Journal of Minera ls & Materials Ch ar ac teri zatio n & Engineeri ng, Vol. 10, No.4, pp.387-396, 2011 Printed in the USA. All rights reserved
Influence of Mould Heat Storage Capacity on Properties of Grey Iron
A.V. Adedayo 1,3 and B. Aremo 2
1Department of Materials Science and Engineering, Obafemi Awolowo University,
Ile-Ife, Nigeria
2Centre for Energy Research and Development (CERD), Obafemi Awolowo University,
Ile-Ife, Nigeria
3Department of Metallurgical Engineering, Kwara State Polytechnic, PMB 1375, Ilorin,
Corresponding Author:
Grey cast iron is characterized by presence of a large portion of its carbon in the form of
graphite flakes which are observable in their microstructures. Their properties are
significantly dependent on the micro-constituents of the, cast iron components. A way of
controlling the microstructure of cast iron is through the controlled cooling rates during
solidification. To control cooling rate, the heat storage capacity of the mould is important.
This paper presents the characteristic effects of graphite flake sizes on some mechanical
properties of grey cast iron. Six mould materials with heat storage capacities ranging from
1.52 kJ.m-2.K-1.s-1/2 to 2.16 kJ.m-2.K-1.s-1/2 were prepared and used to cast some grey cast ir on
samples whose microstructures were observed by optical microscopy. Mechanical properties
of the grey iron were evaluated. The results show that the properties increased with the heat
storage capacity of the mould. Also, the microstructures show a dependence on heat storage
capacity of the mould.
Keywords: Graphite, Heat storage capacity, Cementite, Grey Iron, Graphite flakes
Cast Irons (C.I.) normally contain 2-4 wt% of carbon with a high silicon concentration [1-4]
and a greater concentration of impurities than steels. The carbon equivalent (CE) of a C.I.
helps to identify the grey irons which cool into a microstructure containing graphite and the
white irons where the carbon is present mainly as iron carbide and is then referred to as
combined carbon - cementite. The carbon equivalent (CE) is defined as:
388 A.V. Adedayo and B. Aremo Vol.10, No.4
%)( PSi
 (1)
A fast cooling rate and a low carbon and silicon contents favours the formation of white C.I.
where as a low cooling rate or a high carbon and silicon contents promotes grey cast iron [2-
During solidification, the major proportion of the carbon precipitates in the form of graphite
[6] or cementite. Immediately after solidification, the precipitated phase is interspersed in a
matrix of austenite. On subsequent cooling, the carbon concentration of the austenite
decreases as more cementite or graphite precipitates from solid solution through diffusion
controlled process. In conventional C.I., at eutectoid temperature, the austenite decomposes
into ferrite-carbide aggregate called pearlite. However, in G.I., if the cooling rate through the
eutectoid temperature is sufficiently slow, then a completely ferritic matrix is obtained with
the excess carbon being deposited on the already existing graphite.
White cast iron are hard and brittle. G.I are softer with a microstructure of graphite in
transformed austenite and iron carbide matrix. The graphite flakes, which are rosettes in three
dimensions, have a low density and hence compensate for the freezing contraction, thus
giving good castings free from porosity. Both the microstructure and the macrostructure have
great influence on the properties of a material [7,8]. The nature of microstructure mixture
can lead to highly varied properties of the weld. Generally, the properties of a material are
related to its structural make-up [7, 9]. A way to control the structure of the materials is by
thermal treatment and control of solidification and cooling rates [7,9,10].
The flakes of graphite have good damping characteristics and good machinability, because
the graphite acts as a chip-breaker and lubricates the cutting tools. In applications involving
wear, the graphite is beneficial because it helps retain lubricants. However, the flakes of
graphite also are stress concentrators, leading to poor toughness. The recommended applied
tensile stress is therefore only a quarter of its actual ultimate tensile strength.
G.I. with different flake sizes was produced by sand casting using moulding sand of different
heat storage capacities. The composition of the G.I. sample is: 3.2 %C, 2.1 %Si, 0.7 %Mn,
0.1 %S, and 0.6 % P, the rest Fe. This has carbon equivalent (C.E.) of 4.1. This indicates that
the C.I. is hypoeutectic. To prepare moulding sand with different heat storage capacities, new
silica sand was obtained and mixed with bentonite, coal dust and water. To every 25 kg of
silica sand was added 2.25 kg of bentonite, 1.5 kg of coal dust, 1.75 litres of water. The sand
constituents were mixed using Ridsdale continuous muller.
With this composition of silica sand, other five different compositions of silica sand were
prepared by addition of varying percentages of iron filings to the silica sand. This serves to
vary the Heat Storage Capacity (HSC) of the mould material. The HSC is expressed as the
Vol.10, No.4 Influence of Mould Heat Storage Capacity 389
root of the product of the thermal conductivity, the specific heat capacity and the density of
the mould materials. i.e:
..cHSC (2)
where λ is the thermal conductivity, c the specific heat capacity and ρ the density of the
mould materials. For a mould made of different materials, a simple proportion formula was
used to evaluate the resultant HSC i.e.
iiiinetmulticompo cfHSC
.. (3)
where fi is the fraction of component i in a multi-component mould (see Tables 1 and 2). A
control sample, which had no Fe content, was also prepared (see Table 3).
Table 1: Standard composition of the green moulding sand
Materials Weight composition (wt %)
Silica sand 82
Bentonite 7
Coal dust 5
Water 6
Table 2: Physical properties of the mould materials
Materials Thermal conductivity
Specific heat capacity
Silica sand 0.657 2.01 1700
Bentonite 1.035 1.089 1850
Coal dust 0.186 1.31 1200
Water 0.551 4.212 999.9
Iron 63 0.502 7220
Table 3: Specimen with percentages and masses of Fe filings contents
Percentage of
Fe filings
Mass of Fe filings (kg) Heat storage capacity of the
mould, HSC, (kJ.m-2.K-1.s-1/2 )
A - - 1.52
A1 1 0.3 1.65
A2 2 0.6 1.77
A3 3 0.9 1.90
A4 4 1.2 2.03
A5 5 1.5 2.16
390 A.V. Adedayo and B. Aremo Vol.10, No.4
These sand samples were used to prepare moulds of about 60 kg (both the cope and the drag)
and used to cast samples of grey C.I. rods (25 mm diameter and 500 mm in length).
Tensile, impact and hardness test pieces were produced from the cast rods through machining
on the lathe machine. The tensile and impact test specimen were machined to ASTM E8M-88
standard test samples (see Fig.1). During machining, coolants were continuously used to
control the temperature of the specimen. The cutting speed was also extremely low. These
were to guide against any over heating which may lead to change in microstructure of the
samples. The produced samples were then tested for tensile strength, toughness and hardness.
Metallographic samples were also prepared by a gentle grinding on abrasive silicon carbide
papers of successive finer grades 240, 320, 400 and 600 lubricated with water. Polishing of
the specimens was carried out on a 150 mm rotating disc of a METASERV universal
polisher. 7μ and 15 μ diamond pastes were used, while kerosene was used as the solvent.
Having obtained mirror like surface, the polished samples were etched using 2% Nital. The
etched specimens were observed on the Olympus metallurgical microscope with a minisee
optical viewing system connected to the USB port of a computer in the department of
materials science and engineering of the Obafemi Awolowo University. Micro examination
was carried out at a magnification of x100 and images captured for metallographic analysis.
The graphite flakes in the microstructure was characterized as inspired by AFS and ASTM
graphite flake type and size rating charts.
Fig.1 Dimensions of standard test pieces used: (A) tensile test piece, (B) Hardness and
metallographic samples, (C) Impact test piece
Tables 4, 5, 6 and 7 present the raw data obtained from property tests carried out on the
prepared G.I. These tables were used to generate the figures (i.e. Figures 2 to 5). Table 8
presents the variation of length of graphite flake with HSC. This was used to generate Figure
Vol.10, No.4 Influence of Mould Heat Storage Capacity 391
6. In general, the evaluated material properties increased with the HSC. Tensile strength
ranged from 2.14 x 108 N.m-2 to 2.73 x 108 N.m-2, elongation from 13 % to 18 % and
Hardness values ranged from 168 BHN to 194 BHN. The toughness values are between 17.7
J and 20.6 J. However, the microstructures of the prepared G.I. (Figures 7 A, A1, A2, A3,
A4, and A5) reveal that length of the graphite flakes decreased with HSC. It decreased from
0.1 m at 1.52 kJ.m-2.K-1.s-1/2 HSC, to about 0.003125 m at 2.16 kJ.m-2.K-1.s-1/2 HSC .
Table 4: Variation of tensile strength with HSC
Heat storage capacity of
the mould, HSC, (kJ.m-
2.K-1.s-1/2 )
Test 1 Test 2 Test 3 Average
A 1.52 2.18 x 108 2.10 x 108 2.14 x 108 2.14 x 108
A1 1.65 2.18 x 108 2.16 x 108 2.20 x 108 2.18 x 108
A2 1.77 2.24 x 108 2.24 x 108 2.24 x 108 2.24 x 108
A3 1.90 2.33 x 108 2.29 x 108 2.31 x 108 2.31 x 108
A4 2.03 2.44 x 108 2.44 x 108 2.44 x 108 2.44 x 108
A5 2.16 2.73 x 108 2.75 x 108 2.71 x 108 2.73 x 108
Table 5: Variation of hardness number with HSC
Heat storage capacity of
the mould, HSC, (kJ.m-
2.K-1.s-1/2 )
Test 1 Test 2 Test 3 Average
A 1.52 170 167 168 168
A1 1.65 173 172 171 172
A2 1.77 176 174 175 175
A3 1.90 182 183 181 182
A4 2.03 185 186 187 186
A5 2.16 193 194 195 194
Table 6: Variation of elongation with HSC
Heat storage capacity of
the mould, HSC(kJ.m-
2.K-1.s-1/2 )
Test 1 Test 2 Test 3 Average
A 1.52 13 13 13 13
A1 1.65 12.5 13 13.5 13
A2 1.77 13.5 14 13 13.5
A3 1.90 13 15 14 14
A4 2.03 15 14 16 15
A5 2.16 17 18 19 18
392 A.V. Adedayo and B. Aremo Vol.10, No.4
Table 7: Variation of toughness with HSC
Heat storage capacity of
the mould, HSC(kJ.m-
2.K-1.s-1/2 )
Test 1 Test 2 Test 3 Average
A 1.52 18 18 17 17.7
A1 1.65 18 17 19 18
A2 1.77 17 18 20 18.3
A3 1.90 19 18 19 18.6
A4 2.03 21 20 20 20.3
A5 2.16 20 21 21 20.6
Table 8: Variation of length of graphite flake with HSC
Mould material Heat storage capacity of the mould, HSC,
(kJ.m-2.K-1.s-1/2 )
Length of graphite
flakes (m)
A 1.52 0.05 to 0.1
A1 1.65 0.025 to 0.05
A2 1.77 0.025 to 0.05
A3 1.90 0.0125 to 0.025
A4 2.03 0.00625 to 0.0125
A5 2.16 0.003125 to 0.00625
1.4 1.61.822.2
HSC (kJ.m-2.K-1.s-1/2 )
Tensile strength (x 10^8 N.m-
Test 1Test 2Test 3Average
Figure 2: Variation of tensile strength with HSC
Vol.10, No.4 Influence of Mould Heat Storage Capacity 393
1.4 1.61.822.2
HSC (kJ.m-2.K-1.s-1/2 )
Hardness (BHN
Test 1Test 2Test 3Average
Figure 3: Variation of hardness number with HSC
1.4 1.61.822.2
HSC (kJ.m-2.K-1.s-1/2 )
Elongation (%
Test 1Test 2Test 3Average
Figure 4: Variation of elongation with HSC
1.4 1.61.822.2
HSC , (kJ.m-2.K-1.s-1/2 )
Toughness (J
Test 1Test 2Test 3Average
Figure 5: Variation of toughness with HSC
394 A.V. Adedayo and B. Aremo Vol.10, No.4
1.52 1.651.771.92.03 2.16
HSC (kJ.m-2.K-1.s-1/2 )
Length of graphite flakes (
length of graphite flakes
Figure 6: Variation of graphite flakes length with HSC
Figure 7: Microstructures (x100) of G.I. cast by mould material A, A1, A2, A3, A4,
and A5 respectively
Generally, during solidification of G.I., primary austenite (γ) will separate out first until the
eutectic temperature is reached. At eutectic temperature, eutectic consisting of austenite and
graphite would form [1, 11]. This is due to the presence of sufficient silicon which increases
the instability of cementite. The eutectic develops from nuclei and is in the form of
approximately spherical particles known as eutectic cells. Graphite appears to be in the form
of separate flakes, but in fact the eutectic cells are three dimensional and roughly spherical in
Vol.10, No.4 Influence of Mould Heat Storage Capacity 395
shape [11]. Rapid cooling, which produces a greater degree of under cooling initiates the
formation of a greater number of eutectic cells and also more frequent branching in the
eutectic graphite “leaves”, giving much finer graphite flakes. The smaller the eutectic cells,
the finer the graphite flakes. This explains the observed trend in gradual change from coarse
to fine graphite flakes in the microstructure. Also, if subsequent cooling proceed slowly
enough, at the eutectoid transformation, instead of forming cementite (Fe3C), the carbon
diffuses to the nearest graphite flake and precipitates there as additional graphite [12]. The
rejection of carbon in the austenite phase is a process controlled by diffusion, which in turn,
is temperature dependent.
During graphitization, free carbon is precipitated in the iron or chemically combined carbon
(Fe3C) is changed to free carbon (or graphite), thus leading to a reduction in quantity of
cementite present in the G.I. microstructure. Actually, fine dispersion of iron carbide (Fe3C)
can be responsible for straining ferrite matrix in G.I. which can lead to increased hardness
and strength. Strengthening of bainitic structures has been reported to be due to straining of
ferrite matrix by iron carbide [13]. Generally, a fast cooling rate promotes formation of
cementite. This is due to suppression of the graphitization process. A slower cooling rate
allows sufficient time for graphitization. This, most often lead to a mixed microstructure
consisting of cementite and graphite [5]. A higher HSC value suppresses this carbon rejection
process through rapid cooling. The iron content of the mould induced this change in HSC of
the mould.
Further more, the three dimensional graphite flakes that formed during eutectic reaction,
dispersed in a matrix of ferrite, pearlite or other iron-based structures have no appreciable
strength, they act essentially as voids in the structure. The pointed edges of the flakes act as
preexisting notches or cracks initiation site[3, 14], thus giving the material a reduced strength.
Fracture mechanics have identified a relationship between crack size and material properties
[15]. In general, higher values of crack length lead to a reduced material strength.
The research shows that the HSC of the mould has significant effects on the microstructure
and properties of G.I. This suggests that HSC of a mould could be varied as required to effect
changes in the microstructure, tensile strength, elongation, hardness and toughness of G.I.
Also, the effects of graphite flake sizes on some G.I. mechanical properties give an inverse
characteristic trend.
Nigerian Machine Tools Limited, Osogbo, Osun State, Nigeria is acknowledged for
assistance in mould preparation, melting and casting of the grey cast iron samples.
396 A.V. Adedayo and B. Aremo Vol.10, No.4
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