Results


The cooling curves obtained from the TA cups all showed similar results for the solidification step in terms of minimum temperature and recalescence during the eutectic solidifica- tion. The measured nodule counts, N, are in the range 250- 280 mm-2


, and listed in Table 2. This similarity in the nodule count of the various cast metals gives adequate conditions for comparing the characteristics of the solid-solid transfor-


mation. As an example, Figure 2-a shows the cooling curve recorded from the TA cup cast with alloy NP6, as well as the time derivative of this curve. From the latter, it is seen that the solid state transformation relates to a clear peak that starts at a temperature significantly higher than the plateau observed on the cooling curve. Because the microstructure of all TA cups showed some ferrite to be present (see Table 2), it was assumed that the start temperature relates to the formation of this phase and was accordingly denoted as Tα,exp


. Table 1. Alloy Chemical Composition (wt. %)*


Note: * Numbers between brackets give the accuracy for each element. Tα


bounds of the three phase fields in the stable and metastable systems, respectively. Table 2. Characteristics of the Cooling Curves and Metallographic Results from TA Cups


and Tp


are the calculated lower


Time (s) (a)


Figure 2. (a) Cooling curve and (solid circles).


Ferrite fraction (%) (b)


its time derivative obtained from the TA-cup cast with


(b) relation between the final fraction of ferrite and the characteristics of the eutectoid transformation, Vtrans circles) and Ttrans


International Journal of Metalcasting/Winter 10 NP6; (open 53


Vtrans


(K/s)


Cooling rate (K/s)


Temperature (ºC)


Ttrans


(ºC)


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