August 2009

255

Serial Report

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

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

2.3 Crystallization of primary austenite in grey iron2.3.1 Crystallization thermodynamics of primary austenite The first precipitated phase in hypoeutectic cast iron is austenite,

called primary austenite. Figure 2-10 shows the free energy change

of primary austenite crystallization. When liquid hypoeutectic

cast iron with composition X is undercooled below liquidus

temperature BC', for example temperature T1, primary austenite

starts to crystallize. At this time, the iron atoms concentration

is higher than equilibrium value by (Xa-X); the free energy

of the iron melt with X is ΔG higher than that of equilibrium

mixture of iron melt with Xa and austenite with Xb, thus forming

thermodynamic driving force for primary austenite crystallization.

The austenite formed at beginning contains carbon Xa, the liquid

near to it also has similar carbon, but the region far away from

the austenite still has equilibrium carbon X, thus forming non-

uniformed carbon concentration and chemical potential in carbon.

Under driving by concentration difference, carbon diffuses from

the melt near to austenite towards outside, causing carbon in the

liquid near to austenite to decrease, thus the iron is oversaturated

gradually, leading to continuous austenite precipitation with

temperature decreasing. When temperature is undercooled to T2,

the concentration condition for new austenite to precipitate is

ready; austenite will grow in the front of original austenite.

2.3.2 Nucleation of primary austeniteNuclei of primary austenite are first formed on the wall of mould

cavity, since the biggest undercooling is resulted in here due to fast

heat transfer of mould wall. When the undercooling is greater than

that demanded for austenite nucleation, the nucleation is started.

The constitutional undercooling due to enrichment of C, Mn, S

and P elements in front of liquid/solid interface further promotes

nucleation process. Temperature fluctuation and liquid flow cause

some of dendrites to drop off from wall of mould cavity, which

float to inside and create ‘crystal propagation’ and become new

austenite nuclei. Furthermore, the addition of nucleation materials

is an effective measure to induce austenite nucleation.

Factors influencing austenite nucleation are:

(1). Undercooling condition: experiments have confirmed [21]

that when chemical composition is fixed, fast cooling rate,

high superheating and long holding time all cause difficulty for

austenite nucleation. This is because, on one hand, with increasing

cooling rate, the nucleation temperature of austenite is decreased

(that is undercooling is increased), giving rise to reduced number

of nuclei. On the other hand, high superheating and long holding

time will destroy existing heterogeneous nuclei or make them less

effective, giving rise to decreased number of nuclei.

However, Stefanescu[22] obtained an opposite relation:

N = 2.45ΔT 0.93

Where, N - number of austenite grains (N/mm2)

ΔT - undercooling (K)

That is, with increasing undercooling, the number of austenite

grains is increased.

(2). Addition of heterogeneous nuclei: the substances

able to become heterogeneous nuclei need not only to satisfy

Chapter 2

Grey Iron (II)

Fig. 2-10: Free energy change of primary austenite crystallization

256

CHINA FOUNDRY Vol.6 No.3

then branches and forms a dendrite (see Fig.2-13). The growth

processes of primary, secondary and third crystal axis are: at the

beginning stage of growth, the fast growth at tips of the octahedra

consisting of (001) plane leads to a primary dendrite formation.

Due to the natural packing process at the closest plane (001)

and constitutional undercooling at the region of melt close to

the crystal, projections or protrusions (the original places of the

secondary dendrite) grow and form at the primary dendrite axis

periodically, then grow into secondary crystal axes; then, third

crystal axes grow on the secondary dendrites. Secondary and third

dendrites all grow along the [100] direction and perpendicularly

each other. Thus, it is clear that the ‘constitutional undercooling’

causes the morphology of austenite to develop sequentially from

plane crystal into cellular crystal, cellular dendrite, and then

dendrite crystal.

Fig. 2-13: Development process from octahedral crystal to dendrite stem

There exist two types of austenite dendrite morphologies [24]:

(1). Typical dendrite (see Fig. 2-14): typical dendrite has long

primary crystal axis and obvious secondary axis dendrites; the

dendrite frame shows an orientation distribution. There are many

branches within the dendrite; the cavity within the dendrite is

narrow and long, easily causing the formation of type E graphite.

(2). Frame-like dendrite (see Fig. 2-15): this type of dendrite

has short primary axis and no obvious secondary axis; the

dendrites are randomly distributed. The primary crystal axes

are often connected in a certain degree. The cavities within the

dendrite are large and have no orientation. When undercooling

is large, type D graphite is prone to forming within the dendrite

frame.

Typical dendrite is formed when dendrite has an enough

growth space and can grow freely. Frame-like dendrite is formed

by crossing, collision and overlapping (joint) of dendrite stems

and arm ends which are parallel and perpendicular each other.

Although, solute enrichment in the liquid at dendrite growth front

reduces the melting point of the liquid, causing difficulty for

dendrite growth and inhibiting overlapping or jointing of dendrite

arm ends. If there is a plenty of austenite in liquid iron, frame-like

dendrite structure is possible to form.

The author considers that there is no principled difference

between these two dendritic morphologies; these two morphologies

may be caused by different development degree of dendrite stems.

Since the randomness of sample sectioning, different

morphologies [25] from a to i (see Fig. 2-16(b)) can be obtained

from the original continuous three-dimensional dendrites (see Fig.

2-16(a)).

crystallization conformability (that is, mismatch), but also the

interfacial energy relationship [12]. Through calculation, graphite

can be effective substrates for austenite nucleation; titanium has

refining effect on primary austenite for grey iron, since carbides,

nitrides and carbon nitrides can act as nucleation substrate for

austenite [21, 23]. Figure 2-11 illustrates the TiC nucleus existing in

austenite. Since at (001)TiC//(110) austenite crystal face and along

<110>TiC//<111> austenite direction, atom spacing difference

for the two crystals is only 1.5%; there is very good conjugate

relationship between them, see Fig. 2-12, so Ti can increase

nucleus of austenite, thus refine austenite grains. Furthermore,

when there exists extra Ti, the S in iron melt will react with Ti,

not with Mn, forming TiS particles. The nucleation effect of

TiS is not as good as MnS, thus, delaying the eutectic graphite

nucleus formation and extending the precipitation time of primary

austenite. Similar to titanium, vanadium, chrome and zirconium

are easy to form carbides, nitrides and nitride carbides which can

act as nuclei of austenite [21].

Fig. 2-11: TiC nuclei in austenite dendrites (purple)

Fig.2-12: Conjugate relationship of TiC- austenite

2.3.3 Growth of primary austeniteAt equilibrium state, the crystalline morphology of austenite is

near to a spheroid-like polyhedron which consists of crystal planes

having small interfacial energy. At non-equilibrium state, due to

solute difference existing in solution in front of crystallization,

the solute gradient in front of edges or corners of polyhedron is

relatively large, and the solute diffusion velocity is fast. Therefore,

the growth velocity at edges or corners is faster than that at plane

face; the polyhedron gradually grows into star-like morphology,

August 2009

257

Serial Report

Fig.1-16: Variety of two dimensional section of austenite dendrite

Because of the inconveniency in observation of three dimensional morphology, Flemings used two dimensional section to give a quantified description for the fineness and branch morphology. Where

Fig. 2-14: Morphology of typical dendritic austenite

(a) Schematic (b) Hot alkaline etching

Fig.2-15: Morphology of frame-like dendritic austenite

(a) Schematic (b) Hot alkaline etching

(a) Three dimensional schematic (b) Two dimensional section

258

CHINA FOUNDRY Vol.6 No.3

-Dendrite morphology, refers to section area of each egg-

like section (mm2/egg-like section)

Ar-The total section area of austenite dendrites, (mm2)

nr-The number of sectioned austenite dendrites (number)

The smaller the is, the more the branching is. The dendrite

with well-developed secondary dendrites belongs to fine dendrites.

The factors affecting dendrite growth include cooling rate,

chemical composition and undercooling.

Under fast cooling condition, carbon atoms in the liquid at the

side of dendrite have not got enough time to diffuse to far away,

thus restricting the lateral growth of dendrite axis. In addition,

austenite heterogeneous nuclei have no enough time to diffuse

away, therefore increasing the opportunity for new dendrites to

grow from the side of primary dendrite (that is the secondary

or third crystal axes) [26]. In the end, all these promote austenite

to grow into more branched, fine and small dendrites with long

and fine main stem. This type of dendrite is often observed in

thin section castings, die castings or the region close to mould

wall; while in thick section and under slow cooling condition, the

structure is mainly coarse, frame-like dendrites.

Carbon equivalent is another important factor influencing

dendrite growth. With decrease of carbon equivalent, the length of

primary austenite dendrite increases.

Alloying elements exert significant influence on the morphology

of austenite dendrites [23,27,28]. Alloying elements V, Mo, B and Ce

promote secondary dendrites and reduce dendrite spacing; Ti, Cu,

Al, Cr and Bi make dendrites random distribution; Ni enlarges

dendrite spacing, reduces branching and promotes short and coarse

austenite. The influence mechanism of those elements is not clear

up to now; it may be related to undercooling, nucleation and

segregation caused by the elements.

The relationship between austenite dendrite growth velocity and

undercooling is[23]:

v = a△T n

Where v - growth velocity of dendrite tip (mm/s)

△T - undercooling (K)

a - constant

n - 1.5–3.0

Too high superheating and holding for a long time for iron

melt will increase undercooling, thus prolong dendrite length and

cause austenite dendrites more obvious orientation distribution[21].Inoculation of iron melt by SiC reduces undercooling of primary

austenite; at this time, short austenite dendrites can be observed [29].

2.3.4 Macro solidification morphology of primary austeniteAccording to alloying content, nucleation state and cooling

condition, primary austenite can precipitate by exogenous or

endogenous models.

Exogenous solidification refers to the nucleation process in

which austenite nucleates first at the interface between liquid iron

and mould wall; if direction of heat dissipation is very strong,

austenite will grow perpendicularly to wall from edge to centre

(see Fig. 2-17), and the growth velocity will exceed the nucleation

velocity. The fast cooled thin section castings are prone to forming

exogenous solidification morphology. Endogenous solidification

presents that nucleation occurs at both the surface and inside of

casting; crystals precipitate throughout whole casting section (see

Fig. 2-18); at this time, the nucleation far exceeds crystal growth.

The morphology of primary austenite has direct relation with

the number of austenite nuclei. When the iron melt has less nuclei

(for example, high superheating, holding for a long time and low

carbon equivalent) exogenous solidification is prone to occurring.

2.3.5 Factors affecting the amount of austenite The parameters describing austenite characteristics include

secondary dendrite arm spacing, arm average length, amount of

dendrites and orientation. Among them the amount of dendrites is

a predominant factor. This is different from other alloy materials;

the predominant factor controlling cast iron strength is the amount

of austenite, rather than secondary dendrite arm spacing [23].

Two methods are used for calculating the amount of austenite.

First method: austenite percentage in the observation field:

Where D - the amount of austenite (%)

Ar - total area of austenite sectioned (mm2)

AT - total area of field (mm2)

The second method: The percentage of the area braided by the

dendrite main stems and branch stems (excluding the related area

Fig.2-17: Exogenous solidification of primary austenite

Fig. 1-18: Endogenous solidification of primary austenite

August 2009

259

Serial Report

of branches without contribution to strength), called interaction

area, in the observation field [23].

Factors affecting the amount of austenite dendrites:

(1). Chemical composition (a) C, Si, Mn, S and P: at equilibrium state, carbon equivalent is

an important factor influencing the amount of austenite dendrites.

With increase of carbon equivalent, the amount of austenite

dendrites decreases. For the same carbon equivalent, the increased

ratio of silicon to carbon can increase the amount of austenite

dendrite (can increase by 15%). With increasing carbon equivalent,

this effect becomes more obvious, since carbon and silicon have

different influence on the region of austenite precipitation in the

phase diagram. Decreasing sulphur reduces the amount of primary

austenite dendrites and the degree of orientation; conversely,

increasing sulphur increases the amount of primary austenite

dendrite and the degree of orientation. The liquid iron with low

phosphorus has less primary austenite dendrites, large primary

dendrite axis spacing and narrow secondary axis spacing [30]. Mn

can increase the amount of austenite. For the iron of w(Mn)≤2.0%,

with increasing manganese, the amount of austenite dendrites is

increased, as illustrated in Fig. 2-19[31].

(b) Alloying elements: Ti, V, Cr, Mo, Zr, Al, Ce, B and Bi can

increase the dendrite amount in different degree. Their influencing

mechanisms are not totally the same. Some form substrates

for austenite, increasing number of nuclei; other elements (for

example Ce and B) increase undercooling, inhibit eutectic

solidification, and extend the time of austenite precipitation [23, 27, 28,

32]. The elements inhibiting graphitization all increase the austenite

dendrite amount in certain different degree; the effecting degree

is in the same ranking order as their ability forming carbides.

Although belong to the elements decreasing undercooling, Al has

effect of increasing the dendrite amount; nevertheless, its effect is

not as obvious as that of Ti.

(2). Cooling rate: with increasing cooling rate the primary

austenite amount increases,this is because (a) it decreases eutectic

temperature, thus extending growing time of the austenite

dendrites; (b) it increases undercooling, leading to increased

dendrite growth velocity; (c) the diffusion difficulty causes the

solute content at the solid-liquid interface far higher than that of

the internal liquid; thus austenite dendrite can continue to grow

during eutectic transformation.

(3). Inoculation and superheating: reports on the effect of

inoculation on primary dendrites are very few in literatures. The

author considers that only after full understanding the effect of

inoculation on the nucleation and the growth of austenite dendrites

and eutectic cells, it is possible to truly and totally understanding

the inoculation mechanism. Therefore, in addition to studying the

effect of inoculation on graphite, it is important to extensively

study the effect of inoculation on austenite.

Inoculation decreases undercooling, therefore, reduces the

dendrite amount. Inoculating with SiFe alloy containing Ca

and Ba, the dendrite length becomes short; at the same time,

the dendrite amount is decreased. With combined inoculation

treatment by the addition of Al and Ti, austenite nuclei are

significantly increased, giving rise to an increase in primary

austenite amount[23].

The higher the superheating of liquid iron is, the more the

austenite amount in structure is, since high superheating causes

high undercooling, thus leading to significant deviation of

composition from eutectic point.

2.3.6 Influence of primary austenite dendrites on mechanical property of grey ironThe influence of dendrites on the mechanical property of grey iron

is like the reinforced steel bars in reinforced concrete or fibres in

composite acting as supporting frame [23].

The amount and length of dendrites play a non-ignored role for

cast iron strength. The more the dendrite amount is, the stronger

the cast iron is. Figure 2-20 shows the relationship between tensile

strength and austenite dendrites [33]. The influence of secondary

axis spacing on cast iron property is not as significant as that of

the dendrite amount; nevertheless, with decrease of the secondary

axis spacing, the tensile strength also increases [34] and this trend

is decreased with increase of carbon equivalent. If dendrites are

presented as short, small and isolated forms in cast iron structure,

the improvement in tensile strength is not significant. When

dendrites cross each other and form frames, the area of interaction

(b) w(Mn)＝1.685%

Fig.2-19: Effect of Mn on austenite dendrite amount [31]

(a) w(Mn) = 0.825%

260

CHINA FOUNDRY Vol.6 No.3

is increased, which is significantly beneficial for strength

improvement [32].

The distribution, morphology and orientation of austenite are

closely related to crack propagation under load. Endogenous

primary austenite normally is small in diameter and randomly

orientated; thus the induced cracks have a long propagation

distance, therefore has stronger ability of loading [35].

The effect of austenite dendrites in enhancing strength is

to prevent crack propagation during fracture process; when

encountering dendrite, crack either changes propagation direction

or propagates along the outer edge of austenite [35, 36]. Figure 2-21

shows an unfractured structure observed by the author; it is seen

that the crack propagates along the outer edge of the dendrites.

With increasing propagation paths, the consumed energy is increased.

In addition, the three dimensional frame structures can inhibit stress

transferring from one eutectic cell to another cell [24, 37, 38]; There is

no graphite inside dendrites, so the metal inside dendrites is not

weakened, thus can prevent microcrack growth.

The relationship between the amount of primary austenite and

hardness shows a rule in which the more the austenite amount is,

the higher the Brinell hardness is. The reason is that the dendrite

amount is less, eutectic cells are inevitably increased. Large

amount of eutectic graphite causes hardness of metal matrix to

decrease.

2.4 Eutectic crystallization of grey cast ironWhen the temperature of iron melt is slightly below eutectic

temperature, eutectic crystallization occurs and the eutectic phases

consisting of graphite and austenite form. Based on the growth

relationship of the two phases, eutectic system can be classified

into three types: regular eutectic, irregular eutectic and divorced

eutectic (also called abnormal eutectic), as illustrated in Fig. 2-22.

(1) Regular eutectic: The two phases of eutectic grow with

the same velocity, thus, the solid-liquid interface at crystallization

front maintains approximately planar. The micro morphology of

regular eutectic can be lamellar, rod- or fabric-shaped. Part of

type D graphite and coral graphite in grey iron form mainly under

regular eutectic condition.

(2) Irregular eutectic: There exists temperature difference in

the front of the two phases; the difference, even it is very small,

can cause one phase leading the other phase to grow, therefore

forming a zigzag eutectic interface. The two phases of the eutectic

grow closely and cooperatively with each other.

When the two phases, flake graphite and austenite in grey iron

crystallise, graphite always is in a leading position and grows in

liquid phase; attaching to graphite, austenite nucleates and grows

afterwards, therefore belongs to irregular eutectic.

(3) Divorced eutectic: The two phases of divorced eutectic

form separately each other in site and time. The eutectic in

spheroidal graphite cast iron belongs to divorced eutectic.

2.4.1 Thermodynamics of eutectic crystallisationWhen the temperature of a liquid iron is below eutectic line E' C' F'（see Fig. 2-23) and decreases to T 1, the liquid with composition

X'c is supersaturated compared to both the two constituents. Their

Fig. 2-23: Thermodynamic potential curve of Fe-C eutectic crystallisation

Fig.2-20: Dendrite amount vs grey iron tensile strength

Fig. 2-21: Crack propagates along the outer edge of dendrites

Fig. 2-22: Classification of eutectic system

August 2009

261

Serial Report

supersaturation is X'c-Xa and Xb -X'c respectively. The free energy

of austenite and graphite mixture (the amount is determined by the

line in lower part in Fig. 2-23) is lower than that of the liquid with

composition X'c by ΔG; austenite and graphite are more stable

in the whole system, causing liquid to transform to (graphite +

austenite) eutectic.

Once solid phases (eutectic) are precipitated, the composition

of the liquid becomes non-uniform. At graphite-liquid interface

the liquid composition is closed to Xa, while at austenite-liquid

interface, the liquid composition is closed to Xb. The chemical

potential gradient causes the diffusion and migration of carbon

atoms and destroys the meta-stable equilibrium between liquid

and the two solid phases; thus the two phases are continuously

precipitated, until all the liquid is depleted.

2.4.2 Nucleation of eutectic grain (cell) and inoculation of grey ironThe crystallized unit of grey iron at eutectic reaction stage is

eutectic grain of graphite plus austenite. In metal – non-metal

eutectic systems, the solid phases which have high melting point

and ability to vary growth direction, often precipitate first and lead

the growth of eutectic grain. Therefore, graphite is the leading

phase in cast iron; austenite precipitates cooperatively between

the graphite branches, making eutectic grain an approximately

spheroidal solidification front. This eutectic grain, formed by

cooperative growth of graphite-austenite with a graphite nucleus

as centre, is called eutectic cell. It is known from this, those

submicron graphite clusters, residual and un-melted micro graphite

grains or particles, branches of primary graphite [40], high melting

point compounds and gaseous inclusions existing in iron melt,

which can be nuclei of graphite, are also the nuclei of eutectic

cells.

Since the nucleus of eutectic grain is the starting point of growth

of a eutectic cell, the number of eutectic cells reflects the number

of nuclei for graphite formation in the eutectic iron melt. The

factors affecting the number of eutectic cells include the chemical

composition, nucleation state of iron melt and cooling rate.

Carbon and silicon have important effect on the number of

eutectic cells. The closer to eutectic point the iron is, the more the

number of eutectic cells is. Sulphur is another important element

influencing eutectic cells. Low sulphur content is not beneficial

for increasing the number of eutectic cells, since sulphides in iron

melt are important substance of graphite nuclei. Besides, sulphur

reduces the interfacial energy between heterogeneous nuclei and

iron melt, making more nuclei active [10, 41]. When w(S)<0.03%,

the number of eutectic cells is significantly reduced and the

inoculation effect is decreased. Figure 2-24 shows an experimental

result of sulphur effect on the number of eutectic cells carried out

by the author.

For the iron with w(Mn)<2%, with increasing manganese the

number of eutectic cells is increased[31]. Nb is prone to forming

compounds of carbon and nitrogen which act as nuclei for graphite

leading to an increase of eutectic cells. Titanium and vanadium

decrease the number of eutectic cells since vanadium decreases

the activity of carbon, while titanium is prone to replacing sulphur

in MnS and MgS and form sulphides containing titanium whose

nucleation ability is not as effective as MnS and MgS[23]. Nitrogen

in iron melt increases the number of eutectic cells. When nitrogen

is less than 350 × 10-6, its effect is not obvious; when exceeding

a certain amount, nitrogen increases undercooling, thus increases

the number of eutectic cells [42]. In liquid iron, oxygen is prone to

forming various different oxides which act as nuclei for graphite,

therefore, eutectic cells is increased with increasing oxygen

content.

Except for the chemical composition, the nucleation state of

eutectic liquid is an important factor. Long time holding at high

temperature will destroy or reduce original nuclei, causing reduced

eutectic cells and large cell diameter. Inoculation can greatly

improve nucleation state, thus increase the number of eutectic

cells, and see Table 2-4 [43].

Addition amount of inoculant w(CaSi) (%) 0 0.05 0.1 0.2

Eutectic undercooling (℃) 24 15 4 2

The number of eutectic cells (number • cm-2) 55 108 160 215

The effects of various inoculants are very different; their

effectiveness is in following rank in descending order.

(a) w(S)＝0.03% (b) w (S)＝0.07%Fig. 2-24: Effect of sulphur on the number of eutectic cells

Table 2-4: Effect of inoculation on the number of eutectic cells[43]

262

CHINA FOUNDRY Vol.6 No.3

CaSi＞ZrFeSi＞75FeSi＞BaSi＞SrFeSi

FeSi with Sr or Ti has weaker influence on the number of

eutectic cells, while the inoculants with rare earth have the much

better effectiveness; when adding with Al and N, the effect is

even more remarkable [32]. The author found from experiments

Cooling rate has significant influence on the number of eutectic

cells; the fast the cooling, the more the amount of eutectic cells,

see Table 2-5.

Comparing the numbers of Table 2-4 and 2-5, it is seen that

inoculation decreases undercooling, while fast cooling rate

increases undercooling. Nevertheless, both of them obviously

increase the number of eutectic cells, indicating that undercooling

has double effects on eutectic cells.

The amount of eutectic cells directly reflects the refinement

of eutectic grains. According to general rule, fine grains can

increase the property of metal. For a long time, when discussing

the relationship between strength and eutectic cells of grey iron,

it is considered that under the premise of similar composition

and graphite types, tensile strength increases with increasing

that using ferrosilicon with Al, Bi, the number of eutectic cells

can be strongly increased, see Fig. 2-25. When inoculating using

Ti-Fe, the eutectic cells are reduced; however, it is found that the

austenite dendrites are refined, as shown in Fig. 2-26.

(a) Microstructure from base hypo-eutectic melt (b) After inoculation Fig.2-25: Influence of inoculation with FeSi + (Al, Bi) on the number of eutectic cells

(a) Microstructure from the base hypo-eutectic melt (b) After inoculationFig. 2-26: Influence of Ti-Fe inoculation on the number of eutectic cells

eutectic cells, because the graphite flakes become small and fine

with increasing the number of eutectic cells. However, many

experiments have shown that there exists no a fixed increasing

relation between strength and eutectic cells. For example, with

increase of silicon, the number of eutectic cells is significantly

increased, but strength is decreased; the strength of grey iron is

increased with increasing superheating (until 1,500℃), at this

time, the amount of eutectic cells is significantly decreased. The

relationship between the variation of eutectic cells and the strength

increase caused by inoculation does not always have the same

trend. Inoculating using FeSi containing Si and Ba, the strength of

the iron is higher than that using CaSi, however, the eutectic cells

number of the iron is much less than that using CaSi.

Thus it can be seen that there are many factors affecting the

strength of grey iron; the amount of eutectic cells is only one of

them and does not play a significant role. Nevertheless, the effect

of the eutectic cells number on the shrinkage of cast iron is quite

obvious. With increase of the eutectic cells number, the shrinkage

Cooling rate (℃• min-1) 60 120 200 375Eutectic undercooling (℃) 12 14 18 22Number of eutectic cells (N• cm-2) 57 75 94 113

Table 2-5: Effect of cooling rate on the number of eutectic cells [43]

August 2009

263

Serial Report

nearby; and the formation heat from austenite nucleation leads

to decreased temperature gradient in the liquid, even negative

temperature gradient. The negative temperature gradient and

constitutional undercooling make the smooth and planar austenite-

iron liquid interface becoming unstable, leading to preferential,

random projections or protrusions on the interface which protrude

into liquid and form dendrites. When the austenite touches and

connects each other integrally, halo is formed along the lateral side

of graphite flake. However, the tips of graphite always protrude

into liquid iron and never enveloped by austenite. One reason for

this is that within critical undercooling, the growth velocity of

front plane ( ) of graphite is greater than that of austenite, see

Fig.2-29[48]; the other reason is that the volume of the liquid iron

delivering carbon atoms to the tips of graphite flakes is greater

than that to the lateral sides of graphite; thus the tips of graphite

have much better conditions of obtaining carbon atoms.

Fig. 2-28: Austenite grows along the lateral side of graphite by dendrite growth

The precipitation of austenite along lateral side of graphite

inhibits the carbon delivering to lateral side of graphite and

the growth of graphite. At the moment, graphite crystal can

continuously branch through twining and changing growth

direction to carbon-enriched region to branch continually. After

graphite branching, austenite precipitates along the lateral side

of graphite immediately; the two phases grow cooperatively, the

tendency of cast iron is increased. In order to prevent the

formation of shrinkage porosity for small size castings, the number

of eutectic cells should be controlled below 300 – 400/cm2.

The relationship between the number of eutectic cells and

graphitizability generally is: with increase of the eutectic cells

number, the chilling tendency is decreased; when the eutectic cells

number is decreased, the chilling tendency is increased. However,

an exceptional fact to this general rule is that when inoculating

using ferrosilicon containing Sr, the eutectic cells number does not

change much, but the chilling tendency is significantly decreased.

Therefore, ferrosilicon containing Sr is a good inoculant for

producing thin wall grey iron castings without shrinkage porosity.

Inoculation is a process in which a small amount of substance

is added into a melt to change its solidification characteristics

through influencing its nucleation. For grey iron, the essence

of inoculation is considered to be a process influencing eutectic

reaction of iron melt by means of inoculant and increasing the

nucleation of eutectic cells (grains) of graphite and austenite

and improving graphitization, to reach the aim of reducing or

eliminating carbide formation in thin wall castings, reducing or

preventing chilling, reducing section sensitivity and the structure

difference between thin and thick wall sections and promoting the

formation of small and fine type A graphite, therefore enhancing

mechanical properties and improving machinability.

However in recent years, the graphitization function of

traditional inoculation has been expanded; the effects of inoculation

on metal matrix (such as grain size, austenite and eutectoid

transformation) have been paid more attention by many researches

and many complex inoculants have been developed [44-46]. Research

work has found that the addition of graphitizing inoculants with

undercooling-promoting alloying elements (Cr, Mn, Mo, Mg, Ti,

Ce and Sb) can increase undercooling, refine grain size, increase

the austenite amount and promote the formation of pearlites[45]. The

surface active alloying elements (Te, Bi, Sb) added in inoculants

can absorb on the surface of graphite nuclei, restrict the growth of

graphite, thus reaching the aim of refining graphite size, improving

comprehensive mechanical properties, structure uniformity and

increasing structure adjustability. This inoculation principle has

obtained application in the practical production of high carbon cast

iron (for example, brake components).

2.4.3 Growth of eutectic cells – formation of austenite halo outside the flake graphite and continuous graphite branching After graphite precipitates, austenite forms on the lateral side

of graphite flakes. The difference in atom spacing on the (0001)

plane of graphite and on the (111) plane of austenite is 2.3%,

see Fig. 2-27[47]; the lattice mismatch, δ, between the two phases

is very small, only 4.55% [12]. So, with graphite as substrate and

attaching to the lateral plane (0001) of graphite, austenite quickly

nucleates and grows, forming halo round graphite. The growth

way of austenite along the lateral side of graphite is very rarely

reported and discussed. The author found that austenite grows in

the form of dendrite and perpendicularly to the (0001) plane of

graphite, see Fig. 2-28. The growth of austenite along the plane

(0001) of graphite is influenced by the crystallization condition

● – carbon atoms on the (0001) plane

O – iron atoms on the (111) plane of austeniteFig. 2-27: Lattice parameter corresponding relationship between (0001) plane of graphite and (111) plane of austenite [47]

264

CHINA FOUNDRY Vol.6 No.3

v = growth velocity (cm/s), ΔT = undecooling (K),

vG1 = growth velocity of graphite along [ ] direction (has determined

effect on flake characteristics of graphite flakes

vG2 = growth velocity of graphite along [0001] direction (thickening velocity

of graphite flake)

vA = growth velocity of austenite

Fig. 2-29: Relationship between growth velocities of graphite and austenite and undercooling [48]

growth model is illustrated in Fig. 2-30[49]. Hereafter, the

graphite within a eutectic cell is always kept continuous.

Recently Fujikawa[50] proposed an austenite fluctuation

model on graphite branching within eutectic cell. The new

model considers that due to the non-uniform absorption of S,

O or other active elements, at certain positions, the austenite

growth velocity on the lateral side of graphite is possible

faster than graphite, thus causing fluctuated projection or

protrusion; austenite overlaps and connects here and causing

graphite branching, as illustrated in Fig. 2-31.

Compared to primary graphite, the eutectic graphite

has significantly more rotating twins and stronger

branching[51]. This is due to three reasons: (a) the

crystallization of eutectic graphite has higher undercooling

than that of primary graphite; (b) the solidification

shrinkage values of graphite and austenite are different

during eutectic growth, and the created stress is prone

to producing twin defects [52]; (c) the growth process of

graphite is restricted by second phase (austenite). All

these make the eutectic graphite flakes short, branched

and bent compared to primary graphite.

The branching degree of graphite depends on the

undercooling of the iron melt. The branching direction

is determined by instability of interface which is directly

affected by composition, temperature gradient, heat

transfer direction and iron melt flow.

The branching characteristics of graphite during

eutectic growth process have no relation with the amount

Fig. 2-31: Fluctuation model of graphite branching within eutectic cell [50]

Fig. 2-30: Eutectic growth model of austenite-graphite [49]

(a) Overlapping model (b) Fluctuation model Ⅰ (c) Fluctuation model Ⅱ

August 2009

265

Serial Report

various interface shapes are related to branching degree of

graphite[42], the frequency (or severity) of branching is directly

influenced by undercooling.

(1) Spheroidal eutectic cell: this type of eutectic cells that is

formed under large undercooling since large undercooling causes

severe branching, has spheroidal contour; the solid-liquid interface

is smooth, and the austenite and graphite have better cooperation.

In addition, large undercooling makes the growth velocity of

austenite increase, causing the formation of smooth interface.

(2) Zigzag shaped eutectic cells: when undercooling is

small, branching of graphite is decreased; therefore the two-

dimensional section of the eutectic cells shows a zigzag shape.

The reason is that small undercooling makes the austenite growth

velocity slower than that of graphite, the graphite flakes take

leading position and protrude into the liquid, giving rise to jagged

solidification front, therefore zigzag interface. Most of eutectic

cells in grey iron belong to this type.

(3) Bamboo leaf shaped eutectic cells: under extremely

small undercooling, graphite is lack of branching dynamics; at

this time, there are only very few branched graphite flakes with

austenite halo, like bamboo leaf shape.

For different branching frequency, the internal density of

eutectic cell has big difference. No matter which type of eutectic

cells, inside eutectic cell, there exist honeycomb structures in

which some late solidified liquid iron remains, forming isolated

liquid pools, as shown in Fig. 2-33.

of nuclei and the nature of nucleus substance, and are only

affected by the thermal condition near to interface of graphite and

the composition of iron melt. Thus, the length and fineness mainly

depend on frequency of branching.

2.4.4 Classification of eutectic cellsAccording to their contour, which is determined by solid-

liquid interface, eutectic cells can be classified into three types:

spheroidal, zigzag and bamboo-leaf shaped, see Fig. 2-32. The

Fig. 2-32: Classification of eutectic cells in grey iron

(c) Bamboo leaf shaped

Fig. 2-33: Internal structure of eutectic cells

(a) Spheroidal

(b) Zigzag shaped

(a) Poor density

(b) Better density

266

CHINA FOUNDRY Vol.6 No.3

However, due to (a) large amount of austenite dendrites; (b)

the weaker breaking effect of small and short graphite on metal

matrix; (c) the better spheroidal contour of eutectic cell (see

Fig.2-35) etc., type D graphite iron normally has better strength

compared to the iron with the same metal matrix.

Type E graphite is a type of graphite smaller and shorter than

type A graphite; similar with type D graphite, type E is also

distributed between austenite dendrites and type D and E are all

called inter-dendritic graphite. The type D and E graphite are

prone to forming in the cast iron with low carbon equivalent, and

more and well developed austenite dendrites. At this time, eutectic

cells and dendrites grow cross each other (see Fig. 2-36). Since

the amount of eutectic liquid between dendrites is relatively small,

the precipitated eutectic graphite is distributed along dendrite

direction, thus showing obvious orientation (see Fig. 2-37). The

undercooling of forming type E graphite is larger than that of

type A graphite but less than that of type D graphite; the size and

length of type E graphite flakes are between those of type A and D

graphite. Although type E graphite does not belong to undercooled

graphite, it is often associated with type D graphite, as illustrated

in Fig. 2-38.

The shape of type B graphite is like rosette, thus is also called

rosette graphite. In the centre of rosette, graphite is dot-like (or

short flakes) graphite surrounded by bent and curled flake graphite.

This type of graphite often forms in eutectic or near eutectic

Fig. 2-34: Ferritization of structure near to type D graphite (the orange is decomposed ferrite)

2.4.5 Formation of eutectic graphite (types A, D, E, B and coral graphite) The graphite flakes formed during eutectic solidification stage

can be classified into types A, D, E and B according to their

morphology and distribution; these graphite flakes are formed

under different composition and undercooling conditions.

Type A graphite is formed at early stage with eutectic cell,

and under relatively small undercooling and in the iron melt with

strong nucleation ability. Since branching is not very developed,

the graphite flakes distribute uniformly relatively. Type A graphite

is formed under irregular eutectic condition; the graphite flakes

grow, ahead of austenite, and project into iron melt almost like

primary phase[53].

Type D graphite is also called undercooled graphite. Severe

branching due to large undercooling is the main reason for the

formation of type D graphite. With large dispersion, the graphite

flakes are shorter and finer than type A graphite. Type D graphite

flakes distribute randomly within austenite dendrites with flake

size less than 20 µm, most within the range of 2 – 4 µm. The

radius of the graphite ends is very small nearly like sharp pointed

shape. According to the classification of eutectic systems, type D

undercooled graphite is formed under highly cooperative growth

and regular eutectic conditions. The graphite and austenite grow

with almost the same velocity and project into iron liquid at the

same time, thus the growth of graphite is restricted. The graphite

flakes are short and extremely bended, and almost dot shape under

low magnification microscope.

Based on the type D graphite growth mechanism, all the factors

increasing undercooling thus promoting branching of graphite are

beneficial for changing from type A to type D. For example, in the

grey iron cast in chilled mould, continuous cast grey iron products,

low sulphur (w(S)<0.01%) grey iron castings, vacuum melted

castings, and high temperature superheating, the type D graphite is

easy to form[17,30].

The normal manufacturing method for type D graphite iron is

adding Ti into high carbon equivalent iron melt. The addition of Ti

varies with carbon equivalent; when carbon equivalent is low, the

addition of Ti can be low and when carbon equivalent is high, the

addition of Ti is high accordingly. For example, for CE = 4.0%,

the addition of Ti is w(Ti) = 0.1% – 0.15%; for CE = 4.3%, w(Ti)

= 0.15% – 0.20%; for CE = 4.7%, w(Ti) = 0.20% – 0.25%. It was

pointed out that for obtaining type D graphite for hypereutectic

iron in die casting, the content of Ti is an important condition [54]. When CE = 4.5%, w (Ti) = 0.085% and CE = 4.44%, w (Ti)

= 0.075%, optimum strength can be achieved. In order to obtain

necessary microstructure and best combination of tensile strength

and good machinability, the content of Ti is better not to exceed

0.1% [55].

Titanium reduces eutectic temperature of the iron melt and

increases its undercooling trend, thus promoting great branching

of graphite. The large amount of small, short and banded type D

graphite flakes reduce the diffusion distance of carbon, causing the

austenite near to graphite to change to ferrite during the cooling of

casting (see Fig.2-34) leading to decreased mechanical properties.

Fig. 2-35: Eutectic cells of type D graphite

August 2009

267

Serial Report

Fig. 2-36: Eutectic cells and dendrites grow cross each other

Fig. 2-37: Orientated growth of type D graphite

Fig. 2-38: Type D and E graphite precipitated concurrently

grey irons with higher carbon and silicon content, and larger

undercooling (for example piston ring and cylinder liner

castings). Because of large undercooling, the graphite in the

centre of eutectic cell branches greatly, leading to undercooled

dot shape graphite in the centre. The released latent heat of

crystallisation reduces branching tendency of the undercooled

graphite, making graphite to change from type D to type A.

Coral graphite is a graphite cluster consisting of many

graphite fibers, exactly like coral in the sea. Under microscope

observation, the two dimensional morphologies of the

graphite present dot shape with few of them showing short

column shape; the morphologies have round ends and are not

connected each other.

In China, the iron with this type of graphite is also called

dot shape graphite cast iron. Under scanning microscope, three

dimensional structure of coral graphite is highly branched fine

fibrous morphology; most section of the fibrous graphite is

round with diameter < 1 µm [57]. The fibrous graphite consists

of circular cones with basal plane rotating itself, (see Fig.

2-5a); the basal planes rotate round the fibre axis in a un-

stable way and form irregular sector-shaped swirls which

rotate frequently and connect each other[3].

Based on the growth mechanism, Minkoff classified

the fibrous graphite together with vermicular graphite into

intermediate graphite between flake and spheroidal graphite [4].

Formation of coral graphite requires high undercooling

or extremely low sulphur content. For example, in the very

pure Fe-C-Si system with w (S)≤0.001%, coral graphite

is possible to form. When sulphur is increased to w (S) =

0.002%, in order to obtain coral graphite, it needs to increase

undercooling (pouring in graphite mould) or add w (Zr) = 0.5%.

Under these conditions, 100% coral graphite can be obtained

in sand mould cast Φ 25mm sample[58]; under vacuum melting

(0.133 Pa), once cooling rate reaches 0.9 ℃/s, sulphur can

increase to w(S) = 0.01%. For iron melt containing w(Mg)

=0.015% – 0.030% and treating by inoculant containing

sulphur, or for iron melt containing w(Mg) = 0.005% – 0.010%

and using heredity treatment with iron turnings (containing

uniform graphite particles) coral graphite can be produced[60].

The author found that in the non-uniform structure of thick

section vermicular iron castings, such a type of graphite is

possible to exist.

Since coral graphite is very fine and small, although most of

the matrix structure is ferrite, the mechanical property is very

good, tensile strength can reach Rm = 400 MPa and elongation

after fracture A = 2％.

(Be continued)