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technical review & discussion

Experimental Evidence for metallurgical modification associated to chunky graphite in heavy-section ductile iron castings J. Lacaze; CIRIMAT, Université de Toulouse, Toulouse, France; I. Asenjo, S. Méndez, J. Sertucha, P. Larrañaga, R. Suárez; Engineering and Foundry Department, Azterlan, Durango (Bizkaia), Spain

Reviewer: The chunky graphite problem has had many pa- pers published which have related the formation of chunky to the presence of cerium. I wonder if the authors have con- sidered that compounds of cerium, not compound of magne- sium could be what is removed by extended holding? Free cerium could also act to form nodules.

Authors: We do agree that other elements may play a role, as noted in the beginning of the discussion section. The emphasis was put on magnesium because it is by far the main spheroidizing element in the investigated alloy.

Reviewer: The authors only reference one thermal analysis paper; many have been previously published that appear to explain the data quite nicely. Secondly, phase transforma- tion temperatures are known to be a function of cooling rate.

Authors: The paper dealing with thermal analysis was refer- enced only because it contains the information for calculat- ing the stable eutectic temperature. The effect of cooling rate on the characteristics of cooling curves obtained by thermal analysis or differential thermal analysis is effectively so well known that it did not appear necessary to elaborate on that. The curves were shown for comparison purpose, namely discussing the similarities and differences obtained with CHG (now called “with-CHG”) and no-CHG materials.

Reviewer: The authors should document how they “calcu- lated” the eutectic temperature (they would need to know the composition of both the CHG and no-CHG regions in the sample to do so) and whether or not they tried to account for the effect of cooling rate on the eutectic temperature.

Authors: The composition of the cast-iron is given in the section “experimental details”, and it has been verified that it is the same in the CHG and the no-CHG areas (ref- erence #11), i.e. that there is no macrosegregation. The eutectic temperature which is evaluated is that obtained from a phase diagram calculation, denoted “stable eutec- tic temperature” just above, as described in reference #13. Its value is given in the same paragraph of the section “ex- perimental details”.

Reviewer: The data shown in Figure 3 looks like classic nucleation and growth.

Authors: Certainly, and the stable eutectic temperature gives the upper temperature limit for both nucleation and growth.

Reviewer: The DTA records that show two phase transfor- mations (Figs 6&7) may simply be a liquidus and solidus- the recalescence that occurs due to undercooling or differ- ences in inoculation. The reported chemistry is near eutectic and the authors did not determine the actual chemistry of their DTA samples. This needs to be clarified.

Authors: It may be necessary to stress that DTA is not obtained by differentiating a thermal analysis curve. Any peaks on a DTA record such as the two peaks ap- pearing in the eutectic range of this study relate to heat release in the sample. Accordingly, the solidus that is the end of solidification, would not give a peak, though it may possibly be associated to a peak summit.


International Journal of Metalcasting/Winter 2012

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