silicon content than NF3. The time derivative of the DTA signal showed that the ratio of the maximum growth rate of ferrite for alloy NF3 and NP6 to be 18.9/3.6=5.25. This value compares well with the 3.7 value estimated by Ågren10 as the jump of the carbon diffusion coefficient at the Curie temperature, thus supporting the present hypothesis.
Figure 6 also shows that at a cooling rate of about 60 K/ min, as-cast (TA cups) and heat-treated (U3) materials do present very different ferrite contents. This positive effect of heat-treating on the ferrite fraction has been already report- ed5
and was tentatively related to the recrystallization of the matrix upon re-austenitizing, resulting in higher number of rapid diffusion paths for carbon from austenite to graphite. Further, it was noted that high Cu addition leads to the for- mation of intergranular ferrite3
, and this was observed again in the present study as illustrated in Figure 8. In this figure
are shown the microstructures of alloys NF3 and NP5 from samples cooled at two different rates selected so that they show a similar final amount of ferrite. It is clearly seen in Figure 8-b that part of the ferrite is intergranular, while there is no such feature in Figure 8-a.
The effect of heat-treating may also be illustrated by plot- ting Ttrans
as-cast and U3 samples (Figures 9-a and -b). At low fer- rite fraction, it is seen that the pearlitic reaction proceeds at about the same temperature for as-cast and heat-treated
and Vtrans
alloys, but with a higher transformation rate (higher Vtrans values) for the former. On the right side of the graphs, when the transformation is essentially ferritic, it is seen that the transformation temperature is lower and the rate of trans- formation higher for heat-treated samples than for as-cast ones. It is not expected that the austenitizing treatment could
as a function of ferrite fraction for the
Temperature (ºC) Copper content (wt. %)
Figure 6. Evolution of the ferrite fraction with copper content for various cooling conditions.
Figure 7. Comparison of the DTA traces recorded on alloys NF3 and NP6 at 5 K/min. The open arrow indicates the Curie transformation of the ferrite matrix in alloy NF3.
(a)
(b)
Figure 8. Comparison of the microstructure of alloys NF3 and NP5 after cooling at, respectively, 5 K/min and 55.5 K/ min (Nital etchant).
International Journal of Metalcasting/Winter 10 57
Ferrite fraction (%)
DTA signal
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