Reply to the Letter to the Editor
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scussion of our paper,[1] Doru M. Stefanescu first stresses that the presence of carbides in the as-cast samples he investigated with Guo was mentioned in one of their other papers,[2] not the one[3] we referenced. It is agreed that the presence of carbides was clearly stated in the first paper by Guo and Stefanescu;[2] however, the authors mentioned that ‘‘When the segregation of Cr and/or Mn reached a certain level, complex carbides of the (Fe,Mn)3C or (Fe,Cr)3C may form’’ in their second paper[3] on page 439. As the alloys and experimental conditions were exactly the same in both of their studies, it was presumed—perhaps improperly—that both studies therefore dealt with the same samples, and this sentence was understood to mean that the authors indicated the possible presence of some carbides in their samples. Unless stated by D.M. Stefanescu, the conduct of the experiments was very similar in his study with Guo to that of our own work, in that commercial cast irons were used, re-austenitized, and then cooled at controlled rates. In our experiments, the cooling rate varied from 2 to 20 K/min but that of the observed samples were 2 and 5 K/min as clearly indicated in the paper.[1] In the work of Guo and Stefanescu, the cooling rate was either 12 or 24 K/min. The difference between Guo and Stefanescu’s two papers is that in the first one,[2] they used energy-dispersive spectrometry (EDS), while in the second one[3] they carried out microanalyses with wavelength-dispersive spectrometry (WDS).
JACQUES LACAZE is with the CIRIMAT, Universite´ de Toulouse, 4 alle´e Monso, CS 44362, 31030 Toulouse Cedex 4, France. Contact e-mail: [email protected] Manuscript submitted December 14, 2016. Article published online February 23, 2017 2130—VOLUME 48A, MAY 2017
Even if decreasing the cooling rate truly favors ferrite formation, this does not mean it could help smoothing compositional differences due to the eutectoid transformation as claimed by Stefanescu in his discussion. There are two reasons for this: 1—It is known that any chemical heterogeneities such as microsegregation issued from the solidification step are hardly smoothed out by heat treatment in the case of cast irons, and this must be due to the low diffusion coefficient of substitutional solutes (Cr, Cu, Si substitute to iron in the fcc and bcc matrices) as well as to thermodynamic interactions. For a cooling rate of 12 K/ min as illustrated in Figure 1(B) of Stefanescu’s discussion, the eutectoid transformation proceeds at about 1023 K (750 °C) and takes a time of t = 600 seconds. Following Fridberg et al.,[4] an upper estimate of the diffusion coefficient of substitutional solutes D can be taken as 10 times the self-diffusion coefficient of iron for light elements such as silicon, and 5 times for elements such as chromium and copper. The self-diffusion coefficient of iron was given for austenite as 7.0 9 10 5Æexp( 286000/RT) m2 s 1 and for ferromagnetic ferrite as 1.6 9 10 4Æexp( 240000/RT) m2 s 1, where R is the gas constant (8.314 J mol 1 K 1) and T the temperature in
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