Letter to the Editor
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e authors of the paper, discussed here, Freulon et al.,[1] have used two commercial composition ductile iron samples to evaluate composition profiles across ferrite/pearlite boundaries at room temperature. They also performed differential thermal analysis (DTA) to record eutectoid transformation temperatures during cooling at various rates after reaustenitization. They conclude that ‘‘composition profiles across ferrite/pearlite boundaries are smooth and similar to those issued from the solidification step’’ and also that ‘‘no trace of long-range diffusion of substitutional solutes could be detected.’’ As no similar reports for cast irons have been reported in the literature, the authors suggest that ‘‘showing smooth composition profiles must have been considered as not being worthy of publication.’’ They do mention in passing a report by Guo and Stefanescu[2] that disagrees with their conclusions, but they dismiss it summarily, stating that ‘‘Though not giving much detail, the authors indicate that some eutectic carbides were present in the as-cast material they investigated which may better explain the abrupt compositional changes they sometimes observed.’’ There is no mention of such carbides in Reference 2. However, such a sample is mentioned in some previous work by Guo and Stefanescu,[3] which does not appear in the reference list of the paper discussed here. This work also includes extensive details of the experimental method, to which Freulon et al. had access, as they mention the existence of carbides in a sample. The purpose of this discussion is to clarify the apparent discrepancies
DORU M. STEFANESCU is with The University of Alabama, Tuscaloosa, AL 35487, and also with The Ohio State University, Columbus, OH 43210. Contact email: [email protected] Manuscript submitted October 31, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
between the work of Freulon et al. and that of Guo and Stefanescu. First and foremost, there is a fundamental difference between the ductile iron samples characterized in these two papers. In the Freulon et al.’s[1] work, the microprobe traces for chemical composition were determined on DTA samples obtained after reaustenitization and subsequent cooling at a controlled rate. It is not clear what that rate was, as DTA experiments were run at cooling rates ranging from 2 to 20 K/min. These rates are rather slow if one expects to find interface distribution of elements during a phase transformation. Actually, as shown in Figure 1, decreasing the cooling rate from 24 to 12 K/min is sufficient to produce a significant change in the transformation kinetics, from a one step, mostly pearlitic transformation, to a two-step ferritic/pearlitic transformation. It is thus to be expected that during slow cooling to room temperature, diffusion will moderate the compositional differences at the phase interfaces. On the other hand, Guo and Stefanescu[2] used samples that were cooled under controlled rate to a temperature within the eutectoid transformation interval and then quenched in water from various
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