13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- investigation has been done on the factors that affect the ductile to brittle transition behavior. In this paper, the influences of the factors such as temperature, ferrite content, cold deformation, phase size and different precipitates on the ductile to brittle transition and cleavage behaviors of a super duplex stainless steel are discussed with the purpose to increase the understanding on the micro fracture behaviors in duplex stainless steels and provide the information for the reliability and integrity to use the material. 2. Material and experimental The material used is super duplex stainless steel (SDSS): UNS S32750 (Sandvik SAF 2507) with a nominal chemical composition as shown in Table 1: Table 1 Nominal chemical compositions and information of DSS used Materials Cmax Si MnCr Ni Mo N Dimension Rp0.2 (mm) (MPa) SAF 2507 0.03 0.8 1.2 25 7 4 0.3 φ260x14 595 Filler 25.10.4.L 0.02 0.3 0.4 25 9.5 4 0.24 φ2.4 537 Two types of materials were used. One was a tube with an outer diameter of 260 mm and a wall thickness of 14mm. The other type of material was all-weld metal produced with TIG-welding using super duplex filler Sandvik 25.10.4.L. The welding was carried out in a V-groove using 13mm plates in SAF 2507 with 10° beveling and a back strip 6 mm in thickness in SAF 2507. Distance between the plates was 14 mm. Arc energy varied between 0.69 and 0.82 kJ/mm and the shielding gas consisted of Ar + 2%N2. Totally 32 runs were used to build up the all-weld metal. Interpass temperature was below 150°C. Figure 1 shows the typical microstructure of the base material (Fig. 1a) and the weld metal (Fig. 1b). Figure 1 Typical microstructure of SAF 2507; (a). Base material, X200, (b). Weld metal. Two types of toughness tests (impact toughness and CTOD) have been performed in a temperature range from -196°C up to room temperature (RT). Two to three samples/temperature were tested and
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