13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- Figure 7. a) Precipitates in the aged Alloy 617 material at 700°C for 3000h and b) fracture after the impact toughness testing The results and discussion above shows that initiation and propagation of fracture in the aged austenitic stainless steel is local. They can behave very differently in different materials, as explained by the four different crack-types. Local approach to fracture should be applied by considering the heterogeneous mechanical behaviors in these two phases. 5. Conclusions The fracture initiation and propagation in the aged austenitic stainless steel is very local. They behave very differently in these materials at high temperature due to different chemical compositions affecting nucleation, growth and shape of precipitates. The brittle σ-phase can appear in the austenitic stainless steel after 1000 hours at 650°C and then increases in amount. The amount and shape have strong effect on the fracture behaviour, where needle shaped σ-phase which mostly appear at high temperature (700°C) after longer ageing time (3000 hours) lead to a low impact toughness and brittle fractures both locally and on a macro-level in the specimen. The nickel base alloy show higher impact toughness with increasing ageing temperature and time. Local approach to fracture should be applied by considering the heterogeneous mechanical behaviours in these two kinds of materials. Acknowledgements Present study was financially supported by AB Sandvik Material Technology in Sweden and the Swedish National Energy Administration through the Research Consortium of Materials Technology for Thermal Energy Processes, Grant No. KME-501. Agora Materiae and Strategic Faculty Grant AFM at Linköping University are also acknowledged. References [1] R. Viswanathan, K. Coleman, U. Rao, Materials for ultra-supercritical coal-fired power plant boilers, Int. J. Pressure Vessels Piping. 83 (2006) 778-783. [2] J. Pettersson, H. Asteman, J. Svensson, L. Johansson, KCl Induced Corrosion of a 304-type Austenitic Stainless Steel at 600°C; The Role of Potassium, Oxidation of Metals. 64 (2005) 2341. [3] T. Sourmail, Precipitation in creep resistant austenitic stainless steels, Materials Science and Technology. 17 (2001) 1-14. [4] M. Schwind, J. Källqvist, J.-. Nilsson, J. Ågren, H.-. Andrén, σ-PHASE PRECIPITATION IN STABILIZED AUSTENITIC STAINLESS STEELS, Acta Materialia. 48 (2000) 2473-2481. a) b) 900 nm 50 μm
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