effects similarly undercooling for the onset of the eutectic reaction and the shape of the eutectic arrest of both the with- CHG and no-CHG materials.
The micrographs in Figure 4 show the microstructure of both materials after cooling at 10ºK/min. where it is observed that solidification led effectively to the same microstructure con- sisting in a lamellar graphite eutectic at the outer surface of the samples which then transforms to undercooled graphite in the central areas. The presence of a significant amount of austenite dendrites allows the peak above the eutectic tem- perature to be related to primary deposition of austenite, in agreement with the hypoeutectic nature of the alloy. It may be noted in Figure 3 that the start temperature for this peak varies from one sample to another in an “apparently” erratic way, namely apparently without any relationship with the cooling rate or with the material (i.e. with-CHG or no-CHG). A possible explanation could be that the sampling procedure for machining DTA samples induced slight change in the ac- tual carbon content from one sample to another. Such a dif- ference would lead to a change in the observed liquidus tem- perature without affecting the eutectic reaction itself. Such a sensitivity to sampling may simply be related to the fact that the characteristic size of the microstructure in the blocks, either austenite dendrites or CHG cells, is of the same order as the size of the blanks used for DTA.
A further comparison of the microstructure obtained after a DTA run is given in Figure 5 for samples solidified at 2.5ºK/ min. It is seen that lamellar graphite shows the same features for both with-CHG and no-CHG materials mainly consisting of coarse graphite lamellae. This change in the graphite morphol- ogy with respect to Figure 4 is a consequence of the reduction in the cooling rate and thus in the graphite growth rate from liquid.
In Figure 6 we compare the DTA traces recorded during the first and second coolings of the additional trial car- ried out for the no-CHG material. The vertical interrupt- ed line represents, as before, the calculated stable eu- tectic temperature. The main differences between these two records show up during the eutectic reaction, which consists of two and one peaks for the first and second cooling records respectively. The same behaviour was observed for the with-CHG material. It is noteworthy that the first eutectic peak during the first cooling starts at nearly the same temperature as the single eutectic peak during the second cooling. This suggests relating it to an initial eutectic solidification with lamellar graphite also for the first cooling. However, after this short ini- tial transformation, the main eutectic reaction during the first cooling takes place at a much higher undercooling that is more typical of compacted-vermicular (CVG) or spheroidal (SG) graphite.
The graphs in Figure 7 compare the DTA curves recorded for various cooling rates on with-CHG (solid lines) and no- CHG (dotted lines) materials after a very short stay in the liquid state. In these graphs, the reference line corresponding to the stable eutectic temperature has been superimposed as shown in Figure 3. The corresponding microstructures are illustrated in Figure 8. Looking first at the records obtained with no-CHG samples (dotted lines in Figure 7), it is seen that they all show the same features for the eutectic reaction, i.e. a small peak starting at about the stable eutectic tempera- ture and a main peak at significant undercooling. The micro- scopic observations showed effectively very little lamellar graphite as illustrated in Figure 8 (column to the right) and the bulk material has solidified with a mix of vermicular and spheroidal graphite.
(a)
(b)
Figure 3. Effect of cooling rate on solidification of (a) with-CHG and (b) no-CHG materials after holding at 1200°C to eliminate Mg. The vertical dashed line represents the calculated eutectic temperature.
International Journal of Metalcasting/Winter 2012 37
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