Stable Eutectoid Transformation in Nodular Cast Iron: Modeling and Validation

Discussion on “Stable Eutectoid Transformation in Nodular Cast Iron: Modeling and Validation”*

“…In summary, the growth of ferrite at any temperature above the lower limit of the equilibrium three-phase field, as assumed by Carazo et al [1] and in the studies that inspired it [2,3] is impossible upon continuous cooling of a cast iron because it would need long-range diffusion of substitutional solutes ahead of the supposedly growing ferrite. This is true for any cooling rate higher than about 1.2 K/min [8] and for any shape of graphite.…”

“…Only a few models explicitly consider the existence of such an austenite-ferrite-graphite range: the contribution under discussion, [1] those that inspired it [2,3] and one previous study from the present author. [4] For kinetics reasons, this latter work explained that ferrite could not grow within the equilibrium threephase field under continuous cooling; this is in contradiction with the other three reports.…”

“…Among the numerous works performed in this area, one finds models for lamellar graphite irons, [17] compact graphite irons, [3] and spheroidal graphite irons. [1,2,4,18] In contradistinction with the experimental evidence mentioned above, there is a long-lasting controversy on when ferrite can nucleate and initiate the eutectoid transformation in relation to the equilibrium three-phase field. As mentioned previously, there are very few works where the opening of the austenite-ferrite-graphite equilibrium three-phase field due to silicon and other substitutional solutes is actually taken into account.…”

“…As mentioned previously, there are very few works where the opening of the austenite-ferrite-graphite equilibrium three-phase field due to silicon and other substitutional solutes is actually taken into account. The study discussed herein [1] assumes that ferrite may nucleate and grow as soon as the upper limit of the equilibrium three-phase field is reached and that the ferrite then grows by rejecting carbon into austenite. Figure 2 shows the isothermal section of the Fe-C-Si phase diagram at 1084.3 K that is the temperature of the upper limit of the equilibrium three-phase field for an alloy with 2.5 mass pct Si.…”

“…For a long time, such equilibrium ferrite has been shown to appear at austenite grain boundaries and not close to graphite nodules. [5,7] Under such isothermal conditions, since carbon activity is the same everywhere in the material, the growth of ferrite has to be explained by means of diffusion of substitutional solutes and not by carbon transfer as stated by Carazo et al [1] …”