13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Figure 1. Show how the impact toughness is influences by the amount of σ phase at equilibrium after ageing at 650°C and 700°C for 3000 hours. 3.3. Fractures The microstructure investigation reveals that the precipitates strongly affect the fracture behaviour. AISI 304 and Sanicro 28 show intergranular fracture after they have been aged at 650°C for 3000 hours (Fig. 2). However, the fracture morphology is different. Fracture surfaces in AISI 304 shows small and shollow dimples, while fracture surfaces in Sanicro 28 are rather smooth. Fracture surfaces in the nickel base alloy Alloy617 are still ductile and intergranular fracture is rarely seen. Figure 2. Pictures showing fracture surfaces from the impact tested samples, a) AISI 304 and b) Sanicro 28, both materials have been aged at 650°C for 3000h. At 700°C, cleavage fracture can be observed in the stainless steels aged for 3000 hours (fig. 3). Typically step like or layer cleavage planes are found and some dimples can still be observed at the fracture surfaces. 0 20 40 60 80 100 120 140 160 0 10 20 30 40 50 60 Impact strength after ageing Fraction of σ phase for equilibrium 650°C 3000h 700°C 3000h a) b) 40 μm 250 μm
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