While the inclusions did not appear very different in Sample 3, the inclusions frequently had more complicated chemis- tries (See Figure 11 and 12). The analysis in Figure 12 could be explained if the inclusion is actually an agglomeration of oxides, sulfides, and nitrides.
The topological signal created by the acicular fer- rite structure seems to have over powered any signal gener- ated by the prior austenite boundaries.
It was impossible to differentiate the prior austenite bound- aries from the acicular structure of Sample 5 (See Figure 13). The SEM primarily generates its image from topologi- cal changes in the sample when using a secondary electron detector.16
The structure of Sample 6 under the SEM is presented in Figure 14. The inclusions observed had compositions typical for furnace slag and appeared at the outer regions of the prior austenite dendrites. These inclusions were likely engulfed during solidification. Their location within the microstruc- ture does not suggest that they served any function in devel- oping the as-cast structure.
Crystallographic Analysis
Based on the observations from the SEM investigation, three materials were evaluated for crystallographic comparisons with austenite and δ-ferrite. For a material to act as an ef- fective heterogeneous nucleation site, it must be solid at the steel’s solidification temperature, stable within liquid steel, and be wetted by the steel.1,2,5
can be confirmed by looking at material references and through thermodynamic calculations. Obtain- ing wetting data to determine if liquid steel will wet the material is very difficult. The lack of data is due to the difficulty in performing wetting studies with liquid steel. Instead of using wetting data, most re- searchers examine the lattice disregistry between the solidifying phase and the proposed nucleation phase. This is because metals are more likely to wet another material as the crystal structure of the mate-
rial becomes similar to the solidified metal. The most gen- eral way to calculate lattice disregistry between two similar crystal structures is with the Bramfitt’s modification to the Turnbull-Vonegut equation (See Equation 1).5
Equation 1
Figure 11. SEM image of Sample 3. The first two criteria
Figure 12. EDS spectrum from the inclusion in Figure 11.
Figure 13. Representative SEM micrograph of Sample 5. 22
Figure 14. Image of Sample 6 microstructure under SEM. International Journal of Metalcasting/Summer 10
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