Colour Metallography of Cast Iron - Nodular 2

Prévia do material em texto

76
CHINA FOUNDRY Vol.7 No.1
Colour Metallography of Cast Iron
By 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.
Spheroidal Graphite Cast Iron, SG iron in short, refers to the cast 
iron in which graphite precipitates as spheroidal shape during 
solidification of liquid iron. The graphite in common commercial 
cast iron can only be changed from flake to spheroidal shape by 
spheroidising treatment. Since spheroidal graphite reduces the 
cutting effect of stress concentration, the metal matrix strength of 
SG iron can be applied around 70% – 90%, thus the mechanical 
property of SG iron is significantly superior to other cast irons; 
even the tensile strength of SG iron is higher than that carbon 
steel.
Most engineering SG irons have eutectic or hypereutectic 
composition; their microstructure formed during solidification 
experiences similar crystallisation process to that of grey irons. 
However, compared to grey iron, SG iron has significantly higher 
undercooling, its solidification is further far away from equilibrium 
state, and easy to get composition deviation, divorced eutectic etc., 
which are different from grey iron.
3.1 Nucleation of spheroidal 
 graphite
Formation of spheroidal graphite experiences two stages: 
nucleation and growth; nucleation is the first process of graphite 
spheroid formation.
In the centre of every graphite spheroid, there exists an inclusion 
particle or complicated combined inclusion nucleus. The shape 
of particles or nuclei in graphite spheroids is different from each 
other and the size is between 0.5 μm – 3 μm, as illustrated in Fig. 
3-1[1]. 
 Chapter 3
Spheroidal Graphite Cast Iron (I)
Fig.3-1: Nucleus of spheriodal graphite [1]
 (a) Single inclusion (b) Complicated combined inclusions
3.1.1 Nucleation substance 
During melting and subsequent spheroidising and inoculation 
treatment, a large number of non-metallic inclusions is produced 
in iron melt; these primary inclusions are very small in size; 
in following pouring, filling and solidification processes, 
these inclusions collides with each other, float up or subside, 
aggregate and become larger, and then become nuclei of graphite 
precipitation. Substances for graphite nucleation include graphite, 
sulphides, oxides, carbides, intermetallic compounds and gasses 
etc, see Table 3-1[2-38]. Compared to grey iron, the nuclei in 
graphite spheroids are easier to find and confirm; the substances 
for graphite nucleation in Table 3-1 are all detected from the nuclei 
of graphite spheroids. It is normally considered that there is no 
principle difference between the flake and the spheroidal graphite 
February 2010
77
Serial Report
silicon content in micro region, leading to local hyper-eutectic 
composition, ‘carbon peak’ in the micro region; the precipitated 
carbon is called non-equilibrium carbon. This newly formed 
graphite has very high activity [42], and has zero mismatch with 
graphite, therefore carbon atoms are easy to directly deposit on it. 
Beside, non- equilibrium graphite can also come from SiC. SiC is 
a type of silicon based nucleation agent[43, 44] and has melting point 
as high as 2,700 ℃, and does not directly dissolve in iron melt, 
only follows the equation below to dissolve in iron melt: 
 SiC + Fe → FeSi + C (non-equilibrium graphite)
In the equation, the Si in SiC combines with Fe, the released or 
replaced C from SiC by Fe is a type of non-equilibrium graphite 
and can be nuclei for graphite precipitation.
(2) Substrate with salt-like structure carbides
When adding into liquid iron-carbon-silicon alloy, some 
metals in group Ⅰ, Ⅱ and Ⅲ in the periodic table of the elements 
can form salt-like carbides, see Table 3-2. The carbides of group 
Ⅱ elements in Table 3-2 can act as nuclei of graphite, since the 
carbides, combined by ion binding, form non-melting particles 
and suspend in iron melt. In addition, there exists good lattice 
matching between CaC2 and graphite; the distance between CaC2 
crystal (111) planes is 0.341 nm, while the basal plane distance 
of graphite is 0.335 nm. Thus, along c-axis of graphite, the two 
lattices have very small mismatch. During nucleation, graphite is 
easy to grow heterogeneously from liquid on the carbon plane of 
salt-like carbides, see Fig.3-2. 
nuclei; an only slight difference is that there are more reaction 
products of spheroidising elements in the nuclei of graphite 
spheroids.
3.1.2 Mechanism of nucleation
The inclusion in iron melt able to become graphite nucleus 
must not only conform to lattice match relationship between 
heterogeneous nucleus and graphite, but satisfy interfacial energy 
requirement between heterogeneous nucleus and graphite. In 
principle, any nucleus substance must conform to the rule of 
nucleus formation; nevertheless, actual nucleation mechanism is 
different for different nucleation substances.
(1) Graphite 
Since the mismatch of graphite is zero (see Table 2-2), graphite 
is an ideal substrate for graphite crystallisation. Graphite in iron 
melt comes from: 
(a) Un-dissolved graphite: For example, if pig iron during re-
melting, is superheated at lower temperature and held for a short 
time, the original coarse graphite flakes are left due to incomplete 
dissolution. It is found from experiments that with increasing of 
pig iron addition in charge materials, the spheroid or nodule count 
is increased [37].
(b) Addition of crystal graphite: If added graphite is hexagonal 
lattice structure, it will promote graphite nucleation, except the 
graphite with other crystal lattices. Addition of graphite before 
or at the same time with spheroidising treatment all can increase 
graphite spheroid or nodule count. But, addition of graphite 
after spheroidisation will cause deterioration of graphite shape 
and increase of carbides, since the added graphite changes the 
interface property [41]. The ash in natural graphite inhibits or retards 
dissolvability of graphite, destroys the interfacial energy condition 
of nucleation, and thus decreases nucleation effect.
(c) Non-equilibrium graphite: In iron melt, the non-uniform 
distribution of silicon element from inoculant causes local high 
Table 3-1: Inclusion constituent phases in graphite nuclei
Type 
Graphite
Sulphides
Oxides
Compound (or element)
C
MgS
CaS
MgS•CaS
MgS•Re oxysulphide
MgS •(Ca,RE)S
CeS；LaS
Ce3S4
MnS, (Mn, Ca)S
MnS•Al2S3
RE2S3
SiO2
MgO
MgO•SiO2
2MgO•SiO2
3MgO•2SiO2•2H2O
xMgO•ySiO2•2MgS
(Mg,Ca,Al)SiO3, (Mg,Al)SiO3
(Mg,Ca,Al)SiO3
2Al2O3•FeO•10SiO2
Fe2O3
Fe2SiO4
CeO2
Al2O3
Reference 
[2]
[3-10]
[3-4, 8]
[5]
[8-10]
[10-11]
[7,12]
[13]
[3,10]
[6,14-16]
[6,17-18]
[3,6,18]
[6,18]
[19]
[20]
[21]
[21]
[22]
[17-18]
[23]
[17]
[3,5,10]
 Type 
 Carbides
 Nitrides
Intermetallic
compounds
 Others
Reference 
[24]
[25-27]
[2]
[28]
[29]
[30-32]
[5]
[2,33-34]
[35-36]
[35-36]
[16,37-38]
Compound (or element)
CaC2, SrC2, BaC2
Silicon carbide, exists in short time
Al4C3
MgSiN2
Mg3N2
BN
AlN
Ce4Bi3, CeBi3, CeBi, Ce3Bi
Ce2Pb, CePb
CeSb,CeSb2
La2Pb, LaPb
LaBi,La4Bi3, La5Bi3
LaSb, La3Sb2, La2Sb, LaSb2
Mg, Ca bubble
N2 (introduced by rare earth metal) 
bubble
Bi
Table 3-2: Type of salt-like carbides[45]
Position in 
periodic table
of theelements
 Carbide
Group
Ⅰ
NaHC2
KHC2
-
Group
Ⅱ
CaC3
SrC2
BaC2
Group
 Ⅲ
YC2
LaC2
-
78
CHINA FOUNDRY Vol.7 No.1
(3) Oxides
When contacting with air, the liquid iron is often oversaturated 
by oxygen. Liquid iron has higher silicon (4-6 times as higher as 
in steel); silicon is an excellent deoxidation element for liquid iron 
itself, and the deoxidation product is SiO2. Orths
[46] considered 
that SiO2 directly controls the formation of graphite in grey iron; 
crystallization of graphite, to a great extent, is dependent on the 
amount and distribution of SiO2. The number of SiO2 particles is 
influenced by liquid iron temperature, oxygen content, furnace 
atmosphere and slag constituent phases.
During melting of liquid iron, the iron-lining-slag-atmosphere 
actually forms a Fe-C-Si-O liquid solution system[45]. If among 
them C, Si and O are in equilibrium, following equation can be 
written: 
 (SiO2) + 2[C] = [Si] + 2{CO}
Equilibrium constant is: 
 
Below equilibrium temperature, the formed deoxidation product, 
solid SiO2, can play an effective role for graphite nucleation, since 
at this time, the SiO2 has a lattice plane able to act as a substrate. 
Therefore, based on the theory of oxide substrate, it is believed 
that under following conditions, it is possible to increase the 
number of graphite nuclei [45]:
(a) Enough oxygen in liquid iron.
(b) Enough silicon in liquid iron, which makes the content 
product of Si and O2, [Si][O]2 surplus, thus causing SiO2 to 
precipitate.
(c) Cooling rate should not be too fast so as to ensure enough 
time for SiO2 precipitation; cooling rate should not be too slow in 
order to prevent nuclei aggregation, floating up and failure. 
(d) Add elements having high affinity with oxygen (Ca, Al, Ba, 
Ti, RE), supply more foreign nuclei for graphite.
(e) 50 ℃ above the equilibrium reduction temperature of SiO2, 
otherwise SiO2 in liquid iron will be reduced. 
Many experiments have confirmed that there do exist oxides, 
and oxygen has been detected in graphite nuclei[8,47,48], but whether 
a single oxide, representative by SiO2, can become a substrate for 
graphite crystallisation is still debate. 
(4) Sulphide /oxide
It is seen from Table 2-2 that the mismatch between sulphides 
MgS, MnS, CaS and graphite is relatively bigger; a single sulphide 
is very difficult to directly become an effective substrate for 
graphite precipitation. Jacobs etc.[49] measured 1 μm inclusions 
by EDS and proposed that graphite nuclei particles have a duplex 
structure consisting of a sulphide core surrounded by an oxide 
shell: (Ca, Mg)S sulphide in the core and (Mg, Al, Si, Ti)-oxides 
in the outer shell. Many elements have more stable thermodynamic 
potential of forming sulphide than that of forming oxide; thus they 
first form sulphide, which then act as substrate for oxide formation. 
The crystal lattice orientation relationship between these two types 
of compounds: 
 Sulphide (110) // oxide (111)
 Sulphide (1 1 0) // oxide (211) 
 Orientation relationship between graphite and oxide:
 Graphite (001) // oxide (111)
Through a deep study on the duplex structure, Skaland 
confirmed that the constituent phase of outer shell is enstatite 
(MgO•SiO2) and forsterite (2MgO•SiO2), which belong to 
complex orthorhombic system[28]. The lattice mismatch between 
the complex silicates and the (0001) plane of graphite is rather 
large (see Table 3-3), the potential barrier for nucleation is high; 
thereby graphite is not easy to nucleate on them. 
Skaland considered that after Mg treatment, although many 
MgO•SiO2 and 2MgO•SiO2 substrates are produced in liquid 
iron, it can not expect to produce more graphite nuclei. Only after 
inoculation, graphite can precipitate massively.
 When inoculating using ferrosilicon containing Ca, Ba, Sr and 
Al, following reaction occurs on the surface of the inclusions:
 MgO • SiO2 + X = XO • SiO2 + Mg
 2 (2MgO • SiO2) + X + 2A1 = XO • Al2O3 • 2SiO2 + 4Mg
Where , X ― Ca, Sr or Ba.
React ion products , hexagonal s i l icates XO•SiO 2 and 
XO•A12O3•2SiO2, form surface crystal on MgO•SiO2 and 
2MgO•SiO2 substrate; among them high index plane and low-
index plane have different growth velocity. The (001) plane of 
Fig.3-2: Heterogeneous nucleation process of carbide [24, 45] 
Inclusion 
phase
Enstatite
MgO•SiO2
Forsterite
2MgO•SiO2
Orientation
 relationship
(100)n/(001)G
(010)n/(001)G
(001)n/(001)G
(110)n/(001)G
(111)n/(001)G
(100)n/(001)G
(010)n/(001)G
(001)n/(001)G
(101)n/(001)G
(111)n/(001)G
Lattice mismatch
 δ(%)
10.2
8.7
5.9
12.3
10.1
9.9
24.3
15.5
25.5
29.7
Table 3-3: Mismatch between the different lattice planes of 
MgO•SiO2, 2MgO•SiO2 and (001) plane of graphite[28]
kSi,C = = 
log = + 15.47 
[Si][O]2
[Si] 27486
[Si]
[C]2[O]2
[C]2 T
[C]2
February 2010
79
Serial Report
the surface crystal and graphite form coherent or semi-coherent 
interface with low interface energy and have low lattice mismatch 
(see Table 2-2), which is beneficial for graphite nucleation. Figure 
3-3 describes the duplex sulphide/oxide structure nucleus model. 
all lower than that of liquid iron (see Table 3-4) 
and their solubility in liquid iron is extremely 
low; during solidification, they distribute in liquid 
iron as extremely fine liquid particles. When 
temperature is slightly below eutectic temperature, 
plenty of new solid-liquid interfaces are formed; 
as substrate, graphite nucleates and grows on 
those interfaces [5,38]. Cooling rate has significant 
influences on coalescence and coarsening of 
the tiny liquid drops. For fast cooling thin wall 
castings, the tiny drops are not easy to coalesce, so 
the nucleation of Bi is significantly increased [36,38]. 
Through measurements it is found that in the Bi 
treated SG iron samples, there exists Bi except Mg, 
S, Fe and Si in the heterogeneous nuclei of Φ l μm. 
If addition of Bi in combination with RE, Bi 
and RE will form intermetallic compounds, and 
weaken their nucleation promotion effect of single 
addition. 
Using the new established duplex model, it is not 
difficult to explain:
The small amount of elements Al, Ca, Sr, Ba can 
change the surface composition of inclusion MgO•SiO2 
and 2MgO•SiO2 and play a catalyst role for graphite 
nuclei; the trace amount of La and Cr does not increase 
any new nuclei, only activates the existing inclusion 
nuclei [50].
Sulphide is the origin of duplex structure nuclei, 
thus has important effect on the number of nucleation 
events. Too low sulphur content, for example w(S) 
< 0.002％, is not beneficial for formation of a large 
number of nuclei[28].
The mismatch between single SiO2 and graphite 
lattice is quite big (37.1％), thus not suitable as 
substrate for nucleation. However, in the duplex 
structure, it is a constituent of duplex silicate and plays 
an important role in nucleation. 
The research work on substance of spheroidal 
graphite nuclei and related mechanism is continuing 
to go further and deeper. Using Confocal Scanning 
Laser Microscopy (CSLM) with composition analysis, 
Tartera confirmed that except for oxides, there exist 
sulphides of Mg, Ce and La in the outer shell of duplex 
structure[51]. 
(5) Bi and its compound: 
Addition of small amount of element Bi and Bi 
compounds can remarkably increase nodule count of 
SG iron. Melting point of Bi and Bi compounds are 
Fig. 3-3: Duplex structure model consisting of a sulphide 
core and oxide shell [28]
(a) Old model (1976)(b) New model (1993) 
Table 3-4: Melting points of Bi and its compounds
Substance Bi Bi2O3 Bi2S3 Bi2Mg3
Melting point (℃) 271 820 685 823
△T0 — undercooling before inoculation 
△T — undercooling after inoculation
Fig. 3-4: Influence of inoculation on undercooling of SG 
iron and grey iron [52] 
3.1.3 Inoculation of SG iron
(1) Importance of inoculation for spheroidal graphite iron
After spheroidisation, sulphur and oxygen content in the liquid iron 
is significantly reduced and the purity is remarkably enhanced, and 
the nuclei is reduced. Besides, the remaining magnesium in the liquid 
iron makes the undercooling increased, (see Fig.3-4). Thus, the iron 
after spheroidisation but without inoculation, has less graphite nuclei, 
strong chilling tendency and poor mechanical property; therefore after 
spheroidisation, the iron must be inoculated. In addition, inoculation 
will improve the roundness and nodularity, help graphite to become 
spheroidal. Inoculation is an important means to increase nodule counts 
and improve mechanical property of SG iron.
 
80
CHINA FOUNDRY Vol.7 No.1
time of inoculation for SG iron is about 12 – 15 min. Inoculation 
fade is aggravated with increasing holding time, enhancing 
treatment temperature, strong stirring, increasing white iron 
charge, too fine inoculant particle size and high sulphur and 
oxygen content in base iron melt. The fundamental reason of 
inoculation fade is reduction and failure of heterogeneous nuclei. 
Fade mechanism is related to nucleation mechanisms of various 
inclusions, and is a reverse process of nucleation. At present, there 
are following several mechanisms on inoculation fade:
(a) Coarsening of inclusion particles [28]: Fine inclusion particles 
(1– 4 μm) in liquid iron are continuously moving in liquid iron 
with convection; prolonged holding will increase the opportunity 
of particle impingement and make the particle coarser. Particle 
coarsening experiences three processes: (1) impingement, (2) 
diffusion and (3) combination. 
The diameter of coarsened particle is determined by Wagner 
equation: 
 
Where d0 ― Diameter of original particle;
 σ ― Interface energy between particle-liquid;
 D ― Diffusivity of element;
 C ― Volume concentration of element;
 VN ― Mole volume of inclusion phase;
 R― Atmosphere constant; 
 T ― Temperature;
 t ― Liquid holding time.
It is seen that particle coarsening is proportional to holding time. 
Except for time factor, coarsening of particle is related to interface 
energy, diffusivity and the content of element.
(b) Loss of concentration and temperature fluctuation：During 
inoculation there exists a plenty of dispersed high Si micro regions 
and C supersaturated atom micro cluster; in addition, there occur 
plenty of temperature fluctuations. Figure 3-5 shows concentration 
distribution of carbon and silicon from a liquid quenched sample. 
The non-uniformity of composition and temperature (also 
called concentration and temperature fluctuation) exerts active 
promotion for graphite nucleation. As holding time is prolonged, 
the concentration fluctuation phenomenon is gradually lost due to 
diffusion. At the same time, temperature fluctuation is gradually 
reduced, which, in the end, results in reduced number of graphite 
nuclei.
(c) Re-oxidation of liquid iron [55]：Orths considered that 
inoculation is a de-oxidation process of liquid iron, while 
inoculation fade is re-oxidation of liquid iron [46]. De-oxidation 
reaction during inoculation produces plenty of SiO2; the increase 
of combined oxygen makes the dissolved oxygen content 
decreasing, thus leading to an increased nuclei and improved 
graphitisation. For the spheroidised iron, as time is prolonged, 
oxygen in atmosphere re-enters into the treated iron, and the 
dissolved oxygen increases again [55-56]. When re-oxidising, the 
surface of SiO2 is polluted; thus the effect as graphite nucleus 
gradually lost, the number of eutectic cells is significantly reduced. 
Table 3-5 presents the relationship of dissolved oxygen, the 
number of eutectic cells with liquid holding time [55]. 
(2) Difference and similarity in inoculation of spheroidal 
and grey irons 
SG iron and grey iron have the same nucleation substances 
and mechanisms, the inoculation elements used for both irons 
are similar. Most inoculants can be commonly used for both SG 
iron and grey iron. But, since the two liquid irons have different 
sulphur and oxygen content, different nucleation state, different 
composition in nuclei (SG iron contains rare earth Ce, La and Mg) 
and different undercooling, thus the inoculation effect is different.
Since the undercooling of SG iron is far large than that of 
grey iron, the inoculation effect of SG iron is significantly better 
than that of grey iron. For example, pure silicon has very weak 
inoculation effect on grey iron, but shows certain inoculation 
effect on SG iron; ferrosilicon containing Sr significantly increases 
nodule counts in SG iron, but does not increase eutectic cells in 
grey iron. This indicates that a good inoculant for SG iron is not 
necessarily good for grey iron. Loper estimated that there exist 
about 1,000 – 10,000 active nuclei/mm2 in inoculated SG iron, 
among them the number of effective nuclei is much more than that 
in grey iron.
Evaluation of inoculation effect on SG iron is to observe the 
nodule counts increase and the fading time. While evaluation of 
inoculation on grey iron is to test many items including the ability 
of controlling chilling tendency, increasing eutectic cells and 
improving section sensitivity, and mechanical property and fading 
resistance. 
 Measures to increase inoculation include: 
(a) Select effective inoculant. Ferrosilicon based inoculant is better 
to contain small amount of element like Al, Ca, Ce, Sr and Ba etc. 
(b) Ensure the spheroidised iron has necessary sulphur. Too low 
sulphur content is not beneficial for improving nodule counts. It 
was found [53] that for a spheroidised iron containing w(S) = 0.005%, 
after using FeS2 post inoculation, sulphur is increased to w(S) = 
0.012％, nodule shape is not affected, but nodule counts increase 
from 528/mm2 to 585/mm2.
(c) Improve inoculation methods. Reduce the time from 
inoculation to solidification, since for all the inoculants, the 
inoculation effect all reaches the best at the moment of addition, 
and soon after fades; there exists no incubation period for fade. 
(d) Increase cooling rate. According to the new duplex structure 
model of graphite nucleus, (see Fig. 3-3) Skaland designed and 
produced a high effective inoculant, Utraseed [54], in which the 
difference from traditional inoculants is that except for ferrosilicon 
particles, a certain of non-metallic constituent S, O is added 
in to compensate the shortage of S and O in liquid iron after 
spheroidisation, which are necessary for obtaining large number 
of nuclei. The composition of Ultraseed is w(Si) = 70% –76%, 
w(Ca) = 0.75％–1.25％, w(Ce) = 0.75％–1.25％, w(Al) = 0.75％ 
–1.25％, S and O < 1％, balance Fe. This type of inoculant has 
advantages of high nodule counts, good nodule shape, no carbides 
and shrinkage, and is successfully used in ladle transfer treatment, 
wire feeding and iron stream treatment. 
(3) Fading of inoculation 
As holding time is prolonged, the effect of inoculation is 
gradually lost; this is called fading of inoculation. The working 
d3 =d30 + t 
64σDCV2N
9RT
February 2010
81
Serial Report
Table 3-5: Relationship of dissolvedoxygen, eutectic cells number with liquid holding time [55]
3.2 Growth of spheroidal graphite
After nucleation, graphite immediately starts to precipitate on the 
substrate to grow. The final graphite shape formed is dependent 
on growth method influenced by processing conditions. Therefore, 
process control is a key to obtain spheroidal graphite.
3.2.1 Process conditions of spheroidal graphite 
 growth
For obtaining spheroidal graphite from flake one, following 
processing conditions must be satisfied: 
(1) Extreme low sulphur and oxygen content
As long as sulphur content is controlled extremely low, without 
addition of any spheroidising element, spheroidal graphite can 
grow. With w(S) = 0.00001％ and under fast cooling, spheroidal 
graphite can grow; if w(S)≥0.004％, spheroidising element must 
be added for spheroidal graphite growth [57].
Oxygen also is a strong subversive element for graphite to grow 
to spheroid. Normally volume fraction of oxygen in grey liquid 
iron is (35 – 60) × 10-4％; while in SG iron it is only (3 – 8) × 10-4％. 
When using pure iron, carbon and silicon charges and melting 
under vacuum condition for two hours and reducing oxygen to an 
extreme low level, SG iron can be obtained.
(2) Addition of spheroidising element 
Normally sulphur and oxygen in cast iron all exceed the required 
level for obtaining spheroidal graphite, thus for obtaining SG iron, 
the addition of spheroidal element is necessary. Spheroidising 
elements are divided into three groups [57]:
The first group: Mg, Y, Ce, Ca and La, Pr, Sm, Dy, Yb, Ho, Er
The second group: Ba, Li, Cs, Rb, Sr, Tu, K, Na
The third group: Al, Zn, Cd, Sn
The first group has the strongest spheroidising ability; the second 
 Holding time (min) 0→6 11 15 19 23 26 29 32 38
 Temperature (℃) 1,390 1,434 1,460 1,484 1,510 1,508 1,510 1,435 1,350
 Eutectic cells (cm-1/2) 317 250 149 122 105 94 72 23,537 33,889
 Dissolved oxygen① (× 10-4%) 0.53 0.78 0.89 1.22 1.70 1.87 1.22 0.32 0.15
 ↑ ↑ ↑
 SiC spheroidisation inoculation
Treatment 
group is the next; the third one has the weakest spheroidising 
ability. When using Mg as spheroidising element, the elements in 
the third group often have subversive effect on spheroidisation.
The elements with strong spheroidisation (the first and the 
second group) have following common characteristics: have strong 
affinity with sulphur and oxygen and can strongly de-sulphur and 
de-oxygen; have low solubility in iron; have obvious segregation 
tendency during solidification; have certain affinity with carbon.
Among all the spheroidising elements Mg has the strongest 
spheroising ability and obtains the best graphite shape, thus the SG 
iron produced by using pure magnesium has the best mechanical 
property.
The optimum contents of spheroidising elements, such as 
Mg, Ce, Y and Ca, in SG iron depend on the cooling rate of 
castings. For the castings with general thickness, it should 
maintain in: w(Mg) = 0.035% – 0.055%；w(Ce) = 0.07% – 
0.12%; w(Y) = 0.15% – 0.2％. When containing too high amount 
of these elements, graphite is prone to degeneration. When 
Mg exceeds 0.08％, except for graphite spheroids, compound 
MgC2∙Mg2C3 is formed and flake and spiky graphite are formed 
around the compounds, and at this time graphite spheroid changes 
towards exploded shape. Besides, since Mg is consumed, when 
remaining magnesium w(Mg) > 0.055％, the spheroid counts 
are reduced. Similar to Mg, excessive Ce and Y will produce 
subversive effect to graphite spheroidization [58]. 
(3) Restriction of subversive elements 
Sulphur and oxygen are commonly existing subversive element; 
besides, Ti, Al, B, As, Sn, Sb, Pb and Bi are occasionally existing 
subversive element in cast iron. 
According to subversive mechanism, subversive elements can 
be divided into three types, see Table 3-6.
Fig.3-5: Non-uniformity phenomenon of carbon and silicon concentration in liquid iron [7]
 (inoculated at 1,400 ℃, liquid quenched after holding for 20 s)
① Volume fraction
82
CHINA FOUNDRY Vol.7 No.1
 Type 
Consuming
Segregation
Mixing
 Element 
S, O, Se, Te
Ti, Al, B, As, Sn, Sb
Pb, Bi
Subversive mechanism 
1. React with Mg and consume Mg
2. Have subversive effect itself 
1. Enrich in solid-liquid interface; deteriorate growth condition for graphite spheroids
2. Segregate to boundaries of austenite, decrease the melt point of austenite around graphite
1. React with Mg, Ce and Y, consume spheroidising elements
2. Segregate to boundaries of austenite, decease the melt point of austenite, destroy austenite halo. 
 Table 3-6: Classification of subversive elements [59]
Element 
Bi
Pb
Sb
Sn
As
Te
Se
B
Al
Ti
V
Zr
Cu
Mg
Ce
*Reference
0.003
0.004-0.009
0.025
0.13
0.125
0.21
0.04
2
0.003
0.009
0.004
0.04
0.09
0.15
0.08
6
0.003
0.009
0.026
0.01
0.08
0.3
0.04
> 1.5
 8
0.03
0.098
0.09
0.08
9
0.003-0.004
0.002-0.009
 
> 2.5
10
 0.002
 0.002 
 0.01
 0.005
 0.02
 0.002
 0.03
 0.005
 0.08
 0.04
 0.05
 0.0l
13
 0.02
 0.005
 0.02
 0.02
 0.05
 0.0l
 0.1
 0.06
 0.06
 0.02
[61]
 0.002
 0.002
 0.002
 0.15
 0.02
 0.02
 0.03
 0.15
 > 2.2
 3
0.002
0.002
0.01-0.08
0.01-0.08
0.08
1
0.00l-0.003
0.002-0.009
0.01
0.11
0.002-0.005
0.01
0.09
0.08
5
0.002
0.002
0.01
0.05
0.04
0.5-3.5
0.02
 7
Table 3-7: Critical content of some subversive elements in cast iron (mass %)
Critical content
Note: 1. The number of *References with no square brackets used here are the cited number of literatures in reference [60] in this chapter 
 2. When increasing cooling rate, the critical content allows increasing.
Titanium itself does not have strong 
subversive effect, but can strengthen the 
subversive effect of As, Sb, Pb and Bi 
(see Fig. 3-6), since the compounds of 
these elements are reduced by titanium, 
thus increasing the effective content of 
subversive elements. In addition titanium 
increases the activity of subversive 
elements, thus increasing their subversive 
ability [59].
The combined effect of subversive 
elements can be expressed by the value of 
constant K [60]; it is seen easily that Bi and 
P have the strongest subversive effect.
 K = 4.4Ti + 1.6Al + 2.0As + 2.3Sn + 
5.0Sb + 290Pb + 370Bi
Under normal conditions, it should 
keep K ≤ 1 ± 0.1. Critical content of 
subversive elements in liquid iron is given 
in Table 3-7 [60].
 Fig.3-6: Effect of Ti on the critical content of some 
subversive elements [60]
February 2010
83
Serial Report
(4) Ensure necessary cooling rate 
Cooling rate plays an important role for spheroidal graphite 
growth. In pure Fe-C, Fe-C-Si and Ni-C melt, under extremely 
high cooling rate, graphite spheroids can be obtained without 
Mg treatment. For example, for vacuum melted pure Fe-C alloy, 
when cooling rate reaching 20 ℃/s, some graphite can grow into 
spheroids; while if melting under atmosphere, the required cooling 
rate is 80 – 100 ℃/s. For iron melt containing sulphur, the allowed 
sulphur content is limited by cooling rate. For w(S) < 0.001％, the 
required cooling rate is 30 – 40 ℃/s; when sulphurincreases to 
0.007％, the required cooling rate needs to be increased to 60 ℃/s 
for graphite growing to spheroids [62].
3.2.2 Crystallisation conditions of spheroidal crystal 
 growth
Spheroid growth is a growth method of crystal in melt, and exists 
in many types of liquid, for example, in the industrious polymer 
and graphite in cast iron etc. The theory of spheroidal crystal 
growth is the foundation of spheroidisation of graphite. The 
conditions of spheroidal crystal growth are [57, 63-64]: 
(1) Formation of large number of symmetrical and radial small 
angle branches
If large numbers of symmetrical and small angle branches are 
formed on germs or embryos in three dimensional space, which 
will be beneficial for formation of a polycrystal spheroid; in 
Fig. 3-8: A model of spheroidal crystal development from regular branching [64]
(a) Single crystal vc>va (b) Flake poly-crystal vc<<va (c) Spheroidal poly-crystal vc>>va
(a) Spheroid consisting of large number of regular branches;
(b) Circle frame consisting of branches; (c) Feather-like branching
θ― branching angle
Fig. 3-7: Crystal of flake and spheroidal graphite [57]
of feather-like branches make a solid 
spheroidal crystal. During branching, 
each branch, on one hand, grows 
along radial direction, on the other 
hand, and coarsens on cross direction, 
thus growing to angle cone. In order 
to grow to a spheroid not other 
degenerate shapes, basal plane (along 
c-axis) growth velocity should exceed 
the growth velocity of prism plane 
(a-axis) va: vc > va
A model proposed by Double and 
Hellawell [66] also shows (see Fig. 
3-9) that graphite spheroid consists 
of large number of compacted circle 
cones (also cal led angle cones) 
growing from the centre towards 
the outer. It can consider that each 
circle cone is a ‘regular branch’. 
Outline of typical single angle cone 
can be seen though observing the 
internal structure of spheroids under 
transmission microscope, see Fig.3-
10. The morphology of boundaries 
of angle cones is illustrated in Fig.3-
10(b). It is seen clearly that graphite 
spheroid is a poly-crystal structure. In 
addition, it also is seen that graphite 
branches continuously along [0001] 
direction. 
contrast, if graphite does not branch, a flake polycrystal that grows 
along basal direction, is formed in the end, see Fig. 3-7.
In order to form large number of symmetrical and small angle 
branches, there must exist large number of defects in graphite 
crystal, because defects can form new steps and new growth 
direction. From crystallisation, breaking of graphite along basal 
plane and its sector shaped non-crystal branching are the basis of 
screw dislocations [65]. Small angle branching is related to the types 
of lattice defects. These defects are formed during activation of 
thermodynamics, the activation energy is increased with decrease 
of undercooling[64], thus strong undercooling can increase lattice 
defects.
(2) Small angle branching should have regular branching 
characteristics
Although the concept ‘regular branching’ was put forward 
quite earlier, it has never been used for description of spheroidal 
graphite growth. Regular branching plays an important role in 
formation of spheroidal crystal [64]. Each branching crystal (that 
is small angle branching single crystal) has almost the same 
growth habit and velocity, and maintains a special orientation with 
original crystal. The development process from regular branching 
to a spheroidal crystal is illustrated in Fig.3-8. First, a circle frame 
made of branches is formed, then large numbers of feather-like 
branches with the same orientation are formed; the large numbers 
84
CHINA FOUNDRY Vol.7 No.1
example, no sulphur in the iron), growth curves of vst and v2D move 
to right, reflecting even higher undercooling (see Fig. 3-11b), vsc > v2D; 
graphite growth follows vsc, giving rise to spheroidal graphite 
growth.
When spheroidising elements exist (see Fig. 3-11c), the Mg 
dispersed on crystal surface makes rotating twin steps to move 
slowly, causing growth to be inhibited or retarded, or even 
totally failed. At this time, only two-dimensional nucleation v2D 
and screw defect growth vsc remain available. vsc and v2D are all 
shifted to right, keeping vsc > v2D; thus graphite grows and forms a 
complete spheroidal shape.
Crystallisation of any metal always happens in undercooled 
liquid; undercooling is a decisive factor influencing phase 
transformation driving force. It influences crystallisation from 
nucleation and influences growth morphology from instability of 
solid-liquid interface as well.
It was calculated that for graphite spheroidisation, undercooling 
△T must be over 29 – 35 ℃. The undercooling difference 
between flake and SG irons is about 15 – 25 ℃ [67-68].
The undercooling during solidification of cast iron consists of 
two types: (a) thermal undercooling △T t and (b) constitutional 
undercooling △TC.
3.2.3 Defect growth of graphite spheroid and effect of 
undercooling
A comprehensive summary has been made on the theory of 
spheroidal graphite formation [67]. In current theories defect growth 
theory more conforms to the crystallisation principle of spheroidal 
crystal, therefore is recognised by more scholars. However, due to 
the complexity of spheroidal graphite formation, some phenomena 
can not be explained and clarified yet; there may be other reasons 
for the formation of graphite spheroids. 
 According to the theory of defect formation, the theory of 
non-defect nucleation and growth, and the effect of inclusions on 
graphite growth, the kinetic curves of growth velocity on different 
lattice planes can be described. Morphologies of graphite are 
dependent on the growth velocity of (0001) and (1010) planes and 
affected by the undercooling (△T ) at interfaces decisively[68-69]. 
The relationship between undercooling (△T) and growth velocity v 
under different circumstances is shown in Fig.3-11. When the melt 
is polluted by sulphur (see Fig. 3-11a), the growth velocity related 
situations are changed; under small undercooling, rotating twin 
step growth velocity vst is faster than two-dimensional nucleation 
growth velocity v2D, or faster than screw dislocation growth 
velocity, graphite grows to flake. Under very pure conditions (for 
(a) Grown spheroid, (b) Single cone crystal, (c) Relation between apex/vertex φand fold angle θ
 (a) Single crystal structure of angle cone (b) Boundary of angle cones
Fig. 3-9: Screw growth model of spheroidal graphite
Fig. 3-10: Internal structure of graphite spheroid under TEM [1]
February 2010
85
Serial Report
The undercooling formed due to change of solidification 
rate is called thermal undercooling. The faster the cooling rate, 
the bigger the undercooling, since under fast cooling, atoms 
diffuse difficultly, thus causes crystallisation to occur at lower 
temperature. The reason why thermal undercooling is beneficial 
for graphite spheroidal growth is that fast cooling causes carbon 
atoms in graphite crystal to extremely quick shrink, giving rise 
to local stress concentration and increased crystal defects. In 
addition, fast cooling causes voids or vacancies in graphite crystal 
clustering or aggregation; then the voids clusters collapse, creating 
new defects and resulting in increase of screw dislocations. 
The second undercooling is constitutional undercooling (△T c) 
consists of two parts[68]:
Fig.3-12: Undercooling caused by chemical composition [68]
 (a) ― Constitutional undercooling
 (b) ― Constitutional undercoolingplus kinetic undercooling
 gcs ― Liquidus temperature 
 g ― Temperature gradient in the liquid
Fig. 3-13: Influence of cooling rate and remaining 
Mg content on undercooling of a SG iron [69]
1: w(Mg) = 0. 005％,2: w(Mg) = 0.016％,3: w(Mg) = 0.049％
 △T c = △T cs + △Tk
 Where 
△T cs ― constitutional undercooling (caused by solute 
redistribution during crystal growth);
△Tk ― kinetic undercooling (produced by absorption of active 
elements on the surface of graphite, thus delaying crystallisation).
Superposition of the two types of undercooling is illustrated in 
Fig. 3-12.
Therefore, the undercooling of any casting consists of three 
parts: thermal, constitutional and kinetic undercooling:
 △T = △T t + △T cs + △Tk
Figure 3-13 shows the influence of cooling rate and remaining 
Mg content on undercooling of a SG iron, which reflects the 
influence of thermal undercooling △T t and kinetic undercooling 
△Tk on total undercooling.
 Under normal casting conditions, it is difficult to obtain the 
necessary undercooling for graphite spheroidisation to occur, 
from the cooling rate alone. Therefore, it is necessary to increase 
undercooling by altering the composition. The influence of various 
elements on the undercooling of an iron melt is divided into three 
types, see Table 3-8.
The influence of the three types of elements on the growth 
process of graphite is: 
First type: elements causing kinetic undercooling. 
Elements such as Mg, Y, Ce, La, etc, belong to this type. The 
functions of these elements are: (1) Deoxidise and desulphurise; 
eliminate the retarding or blocking effect on the outlets of screw 
dislocations, activate the growth steps, and ensure carbon atoms 
continue to deposit, thus making growth velocity along the c-axis 
greater than that along the a-axis, vc > va. (2) Significantly purify 
the iron melt, thus increasing kinetic undercooling. (3) The stress 
due to elements embedding in graphite lattice causes high density 
of screw dislocation [70]. Since Mg and Ce are also active elements, 
which are easily absorbed onto the graphite interface, excessive 
Mg and Ce can retard or block the graphite steps, in the same way 
as S and O do, therefore resulting in a poor graphite shape. 
Second type: elements causing constitutional undercooling.
Si is the main element causing constitutional undercooling [68]; 
Si is not absorbed on the graphite crystal surface and does not react 
with graphite. This type of element, which is enriched in the liquid 
in front of graphite growth, increases constitutional undercooling 
and instability of graphite growth, causing the graphite to branch 
strongly. Experiments found that in a pure Fe-C alloy without 
Fig. 3-11: Under different circumstances, effect of undercooling on growth velocity of graphite (1010) and (0001) planes, v
 (a) Polluted environment (b) Pure environment (c) Environment with spheroidising elements
A- Growth velocity on the rotating twin step vst; B- Two dimensional nucleation velocity v2D; C- Screw defect growth velocity vsc
86
CHINA FOUNDRY Vol.7 No.1
extremely unstable conditions [74]. With increasing undercooling, 
various defects increase and the instability of growth increases 
significantly, thus the growth mode of graphite gradually changes 
from type (1) to type (7); growth gradually develops towards 
conical pyramids. Figure 3-14 shows the relationship between 
undercooling (ΔT) and unstable growth of graphite.
Minkoff considered that for some hexagonal substances such 
as graphite, silicon, diamond and BN (boron nitride), their growth 
morphologies are all related to undercooling. Under high interface 
undercooling, sub-structures are induced by thermal stress, 
which make the layers of pyramidal crystals unstable, increasing 
branching tendency, thus promoting the formation of spheroids; 
see Fig. 3-15. Elliott suggested [75] that under certain conditions 
of cooling rate, desirable graphite morphology could be obtained 
by adjusting composition and carefully balancing trace element 
content to control undercooling.
2. Growth velocity of basal plane greater than that of prism 
plane
 Increased undercooling will readily increase various point, line 
absorption of S and O onto the prism plane, increases the growth 
velocity of the prism plane, and makes va > vc. Table 3-9 shows 
the relationship between the growing thickness of various crystal 
planes and S content. 
sulphur, Si promotes the formation of strongly branched coral 
graphite. Also, when pouring low sulphur (w(S) = 0.007％) and 
high Si (w(Si) = 11.22％) liquid iron into thin section castings, 
spheroidal graphite can be obtained [65]. In addition to Si, Bi has 
a similar effect. In a Ni-C alloy treated with w(B) = 4％, graphite 
precipitates as spheroids [57].
Third type: active elements S, O.
Elements S and O are subversive elements, which inhibit 
graphite spheroidisation; they are prone to being absorbed on the 
graphite crystal face, and increase the graphite growth temperature, 
thus making the graphite crystallisation temperature closer to 
equilibrium temperature. S and O decrease kinetic undercooling 
and cause graphite to grow towards the stable type, thus decreasing 
the branching tendency [68]. S and O are absorbed preferentially on 
the outlets of screw dislocations (has the lowest potential there); 
they inhibit the growth of screw dislocations and promote the 
development of rotating twins [71]. In addition, the bond energy 
forming covalent bonds C-O and C-S between S and O with C 
atoms is far greater than that of C-C [72], which accelerates the 
 Type
1. Active spheroidisation elements
2. Elements causing 
 constitutional undercooling 
3. Active impurity elements
Element
Group Ⅰ, Ⅱ, Ⅲ and La 
and Ce rare earths 
Group Ⅳ (mainly Si and B)
Group Ⅵ (mainly S and O)
 Present form
Strong absorption on graphite crystal surface and 
close combination with S and O
No absorption on graphite crystal surface, but 
enrichment in the liquid in graphite growth front
Absorption on graphite crystal surface
 Effect
Increase kinetic 
undercooling
Increase constitutional
undercooling
Reduce kinetic
undercooling
Table 3-8: Effect of elements on the undercooling of iron melt
 w(S)(%) 0 0.04 0.10 0.40
Basal plane/prism plane 1.8 1.7 1.0 0.5
 vc and va vc> va vc> va vc= va vc< va
 
Table 3-9： Relationship between growing thickness of 
various crystal planes and S content [72]
Composition Bi Sb Pb Ce As Sn Al Ti
hypo-eutectic 4,636 3,000 2,740 1,995 1,400 1,420 832 0
hyper-eutectic 2,896 2,000 1,544 980 1,060 1,010 840 0
Table 3-10: Activity of some surface-active elements in the iron melt (unit ×10-5 N／cm)
 The relationship between spheroidal graphite growth and 
undercooling is apparent. With increased undercooling, the 
following variations of graphite growth occur:
1. Instability of graphite growth increases 
The instability of graphite growth refers to the growing trend 
of graphite not following hexagonal ring plane spreading growth. 
According to the severity of undercooling, there exist seven modes 
of unstable growth of graphite [68]: (1) Primary graphite branches 
on the (1010) plane in the form of dendrites. (2) Eutectic graphite 
branches on the edges of the graphite. (3) Step instability occurs on 
the graphite surface and branches are extended.(4) The branches 
extending from steps leave the crystal surface, protrude into the 
liquid and continue to grow. (5) The graphite protruding into the 
liquid grows by rotating around itself. (6) Cone pyramid branches 
grow on (0001) plane. (7) Pyramid branches extend with (1010) 
plane as their outer surface. Among them, the pyramid crystal is a 
spiral polyhedral cone consisting of many hexagons, and formed 
by rotating twin dislocation transmitting into screw dislocation; 
pyramid crystals are the result of graphite growing under 
In addition to S and O, many trace subversive elements such 
as Bi, Pb, Sb and As also belong to the group of surface active 
elements. Their subversive effect is related to the surface activity 
value; the higher the surface activity, the stronger the subversive 
effect [73]. Table 3-10 lists the activity of some surface-active 
elements in the iron melt. Whether the subversive mechanism of 
those elements is similar to that S and O still need further study.
February 2010
87
Serial Report
Fig. 3-15: Relationship of growth morphologies of ice, graphite, silicon, 
diamond and boron nitride (BN) with undercooling[74]
(morphology of ice is not affected by undercooling) 
Fig. 3-16: Relationship between undercooling 
and graphite morphology [77]
Fig.3-14: Relationship between undercooling (ΔT) and 
unstable growth of graphite [68]
a → b→ c→ d undercooling gradually increases
The ratio of growth velocity of basal plane to 
growth velocity of prism plane of graphite crystal 
is closely related to graphite growth morphologies. 
Only when vc > va graphite is possible to grow into 
rod, angle cone, pyramid and even spheroidal 
graphite consisting of many cones; otherwise, 
graphite can only grow into flakes. It can be seen 
from Table 3-11, with changing of va/vc > 1 to va/
vc < 1, graphite changes from type A→ type B → 
undercooled D type → coral → vermicular → 
spheroidal shape. If vc is far greater than va, and 
with irregular branching, graphite will change from 
spheroidal to exploded and over-treated graphite.
Changing solidification velocity can result in 
different undercooling, which can be used to control 
graphite morphology. With increasing solidification, 
graphite shape changes from flake to spheroidal, 
see Fig. 3-16. When undercooling ΔT is smaller 
than critical undercooling ΔT 0, the growth velocity 
on prism plane is greater than that of basal plane, 
graphite grows into flake, otherwise, when vc > va, 
graphite grows to spheroidal morphology.
It is known from production practices that the 
processing measures to make graphite grow as 
spheroids are: (a) change chemical composition; (b) 
control cooling rate. Among chemical compositions, 
the important elements influencing graphite growth 
are those that can change the undercooling tendency 
of liquid iron. Other factors that significantly 
change the undercooling tendency of liquid iron are 
section thickness of the casting, mould material, 
pouring temperature and pouring rate. In principle, 
the essence of these process conditions is to alter 
the undercooling status of graphite crystallization. 
Using a schematic diagram, see Fig. 3-17, the author 
summarized the relationship between processing 
conditions for obtaining spheroidal graphite iron 
and defect growth mechanism influencing graphite 
spheroidisation. It can be seen from the figure that 
the aim of spheroidisation is to change kinetic 
and plane defects during the growth of graphite, thus increasing the density of 
screw dislocations, resulting in vc > va. If it can be ensured that every single 
graphite crystal has vc > va, then ‘regular branching’ (particular branching) 
conditions are easily satisfied.
88
CHINA FOUNDRY Vol.7 No.1
Table 3-11: Relationship between the ratio of growth velocities of graphite 
prism to basal plane va/vc and graphite morphology[76]
undercooling of the liquid iron. As for the reason why inoculation can 
improve spheroidisation effect, the author considered that it might be 
due to the constitutional undercooling of local regions resulting from 
Si concentration fluctuation. 
Nevertheless, the following situations exist in high temperature iron 
melts [68]:
(1) The growth of a spheroidal crystal is an unstable growth process, 
which is very sensitive to any small variation in physical 
environment, temperature, and composition. 
(2) The composition of liquid iron is very difficult to 
control accurately, especially for those trace elements 
having an important effect on graphite growth.
(3) Below the temperature at which graphite growth 
occurs, some elements will react with each other, causing 
composition of the iron to change continuously, within a 
small range. 
(4) Segregation of elements in castings will influence 
the distribution of trace elements, thus causing important 
variations of graphite morphology.
(5) In addition to acting with graphite, the elements Mg 
and Ce (added to increase undercooling), will also react 
locally with other elements in the liquid iron surrounding 
the graphite. This will cause a concentration of the 
active elements Mg and Ce around the graphite, to vary 
continuously.
All these factors make the formation of spheroidal 
graphite very complex. In addition to undercooling, 
absorption and interface phenomena also inevitably 
influence the formation process of graphite spheroids. 
Therefore, the author has recognized that the model in 
Figure 3-17 cannot explain all the phenomena involved in 
graphite spheroidisation.
 Fig. 3-17: Relationship between 
 processing conditions and 
 growth mechanisms of 
 spheroidal graphite 
Be continued