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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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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
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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]
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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].
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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
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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.
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(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.
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CHINA FOUNDRY Vol.8 No.1
(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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