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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