Technical Review and Discussion
Eutectic Cells and Nodule Count— An Index Of Molten Iron Quality E Fraś, University of Science and Technology, Cracow, Poland, H. López, University of Wisconsin-Milwaukee, USA.
Reviewer: The equations derived in the paper all require experimental input to tune the parameters. The value of the nucleation parameters changes for each inoculant and need to be determined for each one. As a result, the equations are not able to predict the behaviour of an inoculant without prior experimentation. Theoretical models should be able to make general predictions.
Authors: There are numerous fundamental laws which con- tain various parameters such as the First Fourier law. This equation contains the thermal conductivity parameter k.
Authors: Russian [20] and German works [22] indicate that the size of graphite particles in liquid iron varies be- tween 10-7
and 10-4 cm. Their relative weight and volume
is so small that their effect on the content in the carbon in solution is negligible. Consequently, there is no effect on the liquidus temperature. This can be illustrated through the fol- lowing example: From calculations, the number of graph- ite nuclei equals the number of graphite nodule and in thin walled castings the nodule densities can reach Nn = 1400 mm-2
=2.6x108 g/cm3 cm-3
particle size l = 10-4 cm3
. Taking into account the largest graphite cm, its volume V = π l3
and its mass M = Vρ = 1.7 10-4 g (ρ = iron density = 7.1 Nn/ 6 = 7.9 10-5
)., Considering that the carbon content in a 1 cm3 of a casting is 3.7 wt%. This, in turn, means that in 7.1 g there is 0.2677 g of carbon in solution. Hence, carbon content of C = 0.2677/7.1 = 3.7 wt %. The carbon in the graphite par- ticles is barely 1.7x10-4 the amount of carbon in solution C = (0.2677- 1.7x10-4
)//7.1 = 3.697 wt %. Such a small change
in the carbon (0.003%) in solution is not possible to be de- tected by thermal analysis. In conclusion, thermal analysis can not be used to verify this hypothesis.
The equation is not able to predict the temperature field in a given material without prior experimentation aimed at determining the parameter k. In the proposed work the situation is analogous. Once the nucleation parameters are established for a given inoculant, equations (11 and 12) can be used to calculate the cell or nodule count.
Reviewer : The hypotesis on p.2 (reference 21-23) supposes that not all graphite went into solution. This hypothesis in mainly based on work of [referenced] researchers. If part of the carbon is not in solution then it should have an influence on the liquidus temperature which only reacts on carbon so- lution. Considerable research on thermal analysis has never confirmed this hypothesis. The authors introduce this hypo- tesis in Eq. 4-7.
Reviewer: Hypothesis on page 3 says “nucleation stops at Tm
”. Published research by Stefanescu and Engler show that in ductile iron, the nucleation of new nodules continues dur- ing solidification. Also Skaland has shown this by using the special inoculant (with O and S) which permits to nucleate new small nodules towards the end of the solidification. Seg- regation can play a role here since the composition of the remaining melt continuously changes during solidification.
Authors: Besides the theoretical arguments, the hypothesis used in this work is based on the experimental work of Fras E., (The Kinetics Of Growth Of Eutectic Cells In Gray Cast Iron, AGH, Cracow, (1976)) where the solidification process is interrupted in a gray (C=3.93%, Si=0.97%, Mn=0.01%, P=0.01%,S=0.02%). Also see micrograph in Fig. Ia). This
Fig.I. Number of eutectic cells (a) and (b) after 31 s and (b) after 85 s solidification time. (a)
International Journal of Metalcasting/Summer 10
(b) 59
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