Austenite/ferrite isothermal reaction in a 0.4% Mn spheroidal graphite iron

A 0.4 wt% Mn ductile iron was austenitised at 950°C and isothermally transformed in the ferritic domain at 765,755, 745 and 735 °C. The kinetics of austenite decomposition has been determined for each selected temperature by means of dilatometry. Additional observations have been made through optical and electronic microscopy. The transformation at 765°C was found to be particularly slow, requiring more than one day to be completed. At the other temperatures tested, four hours were found enough to achieve austenite decomposition. For the alloy studied, results showed that stable austenite decomposition into ferrite and graphite is interrupted by the formation of carbides, even for the highest selected temperature which is only 10°C below the lower limit of the three phase field austenite/ferrite/graphite. Particular attention has been paid to the beginning of ferrite growth at 765°C. Ferrite was observed to nucleate systematically at the graphite/austenite interface, and it was observed that growth of ferrite proceeds to a significant extent before graphite nodules are totally surrounded with a ferrite shell. At lower temperatures, i.e. higher undercooling, it is expected the formation of a ferrite halo occurs soon in the transformation process. Carbide precipitation was evidenced in inter-cellular austenitic zones which are certainly associated with low Si and high Mn contents. These results have been analysed in terms of a competition between volume diffusion of carbon through the growing ferrite shell or rejection of carbon in the austenite close to the transformation front.