13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 3.2. Precipitates and its influence on the toughness The toughness is strongly influenced by the precipitates formed during the high-temperature ageing process. Table 3 shows the precipitates appearing in Sanicro 28 at the four ageing conditions. Comparing table 2 and 3 it is clear that the largest impact toughness decrease for Sanicro 28 (50 J to 5 J) is due to increasing amount of sigma phase, σ, with needle shape. The σ-phase is a brittle tetragonal phase and can differ relatively much in composition and occur at different ageing conditions depending on chemical composition of the aged material [3, 9]. This indicates that both temperature and time are important for the impact toughness of Sanicro 28. The Cr,Mo-rich carbides in Sanicro 28 is probably M23C6 and/or M6C depending on ageing condition [3, 10]. Intergranular carbides can be observed in all samples of the tested materials and occur before intragranular precipitation. AISI 304 show similar precipitation progress as Sanicro 28, but the amount of σ-phase seems to be larger already from the first ageing condition and the needle shaped σ-phase appear at all ageing conditions except of 650°C and 1000 hours, AISI 304 also has a larger amount of σ-phase with needle shape at each ageing condition [3, 11, 12]. For the nickel base alloy Alloy 617, γ´ precipitate is a main precipitate which grows at higher temperature and longer times, which probably is the reason for the increase in impact toughness with both temperature and time. Compared to non-aged material ductility decreases when the material is aged due to precipitates that increases the strength. However, when the precipitates grow larger and are evenly distributed the strength decreases and the ductility increases again. Other precipitates that are common but not detected by the EDS investigation in this study due to their small size in these ageing conditions is carbides as M23C6 [6, 7, 13]. Table 3. Precipitates of Sanicro 28 for different ageing conditions. Ageing condition Precipitates Location of precipitates Shape/amount 650°C & 1000h Cr,Mo-rich carbides σ Intergranular Intergranular Rectangular/large Round/small 650°C & 3000h Cr,Mo-rich carbides σ Intergranular Intergranular Rectangular/large Round/small 700°C & 1000h Cr,Mo-rich carbides σ Inter- and intragranular Inter- and intragranular Rectangular/large Round/small, needle/small 700°C & 3000h Cr,Mo-rich carbides σ Inter- and intragranular Inter- and intragranular Rectangular/large Round/large, needle/large Figure 1 illustrates how the amount of σ phase affect the impact toughness for these three materials. The amount of precipitate is given by ThermoCalc-simulations [14]. The impact toughness is decreasing when the fraction of σ phase increases. There are notable differences for the two ageing conditions. At the ageing condition, 700°C for 3000h, the impact toughness is much lower than other conditions. One of the reasons is that this figure only considers the effect of σ-phase since the other phases can also play important roles, another reason is the shape of the precipitates that changes from rectangular to needle shaped. Also the ThermoCalc-simulation is only valid for equilibrium which not necessarily the samples that have been aged and then impact tested are.
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