Influence of solidification structure on creep behavior of nonalloyed steel at high temperatures
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Fig. 1 - - The sulfur print of a continuously cast slab with positions of the specimens indicated: (a) (top) specimens with the primary dendritic arms parallel to the specimen axis, (b) (right) specimens with the primary dendritic arms normal to the specimen axis, and (c) (left) specimens with equiaxed dendritic structure.
the cathetometer. The magnification on the film was 5 times. The core of the differential transformer was fixed to the connecting rod of the weight. The measured strains include the elongation of the connecting gear and were corrected with those measured with the camera. Results: The creep curves are shown in Figure 2 in which the point symbols show the results for the continuously cast material and the lines are those for the worked material.t3] The strain rate gradually decreases with increasing time, and eventually reaches a range with approximately constant strain rate (tests at 4.2 M N / m 2). At 7.1 M N / m 2 and 1350 ~ there is an increase of strain rate at about 110 seconds in the as-cast material and at about 100 seconds in the hot-rolled material which is due to dynamic recrystallization. The dynamic recrystallization is the dominant restoration process for metals with low stacking fault energy like austenite. [4] The specimens of the as-cast material show the same creep strains within the experimental error. On the other hand, the strain of the worked material is smaller than that of the as-cast material. The results show that the creep behavior does not depend on the solidification structures. This may be attributed to the y-c~ and c~-y transformation occurring during cooling and heating by which the original grains are destroyed and grains of similar size are produced. Figure 3 shows etched sections of three samples which were prepared after the creep test. The microsegregation is still present as is evident from the dendritic structure that is clearly visible. Apparently, the difference of the dendritic structure does not have a significant effect on the creep behavior. The difference between the results for the as-cast material and for the hot-rolled material cannot be explained easily. There is some difference in composition of the steels. Particularly, the as-cast material contains about half as much sulfur. Consequently, its sulfide inclusion content is also about half. Furthermore, in the hot-rolled material, the sulfide inclusions are elongated in the direction of the sample axis, whereas in the as-cast samples there is no directionality and VOLUME 19A, NOVEMBER 1988--2857
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