cooling of the pearlitic reaction was not expected as it has been shown that limited additions of Cu, Mn, As or Sn do not affect growth kinetics of pearlite in cast irons.8
The lack
of correlation between copper content and undercooling for the start of the ferritic reaction lead to Figure 6 where the amount of ferrite is plotted as a function of the Cu content for the various cooling conditions available, i.e. for the U series as well as for the as-cast samples. The effect of Cu on the eutectoid reaction is evident. It clearly appears that 0.6 wt. % Cu is a critical limit for cast irons with 0.1 to 0.2 wt. % Mn. Above this limit a significant amount of ferrite could precipitate only at low cooling rates. For low levels of Cu, from 0.1 to 0.6 wt. %, the amount of ferrite decreases slowly for the as-cast material, while it remains nearly constant for U materials (with the possible exception of the U2 series).
This evolution of the final ferrite fraction with Cu in the range 0 to 0.6 wt. % could appear in line with the pre- vious result that a low level of Cu counteracts the effect of manganese.3
that Cu opens the window (Tα
i.e. that contrary to manganese it decreases Tα Tp
the Cu content varied from 0.11 to 0.6 wt. Thus, it is quite possible that the effect observed in Figure 6 is also due to an increase of the start temperature for the ferritic reac- tion. This effect of the transformation temperature appears evident when noting that Tα
(see equations 1 and 2). However, it is seen in Table 1 that the Cu addition was associated with an increase of the Si content in the present series of castings. Thus, both the temperature Tα
and the window (Tα -Tp
A possible reason of this effect could be -Tp
) for the ferritic reaction, less than
) did increase when
for these four alloys.
NP4 to NP7, i.e. for Cu contents from 0.55 to 0.95 wt. %, while the window (Tα
-Tp
) remains at about the same value
, but should also change the growth kinetics of ferrite. In order to verify this effect, the DTA records of the whole series of alloys were compared for the three cool- ing rates available. The records on alloys NF3 (ferritic) and NP6 (pearlitic) for a cooling rate of 5 K/min are plotted in Figure 7. On both curves, the arrests at highest and lowest temperatures relate respectively to the ferritic and pearlitic transformations, as already mentioned in relation with Fig- ure 4. The intermediate bump on the record of alloy NF3 (open arrow) is due to the Curie transformation of ferrite. The comparison of these two records shows that the growth rate of ferrite is much smaller in the case of alloy NP6 than for alloy NF3. As a matter of fact, a continuous decrease of the ferrite growth rate was observed with increased addition of copper from alloy NF3 to alloy NP7 for all three cooling rates investigated with DTA. Such an effect appears far too marked to be due only to the slight temperature difference for the start of the ferritic reaction that is observed. It is here postulated that the effect is mainly due to the sharp decrease of the carbon diffusion coefficient in ferrite at the Curie tem- perature as assessed by Ågren.10
The drastic decrease of the ferrite fraction above 0.6 wt.% Cu suggests that copper does not only affect the start tem- perature and the window for the ferritic reaction as postu- lated previously3
Though the Curie tempera- decreases strongly from alloys
ture could not always be observed on the DTA traces, it is estimated at 744°C (for alloy NF3 from Figure 7 and should be similar for all other alloys as they contain about the same
Cooling rate (K/min) (a)
Cooling rate (K/min) (b)
Figure 5. (a) Evolution of the characteristic transformation temperatures with the cooling rate for alloy NP1; (b) evolution of the undercooling of the ferritic (open symbols and crosses) and pearlitic (solid symbols and plus signs) reactions for all alloys and cooling rates.
56 International Journal of Metalcasting/Winter 10
Temperature (ºC)
Undercooling (ºC)
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