colour metallography

154

CHINA FOUNDRY Vol.8 No.1

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

Chapter

Vermicular Graphite Cast Iron (I)Vermicular graphite cast iron (VG iron for short in the following

sections) is a type of cast iron in which the graphite is intermediate

in shape between fl ake and spheroidal. Compared with the normal

fl ake graphite in grey iron, the graphite in VG iron is shorter and

thicker and shows a curved, more rounded shape. Because its

outer contour is exactly like a worm, hence it is called vermicular

graphite. Since the compactness of the graphite (i.e. the ratio of

width/length, d/ l) in VG iron is far higher than that in grey iron,

it is also called compacted graphite. Considering both names, this

type of graphite is often referred to as C/V graphite internationally.

Compactness of graphite is represented by d/ l; the inverse or

reciprocal of this (i.e. l/d) represents the ‘incompactness’ of

graphite; the bigger l/d is, the less compact the graphite. When l/d equals one, the graphite is at its most compact form, i.e. spheroidal

in shape. The l/d value of spheroidal, vermicular and fl ake graphite

is shown in Fig. 4-1. Vermicular graphite can be divided into three

types according to its size and l/d value, and the corresponding

properties of VG irons are shown in Table 4-1. Type I graphite is

much smaller and narrower than types II and III, and although it is

very compact (l/d = 2-4), because the graphite is very thin, both

UTS and elongation are lower than that for type II. The length

and width of graphite in type II, both increase compared with type

I, but the width is increased more significantly than the length,

thus type II graphite iron has the highest strength and elongation

after fracture, among the three VG irons. Type III has the highest

l/d value of the three types of graphite, resulting in the lowest

compactness and the lowest elongation after fracture.

Fig. 4-1: Three types of graphite and ratio of length/width

Table 4-1: Types of vermicular graphite and corresponding mechanical properties

Graphite typeGraphite size Mechanical properties

Length l (μm) Width d (μm) Ratio l/d UTS (MPa) Elongation (%) HBS

I 20 10 2-4 300-450 2-5 150-240

II 150 50 2-5 350-500 3-9 150-240

III 150 20 3-10 300-450 1-3.5 150-250

Because of the shape feature of vermicular graphite, VG iron

has a good combination of mechanical and physical properties.

VG iron has signifi cantly superior mechanical properties to grey

iron and has better heat conductivity, damping capacity, castability

and machining properties than SG iron.

4.1 Nucleation of vermicular graphite4.1.1 Nucleation of VG and inoculation of VG ironThe graphite in VG iron consists of vermicular graphite and

spheroidal graphite as well; vermicular graphite is approximately

(a) Spheroidal graphite: l/d = 1;

(b) Vermicular graphite: l/d = 2-10;

(c) Flake graphite: l/d ≥50.

(a) (b) (c)

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February 2011Serial Report

70%-85%, with the remainder spheroidal graphite. Vermicular

graphite forms at the eutectic solidifi cation stage and thus belongs

to the eutectic graphite. The isolated, curved and thick vermicular

fl akes observed under an optical microscope are graphite branches

growing in the eutectic cells. Similar to eutectic graphite in grey

iron, it is very diffi cult to fi nd the nucleus of vermicular graphite.

Due to lack of direct evidence for the composition of vermicular

graphite nuclei, the identifi cation of its constituents is still under

consideration. Until now, the assumption on the composition of

vermicular graphite nuclei is only a kind of qualitative analysis.

Stefanescu [1] considered that the substances which form the nuclei

of graphite in VG iron and grey iron, are not fundamentally

different, but the nucleation substances of vermicular graphite

contain more complicated and a greater variety of compounds, as

vermicularisers contain many elements such as Mg, Ce, Ca, Al and

Ti. The number of graphite nuclei in VG iron is slightly higher than

in grey iron, but far less than in SG iron, being approximately one

tenth of the nodule count [1, 2]. For SG iron and grey iron, increasing

the inoculation will increase the number of nuclei; for vermicular

graphite iron, however, it is found that increasing the inoculation

decreases the amount of vermicular graphite and increases the

proportion of nodules. Therefore, to increase the proportion of

vermicular graphite (and reduce the number of nodules), it is

better not to have too high a number of graphite nuclei. For a VG

iron with certain thickness, there exists an optimum number of

nuclei; too low a number will cause carbides to occur easily; too

high a number will decrease the amount of vermicular graphite [3].

Therefore, the inoculation of vermicular iron is not as important

as for SG iron; if no carbides occur in the microstructure after

treatment, inoculation is not necessary [4,5]. However, for a situation

where the addition of inoculant is insufficient, inoculation can

promote the formation of vermicular graphite.

Because secondary inoculation with Fe-Si will cause the graphite

nuclei to significantly exceed the optimum number and change

growth conditions towards the formation of spheroidal graphite,

the problem of chilling and carbides in thin wall vermicular iron

is very diffi cult to overcome by secondary inoculation. Therefore,

to reduce the chilling tendency of thin-wall VG iron, secondary

inoculation with Fe-Si is not suitable. It was suggested using Al

to replace Fe-Si for secondary inoculation [1]; using Al, the chill is

reduced and the number of nuclei is not increased too much, and

this benefi ts the formation of vermicular graphite.

4.1.2 The crystalline “germs” of vermicular graphite

Once graphite nuclei form, carbon atoms will stack upon the

nuclei and form crystalline “germs” or embryos. These crystalline

“germs” of vermicular graphite can be:

(1) Small nodules: in the liquid iron after vermicularisation,

the spheroidising elements are often non-uniformly distributed.

In the segregated regions, spheroidising elements are enriched

to a certain degree and graphite in these regions will grow in a

spheroidal form. Although the residual Mg and Ce in the treated

iron is less than in SG iron, the graphite size is small at the

early growth stage; less spheroidising elements are needed for

spheroidal growth and thus spheroidal graphite can still form.

Many researchers found from their liquid-quenching experiments

that for VG iron, the graphite precipitated at the initial stage of

eutectic solidifi cation is spheroidal [1, 6-10].

(2) Flake graphite: for hypereutectic liquid iron treated with rare

earth ferro-silicon alloy or Re-Mg-Ti alloy, in the spheroidising-

element depletion region, the crystalline germs of vermicular

graphite can be fl ake graphite [11-13].

(3) Worm shaped crystalline germs: in hypoeutectic VG

iron, when austenite dendrites grow, carbon atoms are rejected

from the dendrites and enriched on the austenite interfaces,

thus creating beneficial conditions for graphite nucleation. The

precipitated germs take the shape of the austenite contours [14],

and are 1 μm thick and less than 10 μm long. The author also

observed vermicular graphite growing from worm-shaped germs in

hypoeutectic vermicular graphite iron, see Fig. 4-2.

4.2 Growth of vermicular graphiteRegardless of whether the crystalline “germs” are spheroidal or

flake, in the end, they will grow to form intermediate graphite

between fl ake and spheroidal shape. The shape must change, either

spheroidal “germs” degenerate to vermicular graphite or flake

graphite changes to vermicular.

4.2.1 Processing conditions for the formation of vermicular graphite

For untreated commercial cast iron with its high content of S and

O, and undercooling characteristics, it cannot reach the conditions

for the formation of vermicular graphite. To obtain vermicular

graphite, cast iron needs treating with a vermicularisation alloy.

In principle, all the vermicularisation treatments can be divided

into two types: undertreatment with spheroidising elements and a

combination addition of spheroidising and subversive elements.

(1) Undertreatment with spheroidising elements In this method, the addition rate of spheroidising elements to

liquid iron is lower than that necessary for a full spheroidisation

treatment, thus the graphite in the liquid iron cannot be fully

spheroidised and intermediate (vermicular) graphite is obtained.

The spheroidising elements used in this method are Mg, Ce and

Ca.

Fig. 4-2: Worm-shaped crystalline “germs” in hypoeutectic vermicular graphite iron (Ni-P tracer method)

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Using a small amount of Mg alone to treat liquid iron can

produce vermicular graphite, but this method needs strict control

of the base sulphur content. Nevertheless, nowadays, with

increased use of electrical furnaces, the use of Mg alone is being

recommended. The ‘SinterCast’ method is such a method, in which

fi rst using SiFeMg and 75SiFe to treat liquid iron and control the

residual Mg to the lower limit, then Mg and/or Si are added by

wire feeding if required. The advantages of this process are stable

production; easy to automate and better machinability as no Ti is

added (hard TiN and TiC are detrimental to machining). Pure Ce

or Rare Earth (RE) alloy containing mainly Ce, are also good for

the treatment of vermicular iron. RE alloy (with mainly Ce) has

advantages of low vapor pressure and being easy to control; the

heavier density of the reaction products, means they do not easily

fl oat up and thus do not cause graphite fade. However, Ce induces

a strong chilling tendency and is prone to cause the formation

of carbides in the structure. Each single element of rare earths

has a different ability to form vermicular graphite [15], with the

order of La > Ce > Pr > Nd. La has the strongest ability to form

vermicular graphite and the widest allowable range of residual

content for producing vermicular graphite. Ca has a weaker ability

to form spheroidal graphite than Mg and RE, thus the transition

process from vermicular to spheroidal shape can be extended and

the addition of alloy widened. In addition, Ca has less chilling

tendency than Mg and RE and can be used to obtain vermicular

iron in thin wall sections. The disadvantages of Ca are that it

forms high melting-point oxides and sulphides, which cover the

surface of the alloy and cause the aggregates to stick to each other,

hindering further reaction of the alloy with liquid iron. Also, the

size of a Ca atom is relatively large and diffi cult to diffuse, thus a

high treatment temperature is necessary with Ca.

(2) Combination addition of spheroidising and subversive elements

This is a method which uses the deleterious effect of subversive

elements on spheroidisation to cause spheroidal graphite to

transform to vermicular graphite.

Among the many subversive elements, Ti and Al are commonly

used elements for production of VG iron. Since they have a weak

deteriorative effect and mild inhibition to speroidisation, Ti and Al

allow a wider range of their critical content, and thus are easier to

control. Also, Al decreases chilling tendency and at the same time

does not increase the nucleation rate too much, which is benefi cial

for increasing the amount of vermicular graphite in the structure [1]. The critical content of subversive elements is related to their

equilibrium partition coefficient in austenite; see Fig. 4-3 [16]. It

can be seen from Fig. 4-3 that the smaller the equilibrium partition

coeffi cient, the less the allowed critical content and the stronger

the subversive effect. Al and Ti have a larger equilibrium partition

coefficient than Sb, As, B and Pb, therefore they have a larger

allowed critical content.

S is a typical subversive element. It was recently found that

addition of a small amount of S can change graphite from

spheroidal to vermicular [17-20]. Compared with Ti and Al, S has the

advantage of requiring a small addition; liquid iron with w(Mg) =

0.025%-0.04%, only needs addition of w(S) = 0.005%-0.015%.

The vermicularisers used for the two methods above and their

composition are listed in Table 4-2.

(3) Cooling rateWith increasing cooling rate, thermal undercooling increases;

graphite gradually changes to spheroidal, resulting in an increased

nodule count and lower vermicular graphite ratio. Therefore, the

production of thin-wall vermicular iron is more diffi cult than that

of thicker section iron. If using RE alloy for a casting of 3.5 mm

thickness, all the graphite will be spheroidal [21]; however, for a large

casting with heavy sections of 350-550 mm and a weight of 100 t,

a satisfactory vermicular graphite ratio can still be obtained [22].

4.2.2 Growth mechanism of vermicular graphiteThe non-uniform adsorption of spheroidisaing and subversive

elements on the prism and basal planes of a graphite crystal is

the main reason for graphite morphology transformation. After

vermicularisation, liquid iron contains spheroidising elements and

Also, S does not pollute returns and thus does not produce the

problem of accumulative S. S is added to liquid iron as pyrites

(natural FS2 containing w(S) = 36%) or added as pure S. The

required addition of S is determined by the residual Mg content

and cooling rate of the iron; see Fig.4-4.

Fig. 4-3: Relationship between critical content of subversive elements and their equilibrium partition in austenite

Fig. 4-4: Relationship between residual Mg and addition of S

(1) 5 min; (2) l0 min and (3) 15 min after Mg treatment

(sand mould, section thickness: 10-20 mm)

Add

ition

am

ount

of S

in la

ddle

, w (S

) %

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Vermicularising process

Type of vermiculariser

Composition (mass %)

Undertreatment with spheroidising elements

1. Mg seriesMg5 or Mg5RE1Mg6Ca5RE3

2. Ca seriesCa5 Ca24Mg5RE3 Ca20RE10

Ce90La5Nd4 Ce50La33Nd12Pr4

RE28Si45 La based RE34Si42

RE7Mg8 RE18Mg8 RE25Mg3 RE15Mg5Ca10 RE20Mg1Ca2

RE20Ca10

Combination addition of spheroidising and subversive elements

Mg8Ti10Si50Mg8Ti4A112Ca2Ce0.4Mg5Ti8A12Ca5RE1 Mg5Ti4Ca4RE2

RE10Mg5Ca2A12 RE20Mg5Ti2A12 RE25Mg3Ti5

Mg(RE)+ S

Table 4-2: Types of vermicularisers and their composition vermicularising

Fig. 4-5: Transformation mechanism from a spheroidal “germ” to vermicular graphite [11]

graphite grows epitaxially along the a-axis or grows tangentially

along the a-direction (along the direction tangential to the a-axis).

During eutectic transformation, an austenite shell cannot envelop

the graphite, but grows cooperatively with the graphite towards the

liquid, forming a VG eutectic cell. Figure 4-6 illustrates the three-

dimensional structure of vermicular graphite developing from a

spheroidal crystalline “germ”. Because of non-uniform distribution

of spheroidising and subversive elements, graphite grows

alternatively from the a-direction to the c-direction; the graphite is

twisted and changed in shape, resulting in an undulating surface of

the vermicular graphite, as illustrated in Fig. 4-7.

Fig. 4-6: Vermicular graphite developed from a spheroidal crystalline “germ”

(a) Low magnifi cation

3. RE series

A. Mischmetal

B. RE-ferrosilicon

C. RE-Mg

D. RE-Ca4. Combined alloy

series

A. Mg(Ca) + Ti(Al)

B. RE-Mg (Ca) + Ti (Al)

C. Mg(RE)+S

subversive elements as well, so the infl uences of these elements

are more complicated in vermicular iron than in SG iron.

(1) Transformation from a spheroidal crystalline “germ” to vermicular graphite [11, 23].

The transformation mechanism from a spheroidal “germ”

to vermicular graphite is shown in Fig. 4-5. Because of the

lower content of modification (spheroidising) elements, when a

spheroidal “germ” grows to a certain size, the graphite degenerates.

For the regions with suffi cient spheroidising elements, the (0001)

plane of graphite grows along the c-axis; for the regions with

insuffi cient spheroidising elements or with subversive elements,

a - graphite (1010) prism plane;

c - graphite (0001) basal plane;

γ - austenite shell;

::: - modifi cation elements

(Fig. 4-7)

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(2) Transformation from a flake crystalline “germ” to vermicular graphite

In this case, when modification (spheroidising) elements are

enriched to a certain amount on the growth interface [1010],

growth is significantly interrupted, which causes graphite to

branch continuously, thus changing growth direction; this

a- graphite (1010) prism plane; c-graphite (0001) basal plane; γ-austenite shell, :: - modifi cation elements

Fig. 4-8: Transformation mechanism of graphite from fl ake to vermicular shape [11]

(a) Infl uence mechanism (b) External morphology

Fig. 4-9: Modifi cation elements cause the graphite ends to become rounded

(b) High magnifi cation

Fig. 4-7: The undulating morphology of the surface of vermicular graphite

transformation mechanism is illustrated in Fig. 4-8. At this time,

flake graphite begins to twist and an undulated morphology

appears on the graphite surface. With further increase in the

modification elements, the produced undercooling makes the

(1010) plane unstable and more branches form; this causes the

(0001) plane to continuously tilt, resulting in rounded ends to the

graphite. The infl uencing mechanism of modifi cation elements on

the ends of graphite is shown in Fig. 4-9. The internal structure

of the rounded ends of vermicular graphite is similar to that of

spheroidal graphite, which consists of multi-angle, conical, single

crystals, with the basal plane perpendicular to the radius direction;

see Fig. 4-10. Using an SEM (scanning electron microscope) and

a TEM (transmission electron microscope), and with the help of

a diffraction pattern, Itufuji verifi ed that the surface of vermicular

graphite tips is the basal plane of a graphite crystal; vermicular

graphite and spheroidal graphite have the same sub-structure[24].

Observation of the internal structure of vermicular graphite with

a TEM, found [23] that there are more structures with the basal plane

of the graphite crystal lattice parallel to the length direction of the

graphite fl ake; see Fig. 4-11. Nevertheless, there is still quite a lot

of vermicular graphite consisting of a mixture of graphite cones

and cylinders; see Fig. 4-12. The graphite crystal lattice planes have

a mixed arrangement, indicating that during growth, the graphite

lattice direction ‘a’ and ‘c’ frequently change from one to the other.

a-graphite (1010) prism plane

c-graphite (0001) prism plane

159


Fig. 4-10: The internal structure of a rounded end of vermicular graphite [23]

Fig. 4-12: The internal structure of vermicular graphite consisting of a mixture of graphite cones and cylinders [23]

Fig. 4-11: The internal structure of vermicular graphite with basal plane of the crystal lattice parallel to the length direction of graphite [23]

160


4.3 Crystallisation of the primary phases in VG iron

Under equilibrium cooling conditions, the primary phase of a

hypoeutectic VG iron is austenite, in the form of dendrites, and

that of a hypereutectic VG iron is graphite. However, under

normal casting conditions, the solidification is non-equilibrium,

thus, in a hypereutectic VG iron, the existence of a small amount

of primary austenite dendrites is quite normal.

4.3.1 Primary austenite

The primary precipitated phase in a hypoeutectic VG iron is

austenite, see Fig. 4-15. For a hypereutectic VG iron, if the melt

solidifi es under non-equilibrium conditions, primary austenite is

quite often formed, see Fig. 4-16. In addition, the elements Ce, Ti

and Al in the vermicularising alloy tend to promote the formation

of austenite; this results in a higher probability of forming austenite

dendrites in a VG iron than in an SG iron. For hypereutectic

composition, the amount of austenite dendrites in a VG iron is

signifi cantly more than in an SG iron. This phenomenon has not

been revealed previously. The effect of austenite dendrites on

the mechanical properties of VG iron has not yet been studied.

Nevertheless, it is estimated that the increase of austenite dendrites

in VG irons will be benefi cial for the improvement of mechanical

properties of thin wall hypereutectic and hypoeutectic irons.

4.3.2 Primary graphite nodulesWhen a hypereutectic VG iron solidifies, primary graphite is

formed first. With the modification by Mg and Ce, the early

graphite is in the form of nodules. The surface of primary graphite

nodules in a VG iron is not smooth and clean, but has some lump-

shaped protrusions; this may be related to the fact that the melt

has relatively more Ce, Al and Ti than in SG iron. Compared

(3) Formation of liquid channels and their effect on the growth of vermicular graphite

Jolley fi rst found that liquid channels exist in the austenite shells

during solidification of SG iron[25], whilst Александров first

observed liquid channels in vermicular graphite eutectic cells [9].

Soon after, many researchers confi rmed this phenomenon [7, 8, 9, 26, 27].

It is commonly thought that the formation of liquid channels

in vermicular iron is caused by different growth velocities of the

two phases of the eutectic. The modifi cation elements Mg and Ce

increase undercooling of the liquid and cause the growth velocity

of austenite to be greater than that of vermicular graphite, thus the

austenite surrounding the graphite forms a concave shaped, growth

opening. When the austenite grows further, trace elements Ce, Ca,

Al, Ti etc are enriched in the concave mouth; this decreases the

melting point of the iron and results in the formation of a liquid

channel. Because grey iron has little undercooling, the growth

velocity of graphite is greater than that of austenite, thus no liquid

channel forms at the tip of fl ake graphite; see Fig. 4-13.

Fig. 4-14: The liquid channel phenomenon in vermicular graphite iron

Research work by the author found that liquid channels form

not only at the tips of vermicular graphite, but also at the sides

of vermicular graphite; see Fig 4-14. The formation mechanism

is the same as that of austenite precipitation around spheroidal

graphite in SG iron. One of the differences between VG iron and

SG iron is that there are more subversive elements and impurities

in VG iron and these elements have complicated influences on

Fig. 4-13: Formation of liquid channel at the tips of vermicular graphite

(a) Vermicular graphite (b) Flake graphite

Brown-liquid channel; blue/green or yellow enclosed by blue/green-austenite

austenite growth. Liquid channels have an important infl uence on

the formation of vermicular graphite; on one hand, liquid channels

restrict the growth direction of graphite branches in space [7, 28, 29], and

on the other hand, the segregation of elements in liquid channels

infl uences the growth of graphite.

The diffusion velocity of graphite through a liquid channel to a

crystalline “germ” is 100 times faster than that through an austenite

shell [8]; therefore, the carbon which diffuses through a liquid

channel is the main supplier for the growth of vermicular graphite.

In addition, the spheroidising, subversive, positive and negative

segregation elements in liquid channels are all at a higher level

than those in austenite [30]; this will infl uence the growth velocity

of basal or prism planes of a graphite crystal. When the subversive

elements (such as S, O, Ti and Al) in a liquid channel are too high,

undercooling is reduced, and this causes graphite growth to change

from spheroidal to fl ake shape. If enough spheroidising elements

are contained in the liquid channel, the graphite will grow along

the c-direction, thicken, twist and even grow to a spherical crown.

Because of the non-uniform distribution of spheroidising and

subversive elements, the growth of graphite in a liquid channel

changes from the a-direction to the c-direction repeatedly [31].

161


with that of VG iron, the surface of graphite nodules in SG iron is

smoother and cleaner, especially for an SG iron treated with pure

magnesium. A comparison of the outer surface of graphite nodules

in these two irons is shown in Fig. 4-17. The growth process of

primary graphite nodules and austenite halos in VG iron is similar

to that for SG iron; the austenite halo also consists of several

austenite grains, see Fig. 4-18.

In thick section VG iron, if the melt contains excessive C and

Ce, irregular graphite is often observed [32] in addition to the

(a) Ni-P tracer method (b) Hot alkaline etched

Fig. 4-15: Primary austenite in a hypoeutectic VG iron

Fig. 4-16: Primary austenite in a hypoeutectic VG iron (wall thickness 50 mm)

primary graphite nodules.

The amount and size of primary graphite in a VG iron varies

with wall section thickness; the thicker the section, the less the

number of graphite nodules and the larger the nodule size. The

locations where graphite nodules appear is random, but they are

often pushed to the LTF regions.

Because primary graphite nodules have less volume fraction,

compared to vermicular graphite, and are spheroidal in shape, they

do not have a negative infl uence on mechanical properties.

(a) VG iron (b) SG iron

Fig. 4-17: Comparison of the surface status of primary graphite nodules in VG iron and graphite nodules in SG iron [23]

Edge

162


4.4 Eutectic solidifi cation of VG ironThe whole process of eutectic solidification, from beginning to

end, directly refl ects the formation of eutectic cells, and study of

the inner and outer structures of a eutectic cell can reveal the rule

of eutectic reaction.

4.4.1 Nuclei of eutectic cellsFigure 4-19 shows the two-dimensional morphology of a complete

VG eutectic cell. It can be seen that many isolated and short

4.4.2 Formation process of eutectic cellsBased on the liquid quenching results [33] and colour metallographic

observations by the author, the formation process of a VG eutectic

cell is as follows:

(1) A vermicular graphite ‘germ’ is formed.

(2) Under the effect of interference elements, the vermicular

graphite ‘germ’ degenerates and branches and austenite

precipitates around the graphite ‘germ’; see Fig. 4-21(a).

(3) Liquid channels or small ‘melt-pools’ are formed at the tip

of or around the graphite; see Fig. 4-21(b).

(4) Graphite continues to branch along the liquid channels and

the growth velocity of the a-axis and c-axis often varies according

to the distribution of modification elements; at the same time,

(a) Hot alkaline etched (b) Un-etched (the same fi eld of view)

Fig. 4-19: A eutectic cell of VG iron

Fig. 4-20: Three dimensional morphology of vermicular graphite in eutectic cells

(a) Section thickness 120 mm (b) Section thickness 60 mm

Fig. 4-18: The structure of austenite halo around primary spheroidal graphite in vermicular iron

vermicular graphite flakes, which originated from a common

nucleus, have grown radially outwards; at the same time, austenite

has also expanded outwards. By using a scanning microscope,

it can be seen that the graphite flakes within a eutectic cell are

connected to each other in space, as shown in Fig. 4-20. Since the

graphite in a eutectic cell is the leading phase, the graphite nucleus

is also the nucleus of a eutectic cell. The location of a graphite-

forming nucleus is dependent on heterogeneous nuclei and is quite

random; however, it forms more easily around austenite dendrites.

163


(a) Vermicular graphite ‘germ’ forms around austenite (small cyan blue blocks)

(b) Formation of liquid channel

(c) Formation of eutectic cell

Fig. 4-21: The formation process of a VG eutectic cell

austenite grows correspondingly; see Fig. 4-21(c).

(5) Vermicular graphite fl akes are totally enveloped by austenite

and a complete eutectic cell is formed.

4.4.3 Characteristics of VG eutectic cells During the growth of VG eutectic cells, the relationship between

graphite and austenite is a type of quasi-cooperative, which is

called ‘loose cooperative coupling’ growth, (see Fig. 3-77).

It differs from eutectic growth in grey iron, which is a ‘close

cooperative coupling’ growth, and from SG iron, which is a ‘non-

cooperative divorced’ eutectic growth. The inner structure, outer

contour, size and number of VG eutectic cells are closer to that of

grey iron. However, because of the existence of many different

elements in VG iron, which are non-uniformly distributed, this

causes the austenite at different locations to have different melting

points. At the locations having a low melting point, in addition to

forming liquid channels, small isolated ‘melt-pools’ can also exist.

The author also observed this phenomenon in his research work –

the honeycomb structure, a liquid-solid co-existence structure; see

Fig. 4-22.

The outer contour of a VG eutectic cell shows a spherical-like

shape whilst the outer contour of a eutectic cell in grey iron shows

a zigzag shape. The interface between VG eutectic cells and liquid

(a) Ni-P tracer method (b) Hot alkaline etched

Fig. 4-22: Inner honeycomb structure of a eutectic cell

is relatively fl atter and smoother, with less undulation compared

to that of grey iron eutectic cells. This is because VG iron exhibits

greater eutectic undercooling and the graphite is more branched.

The outer contour of eutectic cells of VG iron is related to their

size. For the small sized eutectic cells, vermicular graphite has

just developed from the ‘germ’ and has less branches, therefore

the outer contour of the eutectic cells is not round and smooth; see

Fig. 4-23.

164


(a) Small eutectic cells

Fig. 4-23: Relationship between the outer contour and size of eutectic cells

(b) A large eutectic cell

To be continued

Titles of The 69th WFC Papers Published in CHINA FOUNDRY Volume 7 No.4 November 2010

383 Advanced manufacturing technologies of large martensitic stainless steel castings with ultra low carbon and high cleanliness

Lou Yanchun and Zhang Zhongqiu

392 Energy conservation and emissions reduction strategies in foundry industry

Li Yuanyuan, Chen Weiping, Huang Dan

400 Effects of fi lter materials on the microstructure and mechanical properties of AZ91

Wu Guohua, Sun Ming, Dai Jichun, et al

408 Application of ceramic short fi ber reinforced Al alloy matrix composite to piston for internal combustion engines

Wu Shenqing and Li Jun

412 What do we do next? To survive, grow and be Distinguished

Yaylali Günay

419 Reduction of greensand emissions by minimum 25% ― Case study

Cornelis Grefhorst, Wim Senden and Resat Ilman

425 The Mystery of Molten Metal

Natalia Sobczak, Jerzy Sobczak, Rajiv Asthana,

et al

438 Investigation of improving wear performance of hypereutectic 15%Cr-2%Mo white irons

R. Reda, A. Nofal, K. Ibrahim, et al

447 Oil quenched malleable iron, the strength of an old material in a “green cast” development and a new future

Cornelis J. van Ettinger

456 Structural and thermophysical properties characterization of continuously reinforced cast Al matrix composite

Brian Gordon, Natalia Sobczak, Małgorzata Warmuzek, et al

463 Advanced casting technologies for lightweight automotive applications

Alan A. Luo, Anil K. Sachdev and Bob R. Powell

165


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