13th International Conference on Fracture June 16–21, 2013, Beijing, China 5 primary voids correspond to the large bainitic packets. The tiny carbide plates (only 0.01*0.04μm in size) precipitated inside the bainite are considered being not able to nucleate small voids. It can be concluded that although the grain sizes of CG specimens are extreme large, yet due to the high temperature (1320℃) heating, the carbon content is unified in the original austenite grain. After fast cooling the lath bainite microstructure keeps uniform in carbon content without local accumulation. The sizes of the carbide precipitation plates inside the bainitic lath are very small (only 0.01*0.04μm in size). The carbide plates with such tiny size can only be broken and nucleate the voids at a high level of plastic strain, however this high plastic strain cannot be reached before the coalescence of the primary voids in a notched Charpy V specimen with high stress triaxiality. And just this high stress triaxiality increases the rate of primary void growth. 3.3 Effects of stress triaxiality on the nucleation and growth of the voids Fig. 1 show that both the strengths at -50℃ and RT measured in tensile tests of specimen CG are lower than those measured in specimen FG, while the Charpy V toughness is higher. The reason is analyzed as follows Fig.7 shows fracture surfaces of CG-specimen (a) and (b) and that of FG-specimen (c) fractured by tensile tests at RT. Comparing Fig.5(a) and Fig. 7(a) for CG-specimen, it can be found that: in the impact test, the fracture is dominated by primary voids, which grow until impingement-resulting coalescence and final rupture, while in the tensile test the final rupture is caused by the secondary voids which connect the neighboring primary voids before their impingement. In FG-specimen, the fracture surfaces for both impact test (Fig. 5(b)) and tensile test (Fig. 7(b)) show rupture being caused by the shear sheets formed by numerous secondary voids which connect the neighboring primary voids long before their impingement. Therefore, the phenomena, that the CG-specimen shows the abnormal superior impact toughness, but the inferior tensile properties, can be attributed to the change of fracture mechanisms from the primary void-coalescence in the notch impact test to the secondary void-connection in the tensile test. In Charpy V tests, the energy, which is spent on the fracture by void growing until impingement-resulting coalescence in the CG-specimen is higher than that spent in the FG-specimen where the fracture is caused by forming shear sheets of the secondary voids to connect the neighboring primary voids before their impingement. However, in tensile tests, both CG-specimen and FG-specimen are fractured with the similar mechanism of connection of the primary voids by the secondary voids. The tensile properties of the CG-specimen are inferior due to its intrinsic inferiority such as large grain size. Then the question is why for the CG-specimen, the fracture mechanism is changed from primary void coalescence in the Charpy V test to the secondary void connection in the tensile test. It is attributed to the difference of the stress triaxialities between these two types of tests. In the Charpy V impact test, because the stress triaxiality at the the notch is high, up to 1.6 by FEM calculation, while in the tensile test it varies from 0.33 to approximate 1.0 from a smooth specimen to a necking one at area reduction of 65 -70%. According to formula (2) Ln(Rc/Ro) = 0.283εf(3σm/2 σeq), for the Charpy V specimen with a high stress triaxiality σm/σeq, the critical void radius Rc is reached at a lower fracture strain εf, which cannot cause the breaking of the tiny carbide plates precipitated inside the bainite laths or cause the breaking of the bainite laths in the CGspecimen. Therefore no (or only a few) secondary voids are produced in the fracture process (Fig. 6(b)). However, in the FG-specimen, the brittle martensite laths are broken at a less strain level and form numerous second voids, which connect the insufficiently growing primary voids and make an early fracture and lower toughness. In the tensile tests, the stress triaxiality is low and the plastic strain, which can reach is high. Even, in the CG-specimen, the secondary voids can be nucleated by the breaking bainitic laths (Fig.2(d)) before the impingement of the primary voids. The final fracture is caused also by the connection of insufficiently growing primary voids by the second voids (As shown in Fig. 7(a)). (a)1320℃ heating specimen (b) 900℃ heating specimen Fig. 7 Fracture surfaces of specimens fractured at RT by tensile tests (a) 1320℃, (b) 900℃
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