Due to the larger size of this casting and therefore the slower cooling rate, the dendritic structure is much coarser in Sample 3 (See Figure 3). Unlike Figure 2, the dendritic structure of the ferrite clusters can be easily discerned. The carbon segregation during solidification, which was explained in the previous paragraph, created a low carbon region within the austenite dendrites. When the casting cooled, the proeutectoid ferrite had an easier time forming at the center of the dendrites. The distribu- tion of the proeutectoid ferrite preserved the as-cast den- dritic structure.
It was impossible to determine the as-cast structure of Sam- ple 4 (See Figure 4).
The prior austenite structure of Sample 5 is visible in Figure 5. It is difficult to resolve the structure due to the acicular ferrite structure dominating the sample contrast. The dark areas within Figure 5 are the prior austenite dendrite boundaries.
The dendritic structure is clearly evident in this ma- terial due to the distribution of ferrite grains which preserved the austenite dendrite structure (See Figure 6). Interestingly, Sample 6 appears to have the largest dendrite size of all the samples.
Electron Microscopy Figure 8. EDS spectrum from location in Figure 7.
Samples 2, 3, 5 and 6 were further examined using the SEM. Inclusions found within dendrites were analyzed with EDS spectroscopy to assist with identifying the phase. One of the difficulties with searching for nucleating phases using only optical microscopy and an SEM is that crystal- lographic information cannot be found. Because some of the inclusions are engulfed by the solidification front and others may be possible nucleation sites, crystallographic information is needed. An investigation should include TEM to capture crystallographic relationships between in- clusions and austenite. The alloys analyzed did not contain retained austenite that could be examined to determine the crystallographic relationship between the austenite den- drites and a particular inclusion. The microstructure that encased the inclusions has gone through the eutectoid reac- tion and lost the crystallographic relationships of the as- cast structure.
Figure 7 is an inclusion near the tip of a prior austenite den- drite tip in Sample 2. The chemical analysis was consistent with alumina (See Figure 8). Due to its location at the tip of a dendrite, this particle was probably engulfed during solidification and did not serve as a nucleating site. Titania, iron oxides, and silicates were also observed within these dendrites. The most interesting finding within this sample was a small particle near the center of a dendrite (See Figure 9). The chemical analysis is consistent with TiN (See Figure 10).
International Journal of Metalcasting/Summer 10
Figure 7. SEM Micrograph of Sample 2 indicating loca- tion of EDS spectra.
Figure 9. Particle near the center of a prior austenite den- drite in Sample 2.
Figure 10. Spectrum from the particle in Figure 9. 21
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