13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- Table 1. Compositions for the different materials in wt%. Material C Si Mn Cr Ni W Co Cu N Mo Fe AISI 304 0.06 0.3 1.4 18.5 8.2 - - - - - Bal. Sanicro 28 0.02 0.43 1.83 27 31 0.02 0.09 0.9 0.05 3.4 Bal. Alloy 617 0.1 - - 22.5 53.8 - 12 - 0.5 9 1.1 2.2. Ageing and toughness tests Standard sample with a dimension of 50x10x10mm (EN 10045-1) was used. The samples were aged at 650°C and 700°C in air-environment for 1000 and 3000 hours before the toughness testing. Impact toughness test has been performed at room temperature (RT), two to three samples at each temperature were tested. 2.3. Scanning electron microscope investigation For the microstructural investigation of deformation and fracture behaviour scanning electron microscopy (SEM), electron channeling contrast imaging (ECCI) and an energy dispersive x-ray system (EDS) were used. An HITACHI SU-70 FEG-SEM and a Zeiss Sigma FEG-SEM were used to perform these microstructural investigations. ECCI uses the orientation dependence of the crystal lattice planes with respect to the incident electron beam to generate an image using backscattered electrons. Dislocations create small distortions in the crystal lattice which affect the backscattered electron intensity, allowing the imperfection to be imaged as a contrast change [8]. 3. Results 3.1. Toughness test results Table 2 shows the results of the impact tests, which reveals two different responses. The nickel base alloy, Alloy 617, is ductile at all ageing conditions and is more ductile than both austenitic stainless steels at all conditions. It seems that the impact toughness increases with both increasing temperature and longer times, but is affected more by temperature according to the energy differences in table 2. For the two austenitic stainless steels the impact toughness decreases with both increasing temperature and longer times. For AISI 304 the temperature has the biggest influence on the impact toughness. The material becomes brittle when the material has been aged at 700°C. Sanicro 28 has relatively high impact toughness at both 650°C and 700°C for 1000 hours, but the impact toughness decreases with increasing temperature, especially at 700°C, the material becomes brittle for about 3000 hours. Table 2. Energy for each alloy at different ageing times and temperatures. Material Energy [J] 650°C & 1000h Energy [J] 650°C & 3000h Energy [J] 700°C & 1000h Energy [J] 700°C & 3000h Energy [J] Reference (RT) AISI 304 75 31 10 6 243 Sanicro 28 100 64 50 5 >300 Alloy 617 127 137 137 151 >300
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