EFFECTS OF ALLOYING ELEMENTS UPON THE EUTECTOID TRANSFORMATION IN AS-CAST SPHEROIDAL GRAPHITE CAST-IRON

Detailed experimental results related to the growth of ferrite and pea rlite during austenite decomposition in S.G. cast iron are presented. They have been obtained by means of quenching samples during direction al freezing and by differential thermal analysis. The kinetics of aust enite decomposition has been quantitatively measured in both cases. Th e effect of cooling rate, nodule count and alloying with low level of copper and/or manganese was investigated. The present results related to the effect of cooling rate and nodule count are in full agreement w ith the literature on the eutectoid transformation in S.G. cast iron. An increase of the former or a decrease of the latter shifts the kinet ics curves for austenite decomposition towards lower temperatures. Mor eover, it was found that adding manganese results in a significant inc rease in the rate of pearlite growth and a decrease in that of ferrite . Copper on the other hand results in a slight increase in the rate of both pearlite and ferrite formation. This latter observation disagree s with the generally accepted pearlite promoter role of copper. Howeve r, similar results have been reported previously, and it is proposed t hat the pearlite promoter effect could be related to high copper addit ion only. Finally, it has been observed that the undercooling for the start of the reaction, calculated with respect to the corresponding te mperature in the phase diagram, increases with the cooling rate for bo th the stable and metastable reactions. It is proposed that these unde rcoolings have two causes, one which should not depend on the cooling rate but on the alloying content of the cast iron, the second which is cooling rate dependent. Quantitatively, these effects are expected to depend on the nature and amount of the additives, silicon. manganese and copper, and on the associated solidification microsegregations.