13th International Conference on Fracture June 16–21, 2013, Beijing, China 9 As shown in Fig. 13 there is not appreciate differences of the densities of high degree misorientation boundaries revealed by EBSD for T5-21 and DM4-1 metals. To be surprised, the density of high degree misorientation boundaries in base metal is appreciably lower than those of weld metals. In followed Fig. 15, by comparing the high misorientation degree boundaries and the fine tear ridges in the cleavage facets, it seems that the latter is produced by the former. Thus, in this study, the resistance to the cleavage cracking is mainly provided by the packet boundary, and the high misorientation degree boundaries offers some resistance and produce the fine tear ridges in the cleavage facets. 4.4 Decisive microstructural constituents Based on the above discussions, it is concluded that the decisive microstructural feature which determines the impact toughness in the notched bar is the bainite packet rather than the region contained by the high misorientation degree boundaries. The bainite boundary provides the main resistance and makes the propagation of a packet-sized crack across the boundary the critical event for the cleavage fracture. The packet size specifies the critical cleavage facet and the local fracture stress σf. The reason why the values of impact toughness measured at -50oC for base metal, and T5-21 are generally higher than those measured for DM4-1, T100 and M100 is attributed to that the size of the bainitic packet of the former group is finer than that of the latter group. Fig. 16 shows a typical comparison between the packet sizes of the DM4-1 weld metal and the T5-21 weld metal. It is apparently that the sizes of packets in T5-21 are appreciably smaller than that in DM4-1 weld metal. The general comparison of the grain or the packet sizes is summarized in Fig. 12. As the lengths of the critical events corresponding to the sizes of packets of the first group of the base metal and T5-21 are smaller than that of the second group of DM4-1, T100 and M100. This is the reason why the impact toughness of the metals of the first group is superior to that of the metals of the second group. 4.5 Effects of nickel addition As indicated in the introduction, Ref.[8] revealed that regardless of packet size and yield strength, the addition of nickel gave rise to a decrease in Charpy V impact transition temperature of about 20oC per percent nickel. He attributed this effect to increasing the cleavage fracture resistance [8]. In this work at first the metals were divided to two groups: the first group including MC and T5-21 which presents superior impact toughness and the second group including DM4-1 which presents inferior toughness. From the composition point of view, the only appreciable distinction in alloy content is the difference in Ni contents: around 6-7% Ni for the first group and lower than 4%Ni for the second group. The effects of Ni addition were focused. Table 5 compares the relative parameters between the two groups. As seen in Table 5, the grain sizes of weld metals with higher Ni contents are finer. Besides the fine grain/packet sizes in the metals of the higher Ni content group, the fine bainitic laths and the higher fractions of the M-A constituent were observed. Because in steels with higher Ni content the flaks of the M-A constituent were found to be rich of austenite and have high plasticity[10], both fine bainitic laths and Ni-rich M-A flakes causes denser higher misorientation boundaries and higher resistance by the appearance of fine tear ridges on the fracture surface. The effect of Ni addition is considered to be decisive in the difference of impact toughness between these two groups. The mechanism of the improvement needs to be investigated in future. Table 5 Parameters relevant to Ni content and toughness ( drawn from Table 5) No. E-50 o C (J) Nickel (%) Average grain size(μm) Width of Bainite lath(nm) Average width of Bainite lath (nm) Area fraction of M-A(%) MC 177 7.58 20 20-80 34 58 T5-21 163 5.74 30 65-186 113 39 DM4-1 24.5 3.79 60 180-850 428 26 Fig 15. Comparing the high misorientation degree boundaries and the fine tear ridges on the fracture surface (a) (b) Fig 16. Sizes of packets in (a)DM4-1 (b) T5-21
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