperatures. Figure 1. Creep rupture times of the base metal, simulated f-HAZ and welded joints of the Gr.91 steel.
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