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Colour Metallography of Cast IronBy Zhou Jiyang, Professor, Dalian University of Technology, China

Translated by Ph.D Liu Jincheng, Fellow of Institute of Cast Metal Engineers, UK

*Note: This book consists of five sections: Chapter 1 Introduction, Chapter 2 Grey Iron, Chapter 3 Spheroidal Graphite Cast Iron, Chapter 4 Vermicular Cast Iron, and Chapter 5 White Cast Iron. CHINA FOUNDRY publishes this book in several parts serially, starting from the first issue of 2009.

Chapter 4

Vermicular Graphite Cast Iron (Ⅱ)

4.5 Solidification of LTF regions during final stage of solidification of VG iron4.5.1 Characteristics of the liquid in the LTF regions

(1) Wider rangeFigure 4-24 compares the size of the LTF regions of three types

of cast iron under the same cooling condition. Since the tips of

graphite in grey iron are in contact with the liquid, the two phases

in the eutectic cells grow in a fully cooperative manner, resulting

in less remaining liquid between the cells. The number of eutectic

cells in SG iron is much more than in VG iron, thus the area of the

LTF regions is also less. Compared with grey iron and SG iron, VG

iron has the largest percentage of the LTF area in the field of view.

(a) Grey iron

(2) With more segregation elements In VG iron, except for the modification elements Mg, Ce and

Ca, there also exist interference elements Ti and Al, which are

easily enriched in the LTF regions.

4.5.2 Microstructure formed in the LTF regions

When the temperature of the liquid in the LTF regions drops to its

crystallisation temperature, the liquid solidifies; depending on its

composition, contamination by impurities and the cooling rate, the

structures formed in the LTF regions are different.

Fig. 4-24: Comparison of the LTF areas of three types of cast iron (sample section thickness 25 mm)

(1) ‘Basket’ net-work structure of pearlite The remaining liquid in the LTF regions is enriched in Ce, Ti

and Mn, which increase undercooling of the liquid, thus promoting

nucleation of austenite. Therefore, austenite dendrites often form

in the LTF regions (see Fig. 4-25); these dendrites contact and

integrate with the austenite grown from eutectic cells. Because

VG iron contains more positive segregation elements than SG iron

or grey iron, during solid phase transformation, the austenite in

the LTF regions has a stronger tendency to form pearlite than in

(b) VG iron

(c) SG iron

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grey iron or SG iron, and the spacing of pearlite is finer (see Fig.

4-26). When the pearlitic structures around eutectic cells connect

with each other, a metallic net-work skeleton is formed, called a

‘basket net-work structure’ [34, 35]; this is illustrated in Fig. 4-27.

The ‘basket net-work structure’ consisting of pearlite (or ferrite)

has a favourable effect on the mechanical properties of VG iron

and enables VG iron to have an ideal combination of properties.

(2) Small graphite nodulesIn the LTF regions, small graphite nodules are often observed

(see Fig. 4-28). And in slow-cooled, thick section VG iron, small

graphite nodules are even more in evidence. The formation of small

nodules is related to a higher content of spheroidizing elements,

Ce and Mg. From their location and size, it is known that small

nodules are formed in the last stage of solidification; different from

small nodules, the isolated and randomly distributed large nodules

are formed in the early stage of solidification. The small nodules

have a beneficial effect on the properties of VG iron.

Fig. 4-25: Austenite dendrites formed in the LTF regions

Fig. 4-26: Fine lamellar pearlitic structure formed in the LTF regions (SEM observation)

Fig. 4-27: The ‘basket’ net-work structure formed in the LTF regions

Fig. 4-28: Small graphite nodules in the LTF regions (section thickness 100 mm)

(3) Flake graphiteIn the LTF regions, when the spheroidizing elements are

insufficient, anti-spheroidizing elements are excessive, or residual

sulphur is too high, flake graphite is easily formed, as illustrated

in Fig. 4-29. This flake graphite markedly weakens the strength of

the metal matrix and is unfavourable to the mechanical properties

of VG iron.

Fig. 4-29: Flake graphite in the LTF regions

(4) CarbidesSince the completion temperature of eutectic solidification

of VG iron, Tes, is higher than that of SG iron and is above the

metastable eutectic temperature, the carbide forming tendency

in the LTF regions of VG iron is weaker than that of SG iron.

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thermal curve reaches point ‘d’ and the liquid in the LTF regions

solidifies; small nodules, vermicular graphite and austenite form,

and solidification is complete.

a – TEN (eutectic begins); b – TEU (a large quantity of eutectic

begins to form); c – TER (eutectic reaction almost complete);

d – TES (eutectic solidification ends)

Fig. 4-31: Formation process of solidification structures of VG iron (corresponding to the points on the curve shown in Fig. 4-30

Nevertheless, if the liquid in the LTF regions contains a too

high level of positive segregation elements, their corresponding

compounds can also form in the LTF regions; TiC is a commonly

observed carbide in VG iron, as shown in Fig. 4-21c.

4.6 Solidification morphology of VG iron4.6.1 Solidification process of VG ironUsing thermal analysis, liquid quenching and colour metallurgy

to observe and analyse the formation, distribution and relationship

of solidification structures, it is possible to effectively study

the solidification process of VG iron. Figure 4-30 shows the

thermal analysis curve of a VG iron with eutectic composition;

the microstructure corresponding to the points on the curve are

shown in Fig. 4-31. It can be seen from these figures that at the

beginning of eutectic solidification (point ‘a’), the structure,

similar to that of SG iron at this point, consists of small graphite

nodules and divorced austenite dendrites. When reaching point

‘b’, the nodule ‘germs’ degenerate towards vermicular graphite,

the eutectic graphite nodule count decreases gradually, a large

number of eutectic cells begin to form, and recalescence appears

on the cooling curve. On reaching point ‘c’, eutectic reaction nears

the final stage, but eutectic cells continue to grow. Finally, the

Fig. 4-30: Thermal analysis curve of a VG iron with eutectic composition

(a) VG ‘germs’ and austenite dendrites precipitate (b) Eutectic cells form

(c) Eutectic cells grow (d) LTF regions solidify

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4.6.3 Solidification morphology of VG iron

Although the morphology characteristics of VG eutectic cells

are closer to those of grey iron, the solidification does not totally

follow the mode of grey iron: i.e. gradually increasing the

thickness of its solid layer (a successive layer-type solidification

mode). The author considered that in comparison with grey

iron, VG iron has 3-5 times wider LTF regions around the

eutectic cells, a smoother outer contour of the eutectic cells and

slightly more eutectic cells, all of which result in the variety of

4.6.2 Characteristics of solidification cooling curve of VG iron

VG iron is a type of intermediate cast iron between grey iron and

SG iron; the characteristic freezing parameters are also in between

grey iron and SG iron, as shown in Fig. 4-32(a). However, for a

VG iron with hypoeutectic composition, the measured cooling

curve shows other characteristics, as illustrated in Fig. 4-32(b).

The obvious difference is that the chilling tendency of the eutectic

reaction of a hypoeutectic VG iron is higher than that of grey

iron and SG iron as well. The reasons for this difference are: (1)

hypoeutectic composition; (2) the vermicularizing elements Mg

and Ce remarkably increase undercooling; (3) highly efficient

inoculants cannot be used because they will increase nodule count

and reduce the percentage of vermicular graphite.

solidification processes [36].

At the edge of a VG iron casting, because of fast cooling, rapid

solidification does not allow the LTF regions to develop and

solidification occurs in a successive layer-type mode, whilst in

the centre of the casting, solidification occurs mainly in a mushy

mode. The influence of wall section thickness has a similar effect

to the above; experimental measurements have shown that for

castings of less than 10 mm thick, solidification is mainly the

successive layer-type mode; for castings with a section thickness

above 25 mm, the solidification occurs mainly in the mushy mode;

see illustrations in Fig. 4-33. In addition, it was found that for

castings with a section thickness up to 50 mm, the size of eutectic

cells increases with increasing section thickness; for castings

thicker than 50 mm, the size of eutectic cells does not show a

significant increase with increasing section thickness. Figure 4-34

shows the solidification microstructure in sections with thicknesses

of 10, 25 and 50 mm, respectively. The outer contour of eutectic

cells in thin-walled VG iron is not very round, whilst in thicker

sections above 25 mm, the eutectic cells show a clear and round

outer contour.

The degree of vermicularization (percentage of vermicular

graphite) has an important influence on the solidification

morphology; see Fig. 4-35. For under-vermicularized VG iron in

which vermicularization is insufficient, there exists more flake

graphite and insufficient vermicular graphite; the solidification is

similar to grey iron, solidifying in the successive layer-type mode.

Conversely, for over-vermicularized VG iron, the nodularity is

too high and in addition to the VG eutectic cells there exists some

SG eutectic cells; under these conditions, the solidification occurs

as a mixture of successive layer-type mode plus mushy mode, as

shown in Fig. 4-35(c). The dendritic structure, vertical to mould

wall, is shown in Fig. 4-36. The picture shows that at the edge

of the casting the structure is austenite with graphite spheroids

enveloped in it, whilst at the centre of the casting, the structure

consists of VG eutectic cells.

4.7 Segregation in VG ironThe micro-segregation of VG iron is more complicated than in

SG iron or grey iron. This is because VG iron contains more

and complex solute elements than either SG iron or grey iron.

The distribution of elements in various structures follows the

common rules of segregation: negative segregation elements

preferentially concentrate in the early-formed austenite, whilst

positive segregation elements are enriched in the LTF regions;

this causes the formation of pearlites, carbides or small graphite

nodules around eutectic cells. The non-uniform distribution of

various elements and structures in thick-section VG iron is more

evident, as shown in Fig. 4-37. Table 4-3 lists the element content

in corresponding regions; it can be seen from the table that the

non-uniform distribution of positive and negative elements is quite

severe. VG iron normally contains a higher Si content than grey

iron and often reaches the level of as-cast ferritic SG iron. The

segregation of such high Si in the centre of eutectic cells causes

ferrite to form, but around the eutectic cells the structure is pearlite.

(a) Hyper-eutectic curves

(b) Hypo-eutectic curves

Fig. 4-32: Cooling curves of grey, VG and SG irons [1]

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References[1] Stefanescu D M, Martinez F and Chen I G. Solidification Behavior

of Hypoeutectic and Eutectic Compacted Graphite Cast Irons: chilling tendency and eutectic cells. AFS Transactions, 1983, 91: 205-216.

[2] Huang Huisong, Sheng Da, Zeng Daben. Vermicular Graphite Cast Iron. Beijing China: Tsinghua University Press, 1982. (in Chinese)

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Fig. 4-33: Influence of section thickness on the macro-solidification structure of VG iron [36]

(a) 10 mm

(b) 25 mm

(c) 50 mm

(d) 75 mm

(e) 100 mm

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Fig. 4-34: Influence of section thickness on the micro-solidification structure of VG iron (Each group has two pictures of the same field of view, the left one is un-etched)

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[12] Nofal A A, Rezk A S and Walt M A. Some Solidification Characteristics of C/V Graphite Cast Iron. The Physical Metallurgy of Cast Iron IV, G. Ohira, T. Kusakawa and E.

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(a) Section thickness 10 mm

(b) Section thickness 25 mm

(c) Section thickness 50 mm

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Niyama, Ed., MRS Conference, Pittsburgh, Pennsylvania, 1989: 81-88.

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Fig. 4-36: Microstructure of over vermicularized VG iron

(a) Under vermicularization (b) Normal vermicularization (c) Over vermicularization Fig. 4-35: Influence of vermicularisation degree (nodularity) on the micro-solidification structure of VG iron

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(a) Hot alkaline etched (b) Nital etched [the same field of view with (a)]Fig. 4-37: The non-uniform phenomena of composition and microstructure distribution in a VG iron

Table 4-3: Element distribution in the solidification structures of VG iron (mass %)

Region in Fig. 4-37Negative segregation element Positive segregation element

Si Ni Cu Cr Mg

A (in the austenite around primary nodules) 2.75 0.062 0.034 0.030 0.0049

B (in VG eutectic cells) 2.68 0.058 0.030 0.027 0.0037

C (in chunky graphite eutectic cells) 2.56 0.053 0.025 0.029 0.0027

D (in the LTF regions) 1.90 0.027 0.008 0.173 0.0170

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[29] Wu Run, Song Weixi. Observation and study of graphite morphologies in rare earth cast iron. Foundry Technology, 1989(1): 42-46. (in Chinese) To be continued