13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- present base metal was 220-235 as shown in Fig. 2. In the conventional high Cr steels, fine-grained HAZ structures 5 μm in the average grain size were observed in the area at about 1.5 mm from the fusion boundary. In the present high B-9Cr steels, however, the fine-grained structures were not formed at a distance of 1.5 mm from the fusion boundary. In this region, the grain size of the base metal was retained and small grains formed only on the prior-austenite grain boundaries (Fig. 4(b)). The grain boundary length was 5.99 cm, KAM was 1.94, and hardness was 243 for the microstructure of Fig. 4(b). At a distance of 0.5 mm from the fusion boundary, recrystallization occurred; however their grain size was more than 10 μm (Fig. 4(c)). The grain boundary length was 4.94 cm, KAM was 2.22, and hardness was 266 for the microstructure of Fig. 4(c). The KAM corresponds well to the hardness. In the conventional high Cr steel welds, Type-IV creep damages form in the fine-grained HAZ with average grain size of about 5 μm; consequently creep strength of welds decreases than base metal [4, 5]. These results mentioned above in Fig. 4 confirm that the HAZ microstructures of high B-9Cr steel are considerably different from those of the conventional high Cr ferritic steels, and soluble free boron is essential for suppression of grain refinement during weld thermal cycle. We consider that the free boron decreases the grain boundary energies, and suppresses the nucleation of γ phase from grain boundaries by diffusional transformation, and causes the martensitic reverse transformation. The similar phenomena of the martensitic reverse transformation that the original grain size and crystal orientation was retained after heating up to AC3 transformation temperature were also reported in the maraging steels [6] and 12Cr turbine rotor steels containing boron [7]. 3.2. Creep strength of dissimilar weld joints Figure 5 shows the relations between stress and creep rupture time of the base metal of high B-9Cr steels and four kinds of the present dissimilar weld joints at 650 ˚C. Open symbols show the rupture times of the base metal and dissimilar weld joints for the MARBN10 steel, and solid symbols show those for the MARBN12 steel. The creep strength of base metal of the high B-9Cr steels was higher than that of the conventional 9Cr steel (Gr.92 steel) due to the grain boundary strengthening effect of boron. The creep strength of base metal of MARBN12 steel with 0.007 % N was higher than that of MARBN10 steel with 0.003 % N. The creep rupture tests of the dissimilar weld joints were conducted at 650 ˚C for 160, 140, 120 and 100 MPa. The dissimilar welds using the high B-9Cr steel show 5-10 times longer creep rupture times than the Gr.92 steel weld at 100 MPa. The differences of the creep strength between dissimilar welds and base metals are small up to 20000h at 650 ˚C, and we are now investigating the creep strength at lower stress conditions; 90 and 80 MPa. The dissimilar welds fractured in the base metal of high B-9Cr steel for the short term test conditions and in the fusion boundary between high B-9Cr steel and Ni base filler wire (Inconel 82) for the long term test conditions. The failure in the fusion boundary was observed at the stresses lower than 120 MPa for the MARBN10 steel welds and lower than 140 MPa for the MARBN12 steel welds. All the present dissimilar weld joints using high B-9Cr steels did not show the Type-IV failure in HAZ and consequently the creep rupture lives were much longer than those of the conventional 9Cr steel welds that showed the Type-IV failure at the present creep test conditions. This means that the microstructures and creep strength of HAZ was considerably improved in the high B-9Cr steel, which contains 0.01 % boron and low nitrogen (<0.01 %), due to the effect of soluble boron.
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