(open circles). This is in agreement with the usual view that the ferritic transformation starts before (i.e. at higher temperature) than the pearlitic one upon cooling.
values of Vtrans
Figure 2-b shows that increase in ferrite content is associated with higher values of Ttrans
(solid circles) and more negative
Figure 3-a illustrates the temperature evolution and its time derivative recorded during one of the heat treatments per- formed in the tubular furnace, including the heating and cool- ing stages. It is seen that the cooling stage shows features similar to those of the records obtained from the TA cups, so that the same characteristics as before could be determined. The results are listed in Table 3 where the measured ferrite fractions are also given. In Figure 3-b the ferrite fractions ver- sus Vtrans
.
are plotted for this series of experiments, where the various symbols used differentiate the cooling conditions (U1 to U4) but not the alloys. It is seen that the evolution is similar to the one observed in Figure 2-b for as-cast alloys, and the same similarity was also noted with Ttrans
temperatures were generally estimated at the intersection of
Finally, Figure 4 shows an example of a DTA record obtained upon cooling for alloy NF3. On most of the DTA records, two peaks could be identified that have been associated to the formation of ferrite for the one at higher temperature and of pearlite for the second one. The associated Tα,exp
where open symbols (and crosses for NF3) are for the ferritic reaction and solid symbols (and plus signs for NF3) are for the pearlitic one. As expected, it is seen that the undercooling of both the ferritic and pearlitic transformations increases with cooling rate. Moreover, in line with previous works, it seems that the undercooling values for the ferritic transformation could extrapolate to zero at zero cooling rate, while a minimum undercooling of 30 to 50°C (86 to 122°F) appears necessary for the pearlitic reaction to start. It is also seen in the figure that the rate of increase of the undercooling with cooling rate is higher for the ferritic reaction than for the pearlitic one. Thus, the two series of points cross each other for a cooling rate of about 150- 200 K/min, beyond which virtually no ferrite would appear. These features have been emphasized in Figure 5b with the interrupted line drawn through each of the two series of points.
The evolution of Tref -Texp
It is also noted that the data in Figure 5-b appears quite scat- tered and this could not be related to the copper content of the alloys, neither for the ferritic reaction nor for the pearlitic one. Indeed, an effect of the copper content on the under-
(Tα,exp and Tp,exp
the base line and signal extrapolations as illustrated for Tα,exp in Figure 4. When pearlite growth showed up as a small peak on the DTA curve, the associated temperature Tp,exp
was bet-
ter characterized by the temperature corresponding to the lo- cal minimum in the DTA signal (Figure 4). The transforma- tion temperatures thus obtained are listed in Table 4.
Discussion
Figure 5-a illustrates, in the case of alloy NP1, the marked lowering of the phase transformation temperatures when the cooling rate is increased. It is seen that the pearlitic transfor- mation starts at a temperature much lower than the ferritic one at a low cooling rate. The difference decreases with increasing cooling rate. Also, it may be noted that the Ttrans shifts from Tα,exp
temperature
and thus characterizes the whole process of austenite decom- position rather than one of the two eutectoid transformations.
to Tp,exp
reference and experimental temperatures for either the ferritic (α) or pearlitic (p) transformations. The reference tempera- tures have been calculated using the relationships given by equations 1 and 2, considering the content of the matrix in each substitutional solute to be 1.05 times the nominal content of the alloy in order to account for the presence of graphite. They are listed in Table 1. It is seen that the reference temper- atures for the ferritic and pearlitic transformations change by more than 15°C (59°F) because of the composition changes.
To account for the composition difference between the alloys, it seems more appropriate to look at the effect of the cooling rate on the undercooling (Tref
-Texp ), where Tref and Texp are the International Journal of Metalcasting/Winter 10 when the cooling rate is increased,
Table 4. Characteristic Temperatures /Tp,exp
) Obtained from the DTA Records
(undercooling) is shown in Figure 5-b
Temperature (ºC)
Figure 4. Example of DTA record (alloy NF3, cooling rate of 10 K/min).
55
DTA signal
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