In studies concerning the solidification mechanism of cast iron, the eutectic part has always been emphasized.
A major problem in its understanding was, however, the fact that this structure could appear in two forms, as an iron-iron-carbide eutectic and as a an iron-graphite eutectic. Moreover, the last one did not look like a eutectic at all.
A subdivision between hypo-eutectic, eutectic and hyper-eutectic was not that convincing either, as in many cases, hypereutectic compositions often behaved as hypo-eutectic ones.
These pecularities would be reason enough to engage between scientists a "battle for the right theory". A battle that would last for more than half a century.
It was not untill 1960 that Standke (1) was able to write that "The eutectic solidification of cast iron, so disputed for years, finally had been solved. Quenching tests had revealed that the eutectic is formed by individual centres, the so-called eutectic cells. From these centres, austenite and graphite grow together into the remaining liquid to to form more or less spherical regions."
This idea is still valid today. In this page, we will look for the real facts that lead to the development of this theory.
Long before the concept of eutectic cells (EC) in cast iron was developed, its presence was noticed and recorded.
Campbell (2) described the formation of a spherical graphite-austenite eutectic forming directly from the melt as far back as 1902!
Figure 1, Campbell's eutectic cells.
In 1906 Benedicks (3) noticed black spots in mottled cast iron. In these spots graphite flakes were found.He came to the conclusion that these spots must have formed directly from the melt and could not have been the result of a decomposition of previous formed carbides. This was not in agreement with the general accepted views at that time.
Wedding (4) established the fact that these graphite containing spherical regions are mainly found at sites where primary austenite dendrites collided or crossed .
Also Gutowsky (5,6) mentioned the formation of eutectic graphite in spherical area's.
Schüz (7) is convinced that he found the actual graphite-ferrite (stable) eutectic and states that this is formed in the shape of circular regions.
Scheil (8) assumes that every graphite nucleus possesses a more or less circular sphere of influence.
The first major step in the development of the idea of eutectic cells was made by Boyles. Although the first results of his research work were known in Europe in the late 30ths (9), it took another ten years to break through (10).
From his experiments, Boyles concluded that solidifcation of cast iron starts with the separation of primary austenite dendrites and -fully independent thereoff- followed by the formation of the graphite-austenite eutectic.
This eutectic expands in all directions to form a more or less circular shape.
The graphite within such cells was interconnected, which had been proved by Mackenzie (11) using serial section techniques to obtain a real 3-D view of the macro-graphite structure.
Figure 2, After Boyles Quenched sample, heat tinted, showing eutectic cell formation during solidification.Right fotograph x20, left photograph, detail x100.
A general acceptance of the EC-theory, however, was only possible after some controversies had been cleared:
-Formation of Undercooled or D-type graphite.
Since Eash's work (12) in 1941, the interest for the carbide-decomposition theory had again increased. Namely the formation of D-graphite was explained as the result of a decomposition of previously formed carbides.
Even in 1954, Morrogh and Williams (13) showed that D or under-cooled graphite was formed by fast decomposition of acicular carbides. Also the first research results on nodular graphite formation seemed to point in the direction of the carbide-theory. It was, however, generally accepted that coarse A flake graphite directly formed from the melt, each graphite flake was formed on a suitable nucleus (14).
Boyles' experiments were accurately repeated by Hultgren et al. (15). Cast iron samples were quenched during solidification at various intervalls. In this way, a relationship could be established between temperature, time, progression of solidification and growth of the various structural components.
Examples are shown in figure 3,4 and 5. The reference cooling curve from fig.3 (obtained without quenching) is superimposed in figure 4 and 5 as a dotted line.
Figure 3, Reference cooling curve obtained during un-interupted solidification.
Figure 4, Cooling curve interupted by quenching just at the start of eutectic solidification.
Figure 5, Cooling curve interupted by quenching half-way during eutectic solidification.
The corresponding micro-structures are shown in fig.6 and 7.
Figure 6, Microstructure of sample that was interupted during solidification acc. to figure 4. Etched, x250.
Figure7, Microstructure of sample that was interupted during solidification acc. to figure 5. Etched, x250.
Liquid that is still present at the time of quenching will be transformed into fine ledeburite, making it possible to distinquish between primary austenite dendrites and eutectic austenite.
These tests lead to the following conclusions:
1)During solidification, certain spots are formed which contain graphite austenite eutectic. These spots (rosettes) are more or less ball-shaped, but their final shape can be influenced by existing primary dendrites.
2)The formation of these spots coincide in the cooling curve with a typical change in the curve.
3)The regular and circular shape of the rosettes indicate that these must have formed from a material that was present between the existing dendrites. Quenching test showed that only fine (quenced) ledeburite was found between the dendrites, meaning that these regions must have been liquid at the time just before quenching.Which also means that these rosettes must have formed directly from the liquid.
4)Cementite eutectic is plate or rod-like, from which it is impossible that circular shaped graphite austenite eutectic forms, which means that the cementite eutectic cannot be the source for graphite formation, including D-graphite.
Bunin (16) examined the 3-D structure of graphite inclusions in cast iron, using serial sectioning techniques, like MacKenzie had done. Examination of fine D-graphite flakes showed that also in this case the, at first sight, individual flakelets are in fact sections through a single graphite skeleton. This meant that also in the case of D-graphite, the liquid must have been the source for its formation.
Figure 8, Three-dimensional graphite shapes found after serial sectioning.MacKenzie, whirl types of A graphite,Bunin Undercooled or D-graphite (bottom right)
BCIRA was able to confirm these outcomes by using Deep-Etch techniques.
As it was also shown in the mean time that cementite never was the site for nodule formation in ductile iron, these BCIRA results meant that the Carbide-theory was completely ruled out as an acceptable theory for graphite formation in cast iron.
Formation of all graphite shapes directly from the melt seemed to be generally accepted now.
Macro-formation of eutectic cells.
The progression of solidification in gray cast iron had been studied by Dunphy and Pellini (17).
Cooling curve measurements, showed that directly from the start of solidification, a network of primary austenite dendrites form throughout the whole wall-thickness. Then, from the mold-wall on, formation of the eutectic starts, progressing in the direction of the haert of the section.
Continued formation of primary dendrites in the heart as well as formation of the eutectic in the outer parts take place simultaneous at a certain time, which explains the typical shape of the cooling curve.
Hughes (18,19) made similar test, using various types of cast iron
The macro structure in fig.9 and 10 clearly shows the successive formation of eutectic cells during the course of solidification.
Figure 9 shows few and small EC at the start of solidification.In fig.10, the size and number of EC have increased with progressing solidification.
The typical macro-solidification of gray iron seemed to be established and is presented schematically in figure 11.Note the highlighting of the independance of primary and eutectic phases.
A last problem was the fact that the relationship between the number of Eutectic Cells and graphite shape appeared not to be very logic. Formation of fine undercooled graphite is accompanied by relatively few EC, coarser A-graphite on the other hand is formed when the number of EC is relatively high. An ingeniuous answer however, was given by Morrogh (20), which made it possible to explain the solidification mechanism of cast iron in a much simpler way. As this explanation has had a huge impact on the development of idea's on graphite formation in cast iron, it will be quoted in full:
"For the same rate of cooling, irons having fine undercooled graphite have fewer and larger eutectic cells than irons having normal flake graphite. It would seem that the rate at which the graphite can crystallize determines the rate at which the eutectic liquid transforms. In irons giving the fine undercooled graphite where the number of eutectic cells is relatively few the rate of crystallization of each graphite skeleton has to be more rapid than in the coarse flake graphite irons since the eutectic transformation takes place from fewer centres in the former case. This higher rate of transformation is obtained by more frequent branching of the graphite skeleton and gives the appearance of finer graphite in a metallographic section. Thus the fewer the nuclei initiating eutectic crystallization, and the larger the eutectic cells as a result, the finer will be the graphite flakes and the more frequently will each graphite crystal branch to provide a fresh crystallization surface. Similarly rapid cooling, even when the degree of nucleation is very high encouragwes frequent branching in order to give a rate of transformation in each cell consistent with the overall rate of heat transfer."
Further developments of the EC theory.
Outside BCIRA much work was also done in studying the formation of eutectic cells.
Ferry and Margerie (21-24) established a relationship between the number of eutectic cells per area (cell count or CC), mechanical properties and cooling velocity.
Patterson (25) accepts the concept of EC and thuss uses his influence on nucleation and under-cooling.
In this way, a general idea about macro and micro solidification of gray cast iron is formed during the early sixties. Idea's that are reproduced by various authors (26,27,28,29) in similar ways, using artificially drawn cooling-curves and which are actually still in use today to explaining the solidification mechanism of cast iron.
Only De Sy (30) held a different view. The formation of a eutectic cells is explained by diffusion and the formation of new nuclei . Fig. 12.
A typical example of the general accepted views at that time (which are still general accepted today) is formed by a publication of Oldfield(26), which explains the solidification of gray cast iron according to figure 13:(blz.178).
At A, formation pf primary austenite dendrites starts. Its formation and growth continues untill B is reached and the remaining liquid has attained the eutectic composition. From this point on, eutectic solidifications starts which continues untill all liquid is transformed (D).
To explain the realtionship between the various structural components and mechanical properties, Oldfield simplifies the solidification mechanism still further by completely leaving out the formation of primary austenite.
Eutectic solidification starts from a number of points or nuclei and proceeds by the growth of the eutectic aggregate of austenite and graphite from these nuclei.Solidification is then, a process of nucleation and growth.
Factors which influence the solidification process must do so either by influencing the nucleation, that is the number of points at which solidification starts, or the manner or rate at which the growth process proceeds. Of course, some variables may influence the nucleation and growth process simultaneously.
In any molten cast iron there are a number of variously sized particles which can potentially function as nuclei, that is, as points for the initiation of solidification.
At a certain temperature, these nuclei can begin to grow, the larger nuclei first, and then on further cooling, the smaller nuclei.
Where the cooling rate is slow, the latent heat evolved by the growth of the large nuclei is sufficient to arrest the fall in temperature of the mass of metal, no further undercooling occurs and no other nuclei begin to grow. However, if the cooling is more rapid the rate of growth of the larger nuclei will be inadequate to arrest the drop in temperature, more undercooling will occur and fresh nuclei will grow.
From 3-D structures, obtained after deep-etching, it was shown that within a EC, the graphite adhered to each other, but was branched in various directions. Oldfield assumed, as Morrogh did, that graphite has the tendency to branch in order to be keep pace with the growth of the austenite phase.
Moreover, formation of new nuclei had to be more difficult than the formation of a new graphite-branch on the existing flake.
Yet another assumption is made by supposing that near the graphite flake a local super-saturated region exists, nescessarry for the formation of a new branch.Under these circumstances, diffusion cannot be the driving force behind growth velocity of the eutectic. So the factor governing the growth of the eutectic will be the growth velocity of the individual flake or the velocity of its branching.
In this way, a simple explanation is found for the formation of the various graphite shapes that occur in cast irons.
The number of branches of every graphite skeleton seems to depend on the radial growth of the eutectic cell. The faster its growth, the more graphite-branches occur and the finer the graphite will be. Growth velocity of the EC increases with increasing under-cooling. So the more undercooling, the finer the graphite will be.
Further research in this area is thus greatly influenced by the ruling idea's on nucleation, under-cooling and the growth opportunities of Eutectic cells.
Also the following factors may have played a role in established these general views:
-The succes story of ductile cast iron ,which increased interest for the influence of small concentrations of minor elements.
-new techniques become available for analyzing small amounts of trace elements,
-Reliable equipment became available to make reproducable cooling-curves in the foundry itself.
-The majority of the research work was directed to immediate practical use. Control of EC formation appeared to be a simple and practical means to achieve that.
End of part I.
Part II will cover the practical use of CC.
Part III will cover the shortcomings of the EC concept.
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W.Standke, Die Giesserei, Vol.47 (1960), pp. 557-58.
(2) Das Zustandsdiagramm des Systems Eisen-Kohlenstoff.
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(17) Solidification of gray iron in sand molds.
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(18) Microscopic and Thermal Analyses of Solidification
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BCIRA Research J., Vol.3 (1955), pp.537-54.
(19) A study of the solidification process in grey and white irons.
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(20) Graphite formation in grey cast irons and related alloys.
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(21) Causes and defects of grain size in pearlitic gray cast iron.
M.Ferry, J-C. Margerie,
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M. Ferry, J-C. Margerie,
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(23) Le grain dans la fonte.
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(24) Influence de la température de coulée sur les caractéristiques mécaniques des fontes grises non alliées, coulées en sable.
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(25) Beitrag zur Kristallisation des lamellaren Eisen-Graphit-Eutektikums in Gußeisen.
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(27) Factors influencing soundness of gray iron castings.
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Production of sound grey-iron castings..
Survey of recent research.
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