13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- 3.3. Weld pool shape The pictures reported on Fig. 4 show that it was possible to obtain an elliptical pool shape by a reduction of the welding speed. However, the reduction of the welding speed did not cause a significant grain curvature as shown on Fig. 5a. On the other hand, a longitudinal grain formed at the center of the weld. At the lowest welding speed (2.54 mm/s), a thicker longitudinal grain was observed and there was still no evidence of grain curvature. a) Welding speed of 5.27 mm/sec (WS 1, HI 1) b) Welding speed of 2.54 mm/sec (WS 3, HI 1) Figure 4. Weld pool shape at two different speed, for the same heat input (142 J/mm) a) Longitudinal section b) Cross section Figure 5. Microstructure of the specimen produced at 3.39 mm/sec (WS 2, HI 1) The effect of a reduction in heat input is shown on Fig. 6. The microstructure observed was produced at the same speed as the ones shown in Fig. 1 (5.27 mm/s) but at a heat input of 129.0 J/mm rather than 140.0 J/mm. Reducing the heat input had the expected effect on grain growth. The grain growing direction tilted toward the top of the weld (radial grain growth). This is beneficial to the weld microstructure as it avoids the formation of a sharp centerline. On Fig. 6a and b, the grains nearly seem equiaxe but this could be an optical illusion caused by the fact that the growing directions of the grain are not parallel to the cross section and the longitudinal plane. To produce radial grain growth, it is necessary to promote the formation of a V shape weld. Such a weld is however more susceptible to incomplete penetration. In fact, the weld produced at a heat input of 112.3 J/mm was not fully penetrated. 1000 m 1000 m
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