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, Milwaukee, WI, USA Copyright © 2010 American Foundry Society Abstract
The present work gives a detailed analysis into one of the most important quality parameters in cast iron, namely the eutectic cells or nodule count. The analysis is based on theoretical fundamentals of cast iron solidification. Expressions are derived and used to relate either the number of cells or the nodule count to factors of technological importance. It is found that such factors are intrinsically related to the chill tendency, pre-shrinkage expansion, shrinkage porosity, shrinkage depression and graphite type. Also, it is shown that the resultant mechanical properties of
Introduction
Gray and ductile iron is widely produced in the metalcast- ing industry. Hence, there are numerous reports related to iron casting processing factors, some of which are of critical importance as they include the solidification of the graphite eutectic. In the austenite-graphite eutectic, cast iron solidifi- cation involves the formation of eutectic cells (Fig.1a-c) that are relatively spherical in shape. The eutectic cells are made of interconnected graphite flakes surrounded by austenite. Alternatively, in ductile iron the graphite nodules are sur- rounded by austenite as shown in Fig.1d.
The importance of the exhibited number of eutectic cells or nodules can be related to the graphite nucleation potential of the liquid iron. In general, increasing the number of eutectic cells or nodule count in iron leads to:
• • • •
a reduction in the chill susceptibility of cast iron;1-4 an increase in the pre-shrinkage expansion;5,6 a reduction in segregation7,8
and changes in the usable properties.9-13
Accordingly, it is evident that the graphite eutectic cell and nodule count have a strong influence on the various tech- nological factors used in the foundry practice. This work further explores the present knowledge on cell and nodule count and the effect of various physical and technological parameters based on solidification fundamentals and from the viewpoint of foundry practice. For simplification pur- poses, the results of numerical modeling for secondary nu-
International Journal of Metalcasting/Summer 10
cast iron are strongly influenced by the foundry practice. In particular, it is established that the eutectic cell density or the nodule count is a quality factor reflecting the physical- chemical state of the liquid cast iron at a given cooling rate. In turn, this determines the graphite nucleation potential of cast iron which can be used to incorporate all of the technological factors associated with melt preparation.
Keywords: cast iron, cell count, nodule count, graphite nucleation, chill
cleation14 and secondary (inverse) chill,15 as well as various
speculations on the types of graphite nucleation substrates were omitted from this work.
Experimental
Metallographic Determinations of Cell or Nodule Count
Metallographic examination consisted of cast iron sample preparation by coarse surface grinding followed by fine pol- ishing. Samples of nodular iron were prepared according to the Ruxanda methodology.16
The nodule count was deter-
mined at 200x using a calibration and detection threshold of 2µm. In flake graphite iron the polished samples were etched with Stead’s reagent (10g copper chloride, 40g magnesium chloride, 20ml hydrochloric acid and 1000 ml 95% ethyl alcohol, etching time ≈ 5 min., magnification 5 to 20x) to reveal the cell boundaries (See Fig. 1c).
The area cell count N or nodule count Nn , (number of cells or nodules per unit area) can be determined from:17 Equation 1
Where; Ni is the number of cells or nodules inside a meas- ured rectangle U (Fig.1c,f), Nw
is the number of cells or nod-
ules that intersect a U side but not its corners, and A is the surface area of U.
35
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