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

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

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

Chapter 5

White Cast Iron (Ⅰ)

White cast iron or ‘white iron’ refers to the type of cast iron in

which all of the carbon exists as carbide; there is no graphite in the

as-cast structure and the fractured surface shows a white colour.

White cast iron can be divided in three classes:

• Normal white cast iron — this iron contains only C, Si, Mn, P

and S, with no other alloying elements.

• Low-alloy white cast iron — the total mass fraction of alloying

elements is less than 5%.

• High-alloy white cast iron — the total mass fraction of

alloying elements is more than 5%.

These three classes of white cast iron have similar crystallization

rules and structures. The as-cast structure contains a large amount

of carbides that make these irons very hard and brittle, and

difficult to machine. These irons are wear resistant due to their

high hardness and find wide applications for abrasion-resistant

components.

5.1 Introduction5.1.1 Normal white cast ironNormal white cast iron, without any alloying elements, is used

mainly in engineering for the following applications:

(1) Abrasion resistant components without especially high wear-

resistant requirements.

(2) White cast iron for the manufacture of malleable iron castings.

The composition of normal white cast iron is listed in Table 5-1.

The composition characteristics for abrasion resistant components

are high carbon and low silicon contents, so as to increase the

amount of carbides to improve wear resistance. However, the

chemical composition of white cast iron for making malleable iron

castings contains higher silicon and lower carbon, to accelerate

graphitization during the annealing process and improve the

morphology of the resultant graphite.

Table 5-1: Typical composition of normal white iron (mass %)

No C Si Mn P S Application

1 3.5 – 4.5 0.4 – 1.2 0.2 – 1.0 0.1 – 0.3 ＜ 0.1 Abrasion resistant castings

2 2.4 – 2.8 1.2 – 1.8 0.3 – 0.6 ＜ 0.1 ＜ 0.2 Malleable iron castings

5.1.2 Low-alloy white cast ironLow-alloy white cast iron occurs when alloying element(s)

are deliberately added, but their total mass fraction is less than

5%. The functions of alloying elements are to increase the

microhardness of carbides, strengthen the metal matrix and further

improve wear resistance. Alloying elements normally used include

chromium, nickel, molybdenum, copper, vanadium, titanium and

boron. Normally, for low-alloy white cast iron, the silicon content

is lower (generally w(Si) = 0.4% – 1.2%) to ensure a ‘white’

structure is obtained; in this case the range of carbon content is

wider and is usually w(C) = 2.4% – 3.6%.

Low-alloy white cast iron is used mainly for abrasion resistant

castings.

5.1.3 High-alloy white cast ironAccording to the type of alloying elements used, high-alloy

white cast irons can be sub-divided into four systems: (1) nickel-

chromium system; (2) chromium-molybdenum system; (3) high

chromium system and (4) tungsten system. These systems, (except

the tungsten system) are included in the Chinese National Standard

for white cast irons; see Table 5-2.

(1) Nickel-chromium system The irons within this system are known internationally as Ni-

hard irons; generally the nickel content is w(Ni) = 3.3% -7.0%.

The predominant characteristics of Ni-hard irons are that they have

high strength and toughness and can be heat treated at a relatively

low temperature, which is favourable for those large castings

which are not suitable for heat treatment at high temperature and

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DT and KmTBCr9Ni5 respectively listed in the Chinese National

Standard.

are prone to cracking. Table 5-3 lists the chemical composition

of Ni-hard cast irons listed in the ASTM standard; types I, II and

IV correspond to grades KmTBNi4Cr2-GT, KmTBNi4Cr2-

Table 5-2: Specification and composition of Chinese abrasion resistant white irons (GB/T8263-1999)

Specification①Composition (mass %)

C Si Mn Cr Mo Ni Cu

KmTBNi4Cr2-DT 2.4 – 3.0 ≤ 0.8 ≤ 2.0 1.5 – 3.0 ≤ 1.0 3.3 – 5.0 …

KmTBNi4Cr2-GT 3.0 – 3.6 ≤ 0.8 ≤ 2.0 1.5 – 3.0 ≤ 1.0 3.3 – 5.0 …

KmTBCr9Ni5 2.5 – 3.6 ≤ 2.0 ≤ 2.0 7.0 – 11.0 ≤ 1.0 4.5 – 7.0 …

KmTBCr2 2.1 – 3.6 ≤ 1.2 ≤ 2.0 1.5 – 3.0 ≤ 1.0 ≤ 1.0 ≤ 1.2

KmTBCr8 2.1 – 3.2 1.5 – 2.2 ≤ 2.0 7.0 – 11.0 ≤ 1.5 ≤ 1.0 ≤ 1.2

KmTBCr12 2.0 – 3.3 ≤ 1.5 ≤ 2.0 11.0 – 14.0 ≤ 3.0 ≤ 2.5 ≤ 1.2

KmTBCr15Mo② 2.0 – 3.3 ≤ 1.2 ≤ 2.0 14.0 – 18.0 ≤ 3.0 ≤ 2.5 ≤ 1.2

KmTBCr20Mo② 2.0 – 3.3 ≤ 1.2 ≤ 2.0 18.0 – 23.0 ≤ 3.0 ≤ 2.5 ≤ 1.2

KmTBCr26 2.0 – 3.3 ≤ 1.2 ≤ 2.0 23.0 – 30.0 ≤ 3.0 ≤ 2.5 ≤ 2.0

Note: Ni-hard irons: w(S)≤0.15%, w(P)≤0.15%; KmTBCr2: w(S)≤0.1%, w(P)≤0.15%. All other specifications: w(S)≤0.06%, w(P)≤0.10%.

① ‘DT’ means low carbon and ‘GT’ means high carbon in Chinese Pinyin by initials. ② Normally, these grades should contain molybdenum.

Table 5-3: Chemical composition of American Ni-hard irons (ASTM A532M-93a) (mass%)

Types Specification C Mn Si Ni Cr Mo P S

A Ni-hard Ⅰ 2.8 – 3.6 ≤ 2.0 ≤ 0.8 3.3 – 5.0 1.4 – 4.0 ≤ 1.0 ≤ 0.3 ≤ 0.15

B Ni-hard Ⅱ 2.4 – 3.0 ≤ 2.0 ≤ 0.8 3.3 – 5.0 1.4 – 4.0 ≤ 1.0 ≤ 0.3 ≤ 0.15

C Ni-hard Ⅲ 2.5 – 3.7 ≤ 2.0 ≤ 0.8 ≤ 4.0 1.0 – 2.5 ≤ 1.0 ≤ 0.3 ≤ 0.15

D Ni-hard Ⅳ 2.5 – 3.6 ≤ 2.0 ≤ 2.0 4.5 – 7.0 7.0 – 11.0 ≤ 1.5 ≤ 0.10 ≤ 0.15

(2) Chromium-molybdenum system Cr-Mo high-alloy white cast irons contain w(Cr) = 7% - 23%

and w(Mo) ≤ 3%. There are mainly four types of Cr-Mo high-

alloy white cast irons in the Chinese National Standard (their

chemical compositions are shown in Table 5-2):

KmTBCr8

KmTBCr12

KmTBCr15Mo

KmTBCr20Mo

Among these, the medium Cr white cast iron (KmTBCr8) is the

wear resistant material with Chinese characteristics, especially

the high Si/C ratio; medium Cr white cast iron and medium Cr-Si

white cast iron (both belong to KmTBCr8) have been widely used

in China. The main features of these irons are the alloying of C

and Cr to give a ratio of Cr/C≈3 and the formed eutectic carbide

is of the type M7C3, thus giving the irons excellent combination of

properties and a higher performance/price ratio.

KmTBCrl2 has limited hardenability, so it is not normally heat

treated, except for stress relief. The as-cast matrix structure is

pearlite (which has good impact fatigue strength) and type M7C3

eutectic carbide.

KmTBCr15Mo is a type of high Cr white cast iron, which has

been studied deeply and is widely used. It is normally air quenched

and tempered and has high hardness, strength and toughness, with

excellent resistance to erosion and impact-abrasion.

KmTBCr20Mo iron has a high Cr content and thus a higher

Cr/C ratio; hence it has better hardenability, hardness, toughness

and corrosion resistance. This iron is suitable for thick section

components used under certain impact and wet abrasive-wear

conditions.

(3) High chromium system The irons under this heading have the highest Cr content within

the high-alloy white cast iron family. High Cr gives these irons

good wear resistance, corrosion resistance, impact toughness and

hardenability, all better than the properties of KmTBCr20Mo

white cast iron; the resistance to corrosive and abrasive wear, and

wear at elevated temperature are also remarkably improved. In an

acidic medium, white cast iron with w(Cr) = 28% has much better

wear resistance and high-temperature oxidation resistance than a

white cast iron with w(Cr) = 15%. The carbon content of this white

cast iron can vary between w(C) = 2.0%-3.3%; increasing the

Cr content and reducing the C content can improve its corrosion

and abrasion resistance. Cr26 high Cr white iron castings are used

mainly after quenching and tempering, but can also be used as-

cast.

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hypo-eutectic, eutectic and hyper-eutectic (see Table 5-4).

When the mass fraction of carbon is between 2.7% - 3.3%, with

increasing tungsten, the carbon supersaturation of the iron changes

from hypoeutectic to eutectic and then to hypereutectic:

Hypoeutectic w(W) ＜ 18%

Eutectic w (W) = 18% – 22%

Hypereutectic w (W) ＞ 22%

(4) Tungsten system White cast irons within the tungsten system represent a new

type of abrasion resistant material, which has higher abrasion

resistance; some casings made from the tungsten system have

a service life close to or equal to the service life of Cr-Ni-Mo

high-alloy white cast iron. According to the content of W and C,

tungsten alloy white cast irons can be divided into three classes:

Table 5-4: Classification and as-cast structure of the tungsten system white cast irons

Type of white cast iron w (W) (%) w(C) (%) W/C As cast structure

Hypoeutectic ＜ 18 2.7 – 3.3

＜ 2 Primary austenite + divorced network-like binary eutectic (M3C+γ)

2–6 Primary austenite + divorced network ternary eutectic (M3C+γ) +

ternary eutectic (M6C+M3C+γ)

Eutectic18 – 22

23

2.7 – 3.3

2.5 Binary eutectic (M3C+γ) + ternary eutectic (M6C+M3C+γ)

Hypereutectic > 22

2.7 – 3.3 >6 Blocky primary M6C + fishbone-like binary eutectic (M6C+γ) +

ternary eutectic (M6C+M23C6+γ)

2.0 – 2.5 >6 Dendritic primary M6C + fishbone-like binary eutectic (M6C+γ)

High tungsten white cast irons have high hardness and good

impact toughness due to the presence of hard, tough, primary

carbide and binary eutectic. Thus, high W white cast iron often

takes eutectic or hypereutectic composition: w(W) = 20% – 30%,

w (C) = 2.0% – 2.5%.

5.1.4 Roles of alloying elements in white cast iron

Carbon: With increasing carbon content, the hardness and wear

resistance of a white cast iron are increased. However, transverse

fracture toughness is decreased and brittleness is increased. The

higher the carbon content, the lower the impact toughness; the

linear relationship is shown in Fig. 5-1. Increasing the carbon

content increases the amount of hard and brittle eutectic carbides,

and also decreases hardenability; thus when choosing the carbon

content, comprehensive consideration should be taken.

Chromium: The main roles of Cr in white cast iron are:

forming carbides, improving corrosion resistance and stabilizing

the structure at high temperature. Increasing both the carbon and

chromium contents will increase the amount of carbides, and thus

improve wear resistance, but will also decrease toughness. The

amount of carbides can be estimated from the following equation:

Mass fraction of carbides = w(C)12.33% + w(Cr)0.55% -15.2%.

When calculating, if w(C) = 3.0%, then 3.0 is put into the

equation to replace C, the same is for Cr. It can be seen from the

equation that the effect of chromium in increasing carbide content

is not as significant as that of carbon. Thus, to increase the amount

of carbide present, it is normal to increase the carbon content. In

the Cr-Mo system irons, the volume fraction of carbides is about

20% - 40%; part of the Cr forms carbides, whilst the remainder

dissolves in the metal matrix to improve hardenability. The

amount of Cr dissolved in the metal matrix [l] is:

Mass fraction of Cr in metal matrix = 1.95 × (Cr/C)% – 2.47%.

With increasing Cr content, the structure and properties of

alloyed white cast iron change substantially; the carbide is

changed from (Fe,Cr)3C to (Fe,Cr)7C3; the hardness of the carbide

is markedly increased and at the same time, the toughness is

improved. Therefore, in addition to higher wear resistance, high Cr

white cast iron also has superior toughness and strength compared

with low-alloy white cast iron. Figure 5-2 shows the relationship

between the mechanical properties and Cr content for a series of

white cast irons; it can be seen from the figure that with increasing

Cr content, both the strength and the deflection vary significantly.

When the mass fraction of Cr is lower than 7%, there exists a

continuous network of M7C3 type carbides, which result in lower

strength and deflection. When the mass fraction of Cr is above 9%,

Fig. 5-1: Relationship between impact toughness and carbon content of high Cr [w(Cr) = 15%] white cast iron

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a discontinuous M7C3 type of carbide is formed, and the strength

and the deflection are both improved. When the mass fraction of

Cr is increased to 12% – 19%, the properties reach their highest

values. If the mass fraction of Cr exceeds 25%, hypereutectic

carbide is formed; the fracture changes to a coarse needle-like

appearance and the mechanical properties are decreased. In

addition, a high Cr content will increase the corrosion resistance

and high-temperature oxidation resistance. Most high Cr white

cast irons have a Cr mass fraction between 11% - 23% and a Cr/C

ratio between 4 - 8.

causes the percentage of retained austenite to increase and hence

has an adverse effect on the wear resistance. Reducing the amount

of C and Cr can decrease the stability of austenite, and at the same

time decrease the amount of martensite, resulting in hardness to

decrease.

Vanadium: Vanadium is a strong carbide promoter and forms

primary carbide or secondary carbide, and increases the degree of

chilling. The strong chilling effect of vanadium can be balanced

with Ni, Cu or by increasing the carbon and silicon contents. In

addition, a small amount of vanadium, for example w(V) = 0.1%

- 0.5% can refine coarse columnar crystals. Because it combines

with carbon in the liquid iron, this reduces the carbon content in

the metal matrix; vanadium increases the martensite transformation

temperature and causes the microstructure to transform into

martensite under casting conditions.

Silicon: Silicon is a restricted element in white cast iron since

it increases carbon activity and thus easily promotes graphite

formation and retards the formation of carbides. In addition,

silicon reduces the hardenability and promotes pearlite formation,

therefore having an adverse effect on the wear resistance. In low

alloy white cast iron, w(Si) is about 1%; in high Cr white iron,

silicon is often controlled w(Si) = 0.4% – 0.7%. Too low a silicon

content (for example, w(Si) ＜ 0.4%) is unfavourable for de-

oxidation. Different from general conclusions, it was reported [4] that

in medium Cr white cast iron, silicon has a tendency of increasing

the amount of carbide (Fe,Cr)7C3.

5.2 Carbides in white cast ironCarbide is an important constituent phase in white cast iron and

its volume fraction can reach as high as 40%; its type, chemical

composition, amount, size, shape and distribution all have an

important influence on the properties of the iron.

The elements which can form carbides are the transition

Fig. 5-2: Influence of Cr on strength and deflection of white iron [2]

Molybdenum: In white cast irons, approximately 50% of the

mass fraction of Mo forms Mo2C, 25% enters other carbides and

the remaining 25% dissolves in the metal matrix. The Mo which

enters the metal matrix improves hardenability of the iron; with

increasing Mo content, the hardenability also increases. The ability

of Mo to improve the hardenability in white cast iron is related to

the Cr/C ratio, as shown in Fig. 5-3. When added together with

any one of Cu, Ni or Cr, or with Cr+Ni together, the effect of

increasing hardenability is more significant. Also, in Ni-Cr type

martensitic white cast iron, Mo has the ability to replace Ni.

Nickel: Nickel is insoluble in carbides and all of it dissolves in

the austenite, thus its only purpose is to improve the hardenability.

The addition of 2.5% Ni to low Cr white cast iron can promote

a fine and hard pearlitic structure. When w(Ni)＞4.5%, the

formation of pearlite can be inhibited. With further increasing in

Ni content [w(Ni)＞6.5%], austenite is stabilised and martensite

transformation occurs at low temperature or in the as-cast state.

For example, Ni-hard iron in the as-cast condition has a structure

of martensite + M7C3 eutectic carbides. For thick section, high

Cr white iron, the addition of w(Ni) = 0.2%-1.5% can inhibit

the formation of pearlite; if Ni and Mo are added together, the

inhibiting effect is more significant.

Copper: In low Cr and high Cr martensitic white cast irons,

copper has the effect of inhibiting the formation of pearlite.

Because of limited solubility in austenite, too much Cu should not

be added; a suitable amount is w(Cu) ＜ 2.5%, thus copper cannot

replace Ni in Ni-hard irons. A combined addition of Cu and Mo

can markedly improve the hardenability. However, excessive Cu

Fig 5-3: Influence of Mo on the hardenability of high Cr white cast iron with different w(Cr)/w(C) ratios[3]

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elements in the periodic table, such as Fe, Mn, Cr, W, Pt, V, Nb,

Ti, etc. The atoms of all these elements have an incompletely

filled d-electron shell. The tendency to form carbides is related

to the degree of incompleteness of their d-electron shell; the

more unfilled vacancies in the d-electron shell the element has,

the stronger the ability to form carbide and the more stable the

carbide. The formation ability in descending order is as follows:

Ti, Nb, Zr, V, Mo, W, Cr and Mn (Fe).

Carbides have a close-packed structure or slightly distorted,

close-packed structure arranged by interaction of these metal and

carbon atoms, which form an interstitial structure consisting of

a metal atom sub-lattice and a carbon atom sub-lattice. The sub-

lattices of metal atoms are obviously different from the metal

lattices from which they are formed, but they still belong to the

typical face-centred, body-centred and close-packed hexagonal (or

complex) structures. If the interstice in a metal sub-lattice is large

enough to contain a carbon atom, a simple close-packed structure

is formed. Therefore, the ratio of carbon atom radius rc to atom

radius of transition metal rM, rc/rM, will determine the type of

carbide formed.

5.2.1 Types of carbides

According to the structure of their crystal lattice, carbides fall into

two types:

(1) Interstitial carbide with a simple, close-packed structure.

When rc/rM ＜ 0.59, carbon atoms are located at the interstices of

the simple lattice, forming an interstitial phase, which is different

from the original metal crystal lattice; the elements Mo, W, V, Ti,

Nb and Zr belong to this type. The formed carbides include:

MC type — WC, VC, TiC, NbC, ZrC

M2C type — W2C, Mo2C

If a variety of transition metals exist at the same time, complex

carbides will form. If three conditions (lattice type, electro-

chemical factor and size factor) are satisfied, the metal atoms in

the carbides can displace each other; for example, TiC-VC system

forms (Ti,V)C; VC-NbC system forms (Nb,V)C; TiC-ZrC system

forms (Ti, Zr)C, etc.

The metal atom M in MC type carbide has a simple face centred

cubic structure, the octahedral interstices all are occupied by

carbon atoms, so M : C = 1 : 1, and the crystal structure type is

that of NaCl, see Fig. 5-4.

(2) Interstitial carbides with a complex hexagonal, close-packed

structure. When rc/rM ＞ 0.59, carbon cannot form a simple, close-

packed interstitial phase, but forms an interstitial compound with a

very complex crystal lattice. The carbides of Cr, Mn and Fe belong

to this complex close-packed structure. Among them, M23C6 and

M6C are complex cubic, M7C3 is complex hexagonal and M3C

has an orthorhombic lattice. Commonly observed carbides with a

complex close-packed structure are:

M3C type — Fe3C, Mn3C or (Cr,Fe)3C, Kc for short;

M7C3 type — Cr7C3, Mn7C3 or (Cr,Fe)7C3, K2 for short;

M23C6 type — Cr23C6, Mn23C6, and ternary carbides Fe21W2C6,

Fe21Mo2C6, (Cr, Fe)23C6, K1 for short.

M6C type — Fe3W3C, Fe4W2C, FeMo3C, Fe4Mo2C ternary

carbides and so on.

(a) M3C type carbide: The carbide most commonly seen

is cementite in normal un-alloyed white cast iron. The crystal

structure of cementite is an orthogonal lattice, with lattice

constants a = 0.45144 nm, b = 0.50787 nm, c = 0.67287 nm [5]. The

crystal structure of cementite is illustrated in Fig. 5-5. Around each

carbon atom there are six iron atoms which form an octahedron;

all the axes of the octahedron are inclined at an angle to each

other, to form a rhombohedral crystal. Because each octahedron

has a carbon atom in it, and each iron atom is shared between

two octahedrons, the atomic ratio of Fe and C in the molecular

formula Fe3C is satisfied exactly. The projection of an octahedron

of cementite is a rhombic, chain-like structure (see Fig.5-6). When

observed as a whole, the rhombus planes are parallel, showing a

lamellar arrangement. In each rhombohedral crystal unit, the Fe-C

atoms are connected by a covalent bond, which is realized by the

covalent electrons of four carbon atoms and 3d-electrons of the

nearest iron atoms at the apexes of the rhombohedral unit. The

other two iron atoms are situated in neighbouring rhombohedral

units where the iron atoms are near to the next carbon atoms,

therefore a strong connection is formed between the layers. In

addition, the electronegative difference between iron and carbon

strengthens the connection of Fe-C, thus the connective force

of Fe-C is about twice as strong as that of Fe-Fe [6]. Whilst the

layers are connected by a metallic bond between iron atoms, the

connection is weak, thus resulting in the strong anisotropy of

cementite. Addition of a third element into an iron-carbon binary

alloy can change the connective strength of the Fe-C bond. The

Fig. 5-4: NaCl type structure

Fig. 5-5: Crystal structure of cementite

M2C carbide possesses a hexagonal, close-packed structure

and examples are W2C, Mo2C, V2C and Nb2C; carbon atoms are

situated at the tetrahedral interstices.

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elements enhancing the Fe-C bond will further stabilize cementite;

whilst the elements that weaken the Fe-C bond cause Fe-C to be

broken-down easily, thus reducing the stability of cementite and

promoting graphitization.

Some elements have limited the solid-solubility in Fe3C and

form alloyed cementite. The elements which can dissolve in Fe3C

are[7]: w(Cr)≤28%, w(Mo)≤14%, w(W)≤2%, w(V)≤3%. The

formed alloyed cementite, (Fe, M)3C, has a high valency, stronger

coherent bond and is more stable [8].

(b) M7C3 type carbide: A typical representative of type M7C3

carbide is Cr7C3, which consists of 56 Cr atoms and 24 carbon

atoms and has an even more complex crystal system than M3C.

The three crystal systems of Cr7C3 are hexagonal, orthogonal and

rhombohedral; their lattice constants are listed in Table 5-5.

The Cr in Cr7C3 can be partially replaced by Fe and Mn; if

replaced above 60% by Fe, then the carbide changes to (Fe,Cr)7C3.

(c) M23C6 type carbide: This is a cubic crystal lattice cell

consisting of 92 atoms; the structure is shown in Fig. 5-7. The

large crystal cell is divided into 8 small cubes; on the apexes

of the small cubes, there alternatively exist atom groups which

become cuboctahedron or cube. Normally, the M in the carbide is

mainly Cr, forming M23C6; sometimes, the M is also mainly Mn.

When containing more Mo and W, Fe21Mo2C6 carbide or Fe21W2C6

carbide is formed. In the structure of Cr23C6, the centre of each

small cube also has an additional atom which can only be replaced

by W. When replaced by W, the crystal type (Fe, W, Cr)23C6 is

formed. The carbon atoms in the Cr23C6 crystal cell are situated

Fig. 5-6: Chain-like crystal structure of cementite[6]

Table 5-5: Crystal type of M7C3 carbide [9, 10]

Crystal system/typeLattice constant

nmDensityg•cm-3

Hexagonal a = 0.688

b = 0.454

6.92 Orthogonal

a = 0.454

b = 0.688

c = 1.194

Rhombohedrala = 1.398

b = 0.452

Fig. 5-7: The structure of (Cr, Fe, W, Mo)23C6 crystal cell unit [8]

Fig. 5-8: The relationship of a C atom and neighbouring metal atoms in the Cr23C6 crystal cell unit [8] (nm)

on the edges of the large cube, and at the same time are located

in between the cuboctahedron and small cube; hence each carbon

atom has 8 neighbouring metal atoms, as shown in Fig. 5-8.

(d) M6C type carbide: This carbide is a complex interstitial,

ternary compound consisting of W, Fe and C, which exists in high

W cast iron and has a micro-hardness above 2,250 HV, and good

strength and toughness properties. The carbides in as-cast, high W

iron consist of M6C + M3C, or M6C + M23C6, or M6C + M7C, but

the main phase is still M6C. This phase is a meta-stable structure; it

will disappear after equilibrium treatment, to be replaced by WC.

M6C has a face-centred cubic lattice consisting of 96 metal atoms

and 16 carbon atoms, with 48 W atoms distributed at the apexes of

octahedrons; the lattice structure is shown in Fig. 5-9. Among the

48 iron atoms, 32 are distributed on the apexes of 8 tetrahedrons;

the centres of the tetrahedrons form a diamond lattice and the

remaining16 Fe atoms are situated in free interstices. In a pure Fe-

W-C system alloy, the composition of M6C is in between Fe4W2C

and Fe3W3C, containing w(W) = 61% - 75%. M6C can dissolve a

large amount of Si [8].

5.2.2 Characteristics of carbides

The characteristics of carbides are high hardness, high elastic

modulus, high melting point, and they are very brittle. In addition,

carbides possess obvious features of metals such as a metallic

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Fig. 5-9: The octahedron in M6C lattice

‘carbon’, and hence carbides have the features of metals.

A high melting point and high hardness are the important

features of carbides; this is because when forming carbides, there

exists a strong cohesive force from the covalent bond formed

by a p-electron of the carbon atom and a d-electron in the metal

atom. The more unfilled vacancies the d-shell has, the stronger the

covalent bond is, and the higher the melting point and hardness

are. The characteristic constants of various carbides are listed

in Table 5-6. Among the carbides, the MC type has the highest

hardness, M7C3 has the next highest and M3C has the lowest,

indicating that, as far as hardness is concerned, the connective

force of the covalent bond is more important than the crystal type.

In addition to high hardness (can reach as high as 2,300 – 2,700

HV), MC also has high oxidation resistance, hence, under high

temperature and service wear conditions, this carbide is highly

valued [10].

lustre, high heat conductivity, and their electrical resistance

decreases with decreasing temperature. When forming carbides,

the electrons of carbon are filled in the d-shell of the metallic

element atoms, resulting in the ‘metallization’ of non-metallic

Table 5-6: The characteristic parameters of carbides [7, 8, 10]

Carbide Crystal typeLattice constant

nm Melting point

ºCHardness

HV

Fe3C Rhombic

a = 0.4514

b = 0.5087

c = 0.6728

1,650 860

Cr7C3 Hexagonala = 0.688

b = 0.4541,780 (decompose) 2,100

Cr23C6 Complex cubic a = 1.064 1,520 (decompose) 1,650

Mo2C Hexagonala = 0.30

c/a = 0.1582,600 (decompose) 1,500

W2C Hexagonala = 0.298

c/a = 0.1578 2,750 2,060 HM

WC bcc a = 0.2901 2,867 2,400

VC fcc a = 0.4130 2,830 2,800

NbC fcc a = 0.4458 3,500 2,400

TiC fcc a = 0.432 3,150 3,200

ZrC fcc a = 0.4687 3,530 2,890

5.3 Crystallisation of primary carbide in white iron

When hypereutectic white cast iron solidifies under equilibrium

conditions, the earliest formed primary phase is primary carbide.

5.3.1 Crystalline thermodynamics and kinetics of primary cementite Fe3C

For a hypereutectic white cast iron, the first precipitated phase

is primary carbide. Figure 5-10 shows the free energy change

of various phases for the Fe-C phase-diagram at temperature T1;

the two inclined lines at the bottom of the figure represent the

free energy of liquid-graphite mixture and of liquid-cementite

mixture, respectively. When liquid iron with a composition X is

undercooled to T1, the carbon content of the liquid X, exceeds the

equilibrium content under meta-stable conditions Xa, forming a

supersaturation (X-Xa); thus a high carbon phase is precipitated.

Whether the precipitated phase is graphite or cementite is

dependent on the thermodynamic and kinetic conditions. Because

△G2 ＞△G1, this means that graphite precipitation will cause

a larger decrease of system thermodynamic potential than

cementite precipitation; therefore the condition is favourable for

graphite precipitation. However, since the mass fraction of carbon

in cementite is only 6.67% and that in graphite is 100%, the

formation of graphite requires carbon atoms to migrate on a large

scale. In addition, cementite is an interstitial compound and when

Note: If a carbide dissolves another element, its hardness will change, for example if it dissolves Fe:(Fe,Cr)3C: 840–1,100 HV; (Fe,Cr)7C3: 1,500–1,800 HV; (Fe,Cr)23C6: 1,140–1,500 HV

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Fig. 5-11: Plate-flake shaped dendrites of primary cementite [11]

forming carbide, iron atoms have no need to diffuse

out from the crystal lattice; therefore, from a kinetic

consideration, forming cementite is easier than forming

graphite.

5.3.2 Crystallization of primary cementite Fe3C

The growth characteristics and morphology of primary

cementite are influenced by the anisotropy of bond

energy between atoms in a crystalline structure.

Although cementite is an interstitial compound, its

growth mode is the same as a solid solution and

follows the dendritic growth mode. Due to the obvious

anisotropy of cementite, the growth velocities along

different directions are very different; the growth

velocity along the longitudinal direction (the forward

direction of the dendrite) is much faster than that

along the transverse direction (vertical to the dendritic

plane). There are large numbers of unsaturated Fe-C

coherent bonds on the edges of the rhombic plane,

which cause cementite to grow preferentially on (010)

plane along [100] direction and thus grow to a plate-

flake like dendrite as illustrated in Fig. 5-11. This

is somewhat similar to austenite, but austenite has a

three-dimensional dendritic structure, whilst cementite

has a two-dimensional structure. The amount, size,

appearance, degree of branching and crystalline

orientation of plate-flake shaped dendritic cementite

are related to the solidification conditions. Figure

5-12 shows the as-cast structure of a thin wall SG iron

casting, formed in a water-cooled mould; on the edge

adjacent to the mould wall, a large amount of primary,

needle-like cementite (the transverse section of plate-

flake shaped cementite) was formed. Besides, when

inverse chill occurs in grey and SG irons, primary

GL — Free energy of liquid phase; Gcm— Free energy of cementite;

Ggr — Free energy of graphite;

△G1 — Free energy change when precipitating cementite;

△G2 — Free energy change when precipitating graphite

Fig. 5-10: Free energy change when precipiting cementite and graphite

carbides are often observed. Because of high carbon, low silicon and fast

cooling, the plate-flakes grow thin and long, displaying a long, fine, needle-

like structure in a two-dimensional plane, as illustrated in Fig. 5-13.

К. П. Бунин described the growth process of primary cementite, as

shown in Fig. 5-14, as follows [5]:

Fig. 5-12: Needle-like primary cementite formed under chilling conditions

Fig. 5-13: Primary cementite in an ‘inverse chill’ structure

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in Fig. 5-15. When primary M7C3 grows, there exists no influence

of austenite around. Even greater undercooling does not cause

twining, thus M7C3 does not branch, and grows along the [0001]

direction, following a single crystal mode; the grain size of M7C3

is far coarser and larger than that of eutectic carbide. This feature

is obviously different from that of flake graphite as primary

graphite still shows a certain amount of branching. Each side

plane of a rod hexagonal crystal of primary carbide M7C3 is very

smooth; hence the growth is inwards as the crystal is enveloped by

its sides. When the enclosed melt solidifies, a eutectic structure or

a small amount of shrinkage is formed; hence the shrinkage holes

are often observed inside a hexagonal single crystal, particularly

in large, coarse primary M7C3 carbides. The size of M7C3 rods is

closely related to cooling rate. When fast cooling occurs, primary

carbides grow into fine rods and they are difficult to distinguish

from the eutectic carbides surrounding them; when slow cooling

occurs, large, coarse rods are formed, which are obviously

different from the surrounding eutectic structure, thus they can

be easily distinguished [10]. The size of hexagonal rods is related

to the chromium content. For example, the primary M7C3 of

hypereutectic white cast iron with w(Cr) = 15% is coarser than that

of a hypereutectic white cast iron with w(Cr) = 26%. The reason

may be due to the different Fe/Cr mass ratio [12].

The precipitation of primary M7C3 influences the solidification

morphology; when the cooling rate is fast , isolated and

disconnected M7C3 rods will solidify on a large scale, with a

‘mushy’ solidification feature. Normal white cast iron or low Cr,

hypoeutectic white cast iron solidifies from the surface towards the

centre in a successive-layer solidification mode.

5.3.4 Crystallization of primary carbide M6C

For a W system white cast iron with Sc ＞1, the W content is as

high as w(W) ＞ 25%, and there exists a large amount of tungsten-

enriched atom groups in the liquid phase. M6C can nucleate

without the need for long distance diffusion of tungsten atoms.

Since M6C is a complex interstitial compound and has a face-

centred cubic lattice, and (111) is an atom close packed plane,

when primary phase M6C grows freely in liquid, the atom close-

Fig 5-14: The growth process of primary carbide (Fe3C) [5]

(a) Protruding branches grow on the edge of a cementite ‘germ’;

impurities gather at the front edges and cause undercooling.

(b) New crystal layers grow on two-dimensional crystal nuclei.

(c) Through a dislocation (mainly screw dislocation)

mechanism, new layers grow on older layers; at the same time,

grooves form between the gaps of protrusions.

(d) The grooves between branches become deep and wide,

and form micro-isolated melt pools. The increased impurities

aggravate the branching tendency and cause zigzag growth.

(e) The layers become thicker, but the velocity of thickening is

far less than the forward velocity. On the surface of flake crystals,

undulated contours of dendrites are formed; the transverse sections

of dendrites are square or T-shaped.

5.3.3 Crystallization of primary carbide M7C3

For a hypereutectic, high Cr cast iron with w(Cr) ＞10%, the

carbide formed changes from M3C to M7C3. There is no report in

the literature about the nucleation mechanism of M7C3; research

work has been more focused on the growth process. M7C3 has two

growth morphologies — rod and plate-flake like. When following

the hexagonal crystal growth system, a rod-shaped morphology

is obtained; if following the rhombic or rhombohedral growth

system, then a plate-flake shaped morphology is easily formed.

Most of the primary carbide in hypereutectic, high Cr white cast

iron follows the hexagonal growth system; because of the obvious

anisotropy of hexagonal crystals, which results in the main growth

direction [0001], the crystal that is formed is a long rod-like

crystal with a hexagonal shape on a transverse section, as shown

Fig. 5-15: Morphology of primary carbide M7C3

(transverse section)

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packed plane (111) is developed very quickly. Along [100]

direction of the apex of polyhedron, M6C crystal grows in a

dendritic mode; at the same time, other planes also grow forward,

and in the end form a regular and symmetrical octahedron (see

Fig. 5-9). Therefore, primary M6C type carbide presents different

morphological blocks. However, except for the effect of phase

structure, the undercooling caused by compositional redistribution

at the interface between crystal and liquid and the precipitation of

a second phase, also have important influences on the growth of

M6C. Besides, during the growth process, a stacking fault is easily

formed due to atom misalignment, which causes an associated

twin structure to form. Thus, if cutting a real, primary M6C crystal,

which grows to a twin crystal, at different sections, various

independent, regular and complex morphologies can be obtained [13, 14]. If a tungsten system white cast iron contains low carbon,

the carbon depletion in the liquid in front of primary M6C is even

more obvious, thus larger constitutional undercooling is formed.

This causes M6C crystals to branch, resulting in a type of primary

M6C carbide which has a primary or secondary axis that branches

like a fork. Normally, this type of structure has high toughness.

The growth of primary M6C is also related to the content of

other elements. In a W-Cr system iron, chromium has a certain

inhibiting effect on the formation of M6C, since with the increase

of Cr, the solubility of W in M6C is increased [15]. The solubility of

Cr in M6C is very low, so the existence of Cr causes M6C to branch

in the [100] direction, making the primary M6C to crystallize in a

dendritic shape, similar to the morphology of primary austenite.

5.4 Crystallisation of primary austenite in white cast iron

Hypoeutectic white cast iron first precipitates primary austenite.

Basically, the crystalline rule of primary austenite in white cast

iron is similar to that of grey iron (see sections 2 and 3 in chapter

2). However, white cast iron contains low carbon and silicon,

high alloying elements and has a high chilling tendency, etc., all

of which cause the primary austenite to have its own morphology

features.

5.4.1 Morphology of primary austenite

Primary austenite dendrites in white cast iron fall into two types

of morphologies, (see Fig. 5-16): ① Long, rod-like dendrites

(called a ‘spiking’ structure). This type of dendrite shows obvious

orientation and a parallel arrangement as illustrated in Fig. 5-16(a);

the dendrites form large, coarse austenite grains. ② Equiaxed

dendrites. This type of dendrite is arranged randomly, without

any orientation; the dendrites form fine austenite grains which are

distributed randomly, as shown in Fig.5-16(b). The morphology

of primary austenite is directly related to its solidification mode.

For white iron castings which solidify in an exogenous mode

(crystals nucleate and grow adjacent to the mould wall), the

primary dendrites show mainly a ‘spiking’ structure. For white

cast iron castings which solidify in an endogenous mode (crystals

nucleate and grow inside the melt), the solidified structure shows

the second type of dendrite morphology. R. Dopp[17] divided

the dendrite morphologies of white cast iron into six types, in a

detailed way, see Fig. 5-17; each type’s own feature is listed in

Table 5-7. White cast iron has mainly type I and II morphologies,

whilst grey iron has mainly type V and VI. In white cast iron, the

effect of austenite morphology is more important, whilst in grey

iron, the amount of dendrites is more important.

5.4.2 Factors influencing the morphology of primary austenite

For a white cast iron with a low degree of carbon saturation (low

carbon equivalent), or with the same carbon saturation, but high

carbon and low silicon, a coarse ‘Spiking’ structure is likely to

be produced. Figure 5-18 shows the effect of carbon content on

the morphology of austenite. The formation of ‘Spiking’ dendrite

morphology is related to the nucleation status of the iron melt; if

the liquid iron has a low level of nucleation, a ‘Spiking’ structure is

easily produced. Compared to cupola melted iron, electric-furnace

melted iron has a lower level of nucleation, thus its solidification

is mainly in the exogenous mode [18]. When furnace charges have

more scrap steel and less pig iron, coarse and orientated dendrites

are increased significantly. High super-heating or long holding

Fig. 5-16: Primary austenite dendrites of white cast iron

(a) Spiking structure (b) Equiaxed grains

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Fig. 5-17: Six morphologies of austenite dendrites in white cast iron [17]

I: Exogenous solidification; II-V: Transition solidification mode; VI: Endogenous solidification

Table 5-7: Types and features of dendrite morphologies

Type Qualitative explanation Quantitative explanation ①

I Exogenous crystal sheaves reach the centre d=0

II On the periphery, exogenous crystal sheaves; in the centre, endogenous crystal sheaves 0≤d≤D/4

III On the periphery, exogenous crystal sheaves; in the centre, endogenous crystal sheaves D/4≤d≤D/2

IV On the periphery, exogenous crystal sheaves; in the centre endogenous crystal sheaves D/2≤d≤D

V Over the whole section, endogenous crystal sheaves d = D

VI Over the whole section, endogenous, irregular crystal sheaves d = D

① d is the diameter of central endogenous growth region; D is the diameter of sample.

time in an electric furnace will decrease the nucleation level of a

liquid iron, encouraging the formation of long, coarse ‘Spiking’

dendrites, as illustrated in Fig. 5-19. Inoculation has a remarkable

effect on the dendrite morphology of white cast iron. The author

found that inoculation with Fe-Ti or Fe-B alloy can increase the

nucleation and produce small grain-shaped, equiaxed dendrites,

thus strengthening endogenous solidification, see Fig. 5-20.

5.4.3 Influence of primary austenite morphology on the defects of white cast iron

Pr imary aus ten i te morphology inf luences the feed ing

characteristics, volume shrinkage distribution and associated

defects of white cast iron. When solidification is mainly

exogenous, coarse and orientated dendritic structures are prone

to form hot tears along the grain boundaries; this occurs because

the inclusions around grain boundaries weaken the strength of the

crystal boundaries. In addition, the boundary shrinkage porosities

due to difficult feeding to boundary regions also contribute to the

formation of tearing. Conversely, fine grain sizes significantly Fig. 5-18: The influence of carbon on primary austenite

morphology [17]

1: Charge is w(pig iron) 20% + w(scrap steel) 80%

2: Charge is w(pig iron) = 25% -75%, balance of scrap steel

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Fig. 5-19: The influence of superheating on austenite dendrite morphology

(a) 1,450ºC (b) 1,550ºC

(a) Without inoculation (1,550ºC) (b) Inoculated with Fe-Ti

(c) Inoculated with Fe-B

Fig. 5-20: The influence of inoculation on austenite dendrite morphology

reduce the sensitivity of forming hot tears. The relationship between

dendrite morphology and hot tearing tendency is shown in Fig. 5-21.

‘Spiking’ dendrite structures are prone to produce hot tearing

defects. In addition, it is easy to form inter-dendritic shrinkage cavities

or porosity in the hot spots; this is because the coarse dendritic

network blocks the feeding channel and inhibits liquid flow. Small

grain dendrites significantly decrease shrinkage cavities.

(a) Sensitive to hot tearing

Fig. 5-21: Relationship between dendrite morphology and hot tearing tendency [17]

(b) Not sensitive to hot tearing

Figure 5-22 illustrates the relationship between

dendrite morphology and shrinkage cavities.

For a grey iron, well-developed austenite dendrites

(for example, ‘Spiking’ structure) can improve the

strength of the iron significantly. However, for a

tempered malleable iron, with increasing coarse

austenite dendrites, the mechanical properties show a

trend of gradual decrease [18].

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(a) ‘Spiking’ dendrites

Fig. 5-22: Relationship between dendrite morphology and shrinkage cavities

(b) Small grain dendrites

To be continued