13th International Conference on Fracture June 16–21, 2013, Beijing, China 2 observed by TEM and identified by selected area diffraction pattern (SADP). Tests were carried out at -50 ℃ and room temperature (RT) by universal test machine SHIMACZU AG-10 TA for tensile tests and by instrumented impact machine CIEM-30CPC for Charpy V impact tests. The average of toughness values measured in three specimens is taken as the test result. Metallographic sections are prepared by griming, polishing fracture surface of charpy specimens at -50oC to characterize the microstructural constituents and measure the grain and packet sizes just under the fracture surface. General feature and the details of fracture facets were observed on the fracture surfaces. The distances Xf from the plastic crack tip to the site of cleavage cracking initiation was measured on the fracture surfaces of specimens which were fractured o lower temperatures with short fibrous crack extension. The distances Xf is a very important parameter by which the local fracture stress σf, fracture strain εpc and the critical stress triaxiality Tc are calculated. The nature of crack-initiating particles was characterized. Critical event, which controls cleavage fracture, is defined as the stage offering the most difficulty during the crack initiation and propagation. The critical event can be revealed by the cracks which retained in the fractured specimen. Metallographic sections were prepared by cutting the specimen perpendicularly to the notch root for observation of retained cracks in fractured specimens. The lengths of retained cracks are also measured by Image software. Table 1 Compositions of experimental metals (wt%) Sample C S P Cr Ni Si+Mn+Mo+Cu+Nb+V BM 0.08 0.002 0.008 0.60 7.58 1.77 T5-21 0.07 0.005 0.005 0.71 5.74 2.71 DM4-1 0.07 0.005 0.008 0.70 3.79 2.46 M100 0.09 0.010 0.006 0.65 6.18 2.82 T100 0.07 0.004 0.008 0.63 6.36 2.7 Table 2 Parameters of simulated heat-treat procedures E (kJ.cm -1 ) tp(s) Tp(℃) th(s) t8/5(s) t5/3(s) 20 15 1320 2 10 24 20 11 900 1 10 24 20 8 730 1 10 24 20 7 600 1 10 24 E:Heat-input, tp:heating time, Tp:peak temperature, th: holding time, t 8/5: the cooling time from 800℃ to 300℃, Table 3 Welding method and working condition. Welding Method No. Heat Input(KJ/mm) Shielding Gas Arc Current(A) TIG T5-21 2.1KJ/mm Ar 300A MAG DM4-1 1.6KJ/mm Ar+5%CO2 280A MAG M100 1.6KJ/mm Ar+5%CO2 280A TIG T100 2.1KJ/mm Ar 300A 3. Mechanisms of abnormal high impact toughness in simulated welding CGHAZ of an 8%Ni high strength steel 3.1 Comparison of experimental results measured in CG-specimen and FG-specimen As seen in Fig. 1, the strengths σy and σb at -50℃ and RT measured in tensile tests of CG-specimen are lower than those measured in FG-specimen. The plasticity in terms of ψ is also higher for FG-specimen. These results consist with the microstructures shown in Fig. 2 which show the extreme larger grains in CGspecimen and middle size grains in FG. This result can be fairly inferred from the Hall-Petch rule. However as shown in Fig. 3, the values of impact toughness measured in CG-specimen at RT and -50℃ are surprisedly higher than those measured in FG-specimen. Although from Fig. 3 and Fig. 4(a), the maximum loads of Charpy V tests measured in FG-specimen are appreciably higher than those that measured in CGspecimen, yet the load drops much faster with the crack propagation. It means that in the FG-specimen the resistance to the crack propagation is lower and at same applied load the deformation then the associated strain is lower than that of CG-specimen. These phenomena make the impact toughness, particularly the crack propagation energy Ep in Fig.3 being higher for the extreme coarse grain specimen. The reason, why
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