13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- The extrapolation was performed three times with rupture data up to 40000 h, 15000 h and 5000 h to verify the stability of the extrapolation. Figure 5a shows the result from the extrapolation. At both 700°C and 650°C the extrapolation overestimates the creep strength of the material when only short time creep data are used. This is common for many parametric methods and it is important that the deviation is not too large. The order of the polynomials and which temperature data sets the model is applied to will also influence the result. These factors are tested until a satisfactory result has been found. The predicted 105 h creep rupture strength at 700°C is about 99 MPa. In Figure 5b, the correlation between the extrapolation and experimental data can be seen. The evaluation satisfies the post evaluation tests (PATs) and other criteria proposed by the European Collaborative Creep Committee (ECCC) [8]. Figure 5 (a). Result from extrapolation with the free temperature model, performed three times with rupture data up to 5000 h, 15000 h and 40000 h, (b). Correlations between the extrapolation with rupture data up to 40000 h and the experimental data. 4.2 Strengthening mechanism In the temperature range up to 700°C, one of the main creep strengthening mechanisms is the interaction between dislocations and precipitates [9]. Figure 6 shows two examples how interaction between the dislocations and precipitates in austenitic stainless steel grade, UNS S31035, creep specimen tested with 210 MPa at 700°C and with a rupture time of 3153 h. Moving dislocations at the nano-sized particles can be seen. Around the intergranular precipitates, the dislocation density is high which indicates that they function as obstacles for the dislocation movements. This increases the creep strength. Smaller nanoprecipitates such as copper rich particles and MX particles are more effective as obstacles for the dislocation movements. However, they have different mechanisms for dislocation crossing. For the copper rich nanoparticles, dislocations cross the particles mainly by climb / bypass of unit dislocations (Fig. 6a). For the MX nanoparticles, deformation might occur by (a) (b)
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