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

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

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

2.5 Crystallization of the LTF during fi nal stage of eutectic solidifi cation of grey ironIn the fi nal stage of eutectic solidifi cation, eutectic cells grow gradually

into large sizes; the liquid iron between the cells enters the last stage

of solidifi cation. At this time, the region of the remaining liquid iron

is called last to freeze volume, LTF in short, as shown in Fig.2-39.

It is diffi cult to quantitatively and strictly defi ne the boundary of the

LTF, since crystallization conditions (composition, cooling rate and

the melt state etc.) in the LTF is obviously different to other regions,

leading to difference in solidification structure which influences

subsequent solid phase transformation. Because the LTF situates in the

boundaries of eutectic cells, the structure, purity, inclusions and density

or compactness in the LTF exert important effects on the mechanical

properties of cast iron.

2.5.1 Crystallisation characteristics of the LTF(a) Heat dissipation and solidification conditions

The volume fraction of the LTF is very small, and the LTF are

surrounded by solidifi ed metal, as solidifi cation in a hot metal mould.

In addition, the graphite projecting into the LTF has good thermal

conductivity, can transfer heat out the regions; thereby the liquid iron

in the LTF has relatively fast solidifi cation velocity.

(b) Strong segregation in liquid composition

The element concentrations in the LTF are very different from that

of mother iron liquid; the carbide formation elements such as Cr, Mn,

V and Mo etc. are enriched; while graphitization elements, Si, Cu, Ni

and Al are signifi cantly depleted.

(c) More inclusions and complicated melt state

The fi nal liquid to solidify often has more non-metallic inclusions

and low melting point metals, which infl uence the solidifi cation of the

LTF. When the LTF is large, it is prone to forming micro shrinkage

porosity.

All the cast irons have such characteristics.

2.5.2 Crystallization of the LTFAccording to the chemical composition, nucleation state and cooling

condition (section thickness), the solidification of the LTF has

following several situations:

(a) Eutectic cells continue to grow

The graphite flakes in eutectic cells continue to project to

and branches in the LTF; at the same time, following graphite,

austenite continues to epitaxially grow, forming zigzag shape and

interpenetrating with neighbour eutectic cells, see Fig. 2-40. Hereafter,

Chapter 2

Grey Iron (Ⅲ)

Fig. 2-39: The LTF during the fi nal stage of eutectic solidifi cation

Fig.2-40: Eutectic cell epitaxial growth in the LTF

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the LTF gradually becomes smaller and smaller, until all the liquid is

depleted, no any other structure forms.

(b) Graphite precipitation

If the volume fraction of the LTF is larger and there exist graphite

nuclei in the region, new graphite nuclei are possible to form and grow

in carbon atom rich area. The graphite presents independently in the

LTF, see Fig. 2-41; while the formed austenite is connected with the

austenite in eutectic cells and diffi cult to be distinguished.

(c) Austenite epitaxial growth

When the carbon is depleted or reduced to a lower value, due to lack

of condition for graphite nucleation and growth, graphite stops growing;

but austenite continues to grow and extend to remaining liquid until all

the liquid is fi nished. At this time, the eutectic cell boundary is wider,

clear and easy to be distinguished, as illustrated in Fig.2-42.

(d) Carbide precipitation

The positive segregation elements (such as Cr, V and Ti) enriched in

LTF reduce the temperature interval between TEG – TEM, (see Fig.1-21).

When the content of those elements is high, it is prone to forming

intergranular carbides. At this time, thick section thickness seems not

to signifi cantly reduce the tendency of carbide formation in the LTF,

since the slow cooling of thick section will aggravate segregation.

Thick section castings without Cr, V, Ti will not occur intergranular

Fig.2-41: Graphite precipitation in the LTF

Fig.2-42: Austenite extends epitaxially to the LTF

carbides, since under the slow cooling, the point S, the end of

solidifi cation, is above the TGM line.

When the cooling rate is increased, the solidification process is

accelerated, the temperatures of the end of solidifi cation, TES, is quickly

decreased below TEM, as illustrated in Fig. 1-17(d), thus promoting

carbide formation.

For iron melt containing low carbon equivalent and high content of

positive segregation elements, it is prone to forming eutectic carbides

between eutectic cells, as shown in Fig. 2-43.

In grey iron containing high content of alloying elements, the

carbides are also distributed, except in eutectic cell boundaries, in

honeycomb structure inside eutectic cells, see Fig. 2-44; this is because

there exists remaining iron liquid in the honeycomb structure, which

solidifies in later stage. All this is resulted from the characteristics

of eutectic cells of grey iron. Besides, carbides in grey cast iron are

also situated between austenite dendrites, see Fig. 2-45. Therefore,

the carbides in grey iron are more dispersed than that in spheroidal

graphite iron.

If carbide formation elements are pushed to centre area of castings, a

large amount of ledeburite is formed in the centre of castings (see Fig.

2-46). At this time, white solidifi cation is formed inside the castings,

called inverse chill.

Fig.2-43: Carbides in the boundaries of eutectic cells

Fig.2-44: Carbides in the honeycomb structure of eutectic cells

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load, the tips will develop into crack initiations that cause the frames to

fracture prematurely. The author observed the microstructures of thirty

tensile samples and found that there occur zigzag shaped metal frames

in most of the samples having low strength (30–50MPa). The metal

network frames of eutectic cells in type D graphite cast iron are mainly

meniscus frames (see Fig. 2-51); maybe, this is the reason that type D

graphite cast iron has higher strength.

(b) Detrimental effect of precipitate phases

Precipitate phases, such as graphite, carbides, phosphorus

eutectics and inclusions in the LTF all are brittle phases; their

morphology and amount have direct infl uence on the strength and

brittleness of the metal frames. The isolated particles (such as TiC,

TiN) have less detrimental effect; but continuous network, angular

or lathy precipitates (for example phosphorus eutectics and

cementite) will cause significant detriment to properties. Figure

2-52 shows crack propagations along carbides.

(c) Effect on solid phase transformation

The positive segregation elements (Mn, Cr. Mo and V etc)

enriched in the LTF increase austenite transformation undercooling

and refi ne transformed structure (see Fig. 2-53), thereby increase

properties of the material. However, if these elements are too

much enriched in the LTF, thus leading to formation of carbides

and phosphorus eutectics; at this time, the detriment effect will

exceed positive effect.

(f) Porosity

Since the volume of final solidifying liquid in the intercellular

regions can not get compensation, small holes, i.e. shrinkage porosity,

are formed in site. For low carbon equivalent hypoeutectic grey irons

(such as grey iron 250, 300), this type of defect is prone to forming in

hot spot area of castings; after machining, small dispersed black spots,

i.e. shrinkage holes, can bee seen on the machined surface.

2.5.3 Infl uence of solidifi cation structure in the LTF on property of grey ironThe structure solidifi ed in the remaining liquid between eutectic cells

has following three infl uences:

(a) Effect of metal frames

If no graphite, carbides or phosphorus eutectics, only austenite is

formed in the liquid in the LTF, metal network frames constructed

from metal matrix will envelope eutectic cells. When under load, the

metal network frames can bear part of the load, release the un-favourite

infl uence of graphite on the mechanical property; the cutting effect of

graphite is restricted within eutectic cells. It is seen from this that metal

frame has signifi cant effect on the mechanical properties of cast iron.

It is considered that the strength characteristics of cast iron are mainly

dependent on the primary austenite and boundaries of eutectic cells

which are not cut by graphite fl akes [63].

The metal network frames can be divided into two types: meniscus

Fig. 2-49: Vertical section of stable and meta-stable Fe-C-P systems (the dash line is with addition of Te) [61]

(a) Meniscus shape (b) Zigzag shapeFig. 2-50: The types of metal frames formed in the LTF

Fig. 2-51: Metal frames in type D graphite cast iron (brown colour)

Fig.2-52: Crack propagation along intergranular carbides in eutectic cell boundaries

and zigzag shape. Spheroidal eutectic cells form meniscus frames and

zigzag eutectic cells form zigzag frames, as illustrated in Fig. 2-50.

It is obvious that due to the tip effect of zigzag frames, when under

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KS=Solute concentration in the centre of dendrite (or eutectic cell)

Solute concentration in the boundary of dendrite (or eutectic cell)

Defi nition:

KS = l no segregation

KS < l positive segregation. Solute concentration in the

boundary of dendrite (or eutectic cell) is higher than in the centre

of dendrite (or eutectic cell)

KS >1 inverse segregation. Solute concentration in the boundary

of dendrite (or eutectic cell) is lower than in the centre of dendrite

(or eutectic cell)

Similarly, segregation ratio S.R can also be used to describe the

segregation degree of solute elements[64, 65].

S.R=Solute concentration in the boundary of dendrite (or eutectic cell)

Solute concentration in the centre of dendrite (or eutectic cell)

S.R >1 positive segregation

S.R <1 inverse segregation

The relation between the two equations: KS=1

S.R

The solute elements in above equations refer to common

elements (Si, Mn, P, S), alloying elements (Cu, Ni, Mo, Cr, V, Ti,

etc.) and other elements studied in cast iron, except Fe.

Table 2-6 lists the KS for the elements in cast iron. Due to

different measurement conditions, the data from different

literatures have quite big difference. It is seen from Table 2-6, the

graphitizing elements (Si, Ni, Cu, Co, Al) have KS>1 and belong

to inverse segregation elements; carbide formation elements have

KS<1 and belong to positive segregation elements. Sulphur and

phosphorus do not belong to carbide formation elements, but

present positive segregation.

Electron probe analyses indicate that phosphorus has serious

segregation and non-uniformity. In grey iron with w(P)=0.11%,

phosphorus eutectics can reach 8.23% – 11.29%; the ratio of the

P content in phosphorus eutectics to the P content in pearlite can

reach as high as 51.4 – 70.6[64].

2.6.2 Factors affecting segregation in cast iron(a) Alloying factor

Different alloy elements have different segregation behaviours;

ElementPrimary

austenite[26]

Eutectic

cell[26]

References[65]① [66]① [67]

Al 1.10 2.50 - - -Si 1.15 1.60 1.17 1.60 1.60Cu 1.10 1.80 1.50 - 1.05Ni 1.15 1.50 1.30 1.20 1.60Co 1.14 1.40 - - -Mn 0.75 0.63 0.75 0.85 0.90Cr 0.85 0.84 0.60 - 0.70W 0.95 0.21 - - -Mo 0.87 0.24 0.40 0.70 -V 0.97 - - - -P 0.69 0.53 - 0.20 -S - - - 0.002 -Ti - - - 0.60 -

Table 2-6: Segregation coeffi cient of elements in cast iron, KS

① The structures when measuring KS was not indicated.

2.6 Segregation in cast ironSimilar to other alloys, segregation in cast iron has macro

segregation and micro segregation.

Macro segregation is a non-uniform distribution of chemical

compositions in macro scale, which is caused by long range fl ow of

liquid in casting and redistribution of solute elements in large area.

Micro segregation is called short range segregation; which is a

phenomenon of chemical composition non-uniform in grain scale.

According to their forms, segregation is divided into cellular,

dendrite and inter-granular segregation. Micro segregation of

solidifi cation is the result of solute redistribution, depending on the

diffusion process of micro area.

Micro segregation in cast iron is more obvious than macro

segregation. The non-uniform distribution of elements during

solidification often causes stuctural diversity, for example, the

region with high silicon is prone to forming ferrite, Mn, Cr, Mo

enriched regions prone to carbides, and phosphorus segregation

regions prone to phosphorus eutectics etc. In addition, segregation

also affects solid phase transformation during cooling or heat

treatment, since segregation caused by non-uniform distribution

of alloying elements will result in difference in stability and

hardenability of austenite. Therefore, segregation in cast iron has

always been paid attention.

2.6.1 Measurement of micro segregationMicro segregation is caused by redistribution of solute elements in

an alloy. For most of the alloys, distribution coeffi cients of solutes,

K0, are all less than 1. For alloys with K0<1, solutes are rejected

from grains, causing constitutional difference between inside and

outside grains.

An important parameter measuring micro segregation is

segregation coefficient KS, which is a ratio of centre solute

concentration to edge concentration in dendrite (or eutectic

cell). The value of KS indicates the degree of difference in solute

concentration inside and outside of a grain.

Fig. 2-53: Fine pearlite structure in the LTF

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this is dependent on their following attributes: Shape of phase diagram: solute element (X) influences the

shape of Fe–C–X phase diagram. When horizontal distance

between solid and liquid lines is increased, the solute concentration

difference between the centre and edge of austenite dendrite is

increased, the segregation becomes severe. Effects of elements with carbon: graphitising elements (Si,

Cu, Ni etc) in cast iron can increase activity of carbon and cause

carbon to be rejected from austenite, but enrich themselves

in austenite grains and present inverse segregation. While,

carbide formation elements (Mn, Cr, V, Mo, etc.) can decrease

thermodynamic activity of carbon, and are prone to combining

with C rejected from austenite, but enrich themselves in the LTF

and show positive segregation. Solid diffusion coefficient: solid diffusion coefficient of

elements has significant influence on segregation degree. The

diffusion coefficient of most elements (except C, N and O) in

solid is very small; the diffusion velocity is far lower than the

velocity of solidifi cation interface, thereby is very diffi cult to reach

constitutional equilibrium. The smaller the diffusion coefficient

in solid, the stronger the segregation. Compared to solid diffusion

coeffi cient, the liquid diffusion coeffi cient has smaller infl uence on

micro segregation. Interactive effect: interactive effect between elements can

change segregation tendency[68]. If third constituent makes

equilibrium distribution coeffi cient of a certain element K0 become

small (when K0<1), the segregation of the element will become

strong.

(b) Process factor Cooling rate: according to general solidification rule, as

cooling rate increases, the segregation degree inside grains

becomes severe, since under low temperature, diffusion of

elements becomes more difficult. However, when cooling rate

reaches rapid cooling rate (103–109K/s), segregation tendency is

decreased greatly, even no segregation occurs. This is because,

at this time, the temperature of dendrite tips quickly drops

to solidus; solid-liquid interface is planar growth rather than

dendrite growth, composition in solid becomes uniform. But for

casting, the infl uence is different; the fast cooling thin wall iron

castings have less segregation, while slow cooling iron castings

(especially spheroidal graphite iron castings) have strong grain

boundary segregation. In heavy section iron castings, the austenite

boundaries, especially eutectic cell boundaries, have significant

tendency of intergranular carbide formation. This is because

positive elements have enough time to be repulsed to the LTF and

combine with carbon there. For example, Ti is prone to forming

titanium carbides, titanium nitrides and titanium carbon-nitrides.

Under fast cooling condition of metal mould, it is found that TiC

is mainly distributed in pearlite while TiN in ferrite. For slow

cooling castings titanium compounds are mainly segregated to

grain boundaries as granular particles [55]. Liquid fl ow during solidifi cation: liquid short range convection

can improve the degree of solute mixing and thin the diffusion

boundary layer at solid-liquid interface, which is beneficial for

precipitation of uniform solid, thus reducing segregation.

Structure factor: fineness of austenite dendrites (or eutectic

cells) infl uences the degree of segregation; extremely fi ne grain

makes the KS value of solute element close to 1 and reduces the

degree of segregation.

2.6.3 Characteristics of micro segregation in grey iron(a) Strong segregation tendency

Cast iron has relatively more solute elements, the interaction

of elements increases segregation degree. Positive segregation

elements such as Mn, Cr, V, W and Mo are repulsed to the LTF

by solidifying solid and combined with carbon in the LTF, thus

making micro segregation further severe. The content of common

elements (such as Si, C, Mn, S, P) in steel is signifi cantly lower

than that in cast iron; thus their segregation tendency is much less

than that in cast iron. For example, Cr in steel has KS=0.94, while

in cast iron KS=0.70[69].

(b) Complicated distribution of solute elements

The large content of solute elements and variety of solidifi cation

structures in cast iron make solute element distribution very

complicated. For example, although primary and eutectic

austenite are all austenite, elements in the two have different

segregation coefficient KS. Within one eutectic cell, the solute

element concentration is also very uneven. This can be seen from

the colour difference in Fig. 2-54. In addition, the composition

difference exists in transverse section of dendrite and longitudinal

direction as well (see Fig. 2-55).

(c) Inverse segregation

For most engineering alloys, the segregation characteristics of

alloying elements show positive segregation. Differently in cast

iron, in addition to positive segregation, some elements present

inverse segregation characteristics. The solute elements in cast

iron are divided into two types: positive and inverse segregation

elements:

Positive segregation elements: Mn, Cr, V, Mo, W, P, S etc.

Inverse segregation elements: Si, Al, Cu, Ni, Co.

It is not diffi cult to see that the segregation characteristics are

closely related to their graphitizability and carbide formation

tendency (except P and S). Positive segregation elements are

Fig.2-54: Segregation within eutectic cell(small white grey area is the LTF having high positive segregation

elements)

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CHINA FOUNDRY Vol.6 No.4

mainly carbide formation elements; while inverse segregation

elements are strong graphitizing elements.

The inverse segregation behaviour of silicon and other

graphitizing elements in cast iron is caused by higher carbon and

silicon content [69].

Low carbon steel is a Fe–C–Si ternary system alloy with lower

carbon and silicon content; in the steels of the Fe–C and Fe–

Si binary alloy systems, the distribution coefficients of solute

element C and Si are K0<1. During solidification, according to

the principle of solute redistribution, although C and Si are also

distributed in liquid phase, the amount segregated to liquid is

relatively small; the Si→C repulsion force is not enough to change

the distribution characteristics in which solid is enriched in Fe,

and liquid is enriched in carbon and silicon. However, for cast iron

with higher C and Si, the silicon in Fe-C-Si system is redistributed

in another way. At this time, the Si→C repulsion force is

remarkably increased; silicon is replaced from carbon-enriched

micro volume, repulsed from liquid by carbon and enters Fe phase

preferentially. With growth of dendrites (eutectic cells), Si is

continuously dissolved in austenite, and the amount of silicon in

liquid is gradually decreased; the segregation of silicon is changed

from previous positive to inverse, while the type or sign of micro

segregation of carbon remains the same. In liquid, the reason

why Si is repulsed by C, not C repulsed by Si is that the valence

electrons on 3s3p orbit of Si atom are far from Si nucleus and

are shielded with two electron orbits K and L; while for carbon

atom, it only has one electron orbit (K) shielding. Therefore, when

forming chemical bond with Fe, silicon is superior to carbon.

The change of segregation direction of silicon in Fe–C–Si alloy is

closely related to carbon content, see Table 2-7. When w(C)>0.35%,

KS>1, the segregation of silicon is changed to inverse segregation.

When silicon is increased, the critical carbon content is also increased.

(a) Segregation along transverse section (b) Segregation along longitudinal direction

Fig. 2-55: Dendrite segregation

Fig. 2-56: Ferrite caused by segregation of silicon in the centre of eutectic cell

(a) The centre of eutectic cell has higher silicon (yellow) (b) Ferrite formed in eutectic cell centre (brown)

Once mass fraction of carbon is less than 0.35%, the segregation

of silicon has nothing to do with its content, showing positive

Table 2-7: Relation of segregation coeffi cient KS of silicon with carbon content [69]

Alloy w(C)(%) KS Type of alloy w(C)(%) KS

C15 steel 0.15 0.91 60MnSi2 steel 0.65 1.20C35 steel 0.34 1.00 Malleable cast iron 2.60 1.51C45 steel 0.48 1.10 Grey iron 3.15 1.60

segregation. The inverse segregation of Cu, Ni, Co and Al is

because the atom bonds of these elements forming with iron are far

stronger than Fe-Fe bond when carbon exists; the thermodynamic

potentials of solid solutions of Cu, Ni, Co and Al forming with Fe

are more stable [26].

2.6.4 Effect of segregation on structure and property

of grey cast ironAt beginning of solidifi cation, the inverse segregation of silicon

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makes the austenite dendrites rich in silicon; the austenite dendrites

are prone to forming dendritic ferrite during subsequent cooling.

During eutectic solidifi cation, silicon is enriched in the centre of

eutectic cells. In iron with higher silicon, ferrite often appears in

eutectic centre (see Fig. 2-56), leading to decreased mechanical

property.

Manganese segregates positively to the boundaries of austenite

dendrites (or eutectics), decreases transformation temperature

of austenite in the regions, increases its stability and makes the

austenite easy to form pearlite.

Sulphur segregates to late solidifi ed grain boundaries and forms

FeS. When manganese is low, isolated FeS is observed.

Phosphorus in cast iron is an element prone to segregation.

When mass fraction of phosphorus is 0.05%, phosphorus is

possible to form phosphorous eutectics in cast iron [70]. For most

iron castings, since phosphorus eutectics are very hard, with

hardness of 800–950HV, and brittle, the phosphorus content

should be restricted. However for improving wear resistance,

phosphorus is often added into cast iron. But when w(P)>0.2%,

shrinkage porosity occurs because of solidification shrinkage

when phosphorus enriched liquid cools to phosphorus eutectic

temperature. When Ni, Cr and Mo elements coexist in cast iron,

the combined positive segregations increase the amount and

tendency of macro shrinkage porosity [71].

Some low melting point elements (Pb, Bi, Sn and Sb etc.) often

segregate to boundaries of dendrites or eutectic cells, causing to

form special structures during solidifi cation. Trace lead of w(Pb)

> 0.005% can cause network graphite to form round austenite

dendrites[72]. Loper found through experiment that for thick section

grey iron castings, if slow cooling through high temperature

region, Wiedmannstaetten graphite can form [73].

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