Journal of Minerals and Materials Characterization and Engineering, 2013, 1, 326-330
Published Online November 2013 (http://www.scirp.org/journal/jmmce)
http://dx.doi.org/10.4236/jmmce.2013.16049
Open Access JMMCE
Influence of Semi-Solid Isothermal Heat Treatment on
Microstructure of Gray Cast Iron
Naglaa Fathy
Department of Physics College of Science, University of Hail, Hail, Saudi Arabia
Received September 22, 2013; revised October 30, 2013; accepted November 13, 2013
Copyright © 2013 Naglaa Fathy et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
Materials serve as an enabling technology contributing to solutions in problems of concern to society. In this research,
the effect of semi-solid isothermal heat treatment on graphite morphologies and matrix structure for gray cast iron were
studied. Microstructure observations and measurements of flaky graphite morphology are reported as a function of iso-
thermal heating time range of 5 to 25 minutes after isothermal heating temperature at 1163˚C. The effect heating time,
on the semisolid microstructures during partial re-melting was investigated. Flack graphite morphology was changed
significantly by isothermal heating of gray cast iron at 1163˚C for heating time above 15 min resulting fine graphite
morphology in matrix structure. Hardness increases with increasing heating time due to the amount of cementite and
fine pearlite matrix for air cooled gray cast iron. The optimum heating treatment condition was achieved at the tem-
perature of 1163˚C for the range of 15 to 20 min.
Keywords: Semi-Solid Heat Treatment; Isothermal; Gray Cast Iron; Microstructure
1. Introduction
At present, the methods of obtaining semi-solid materials
with non-dendritic microstructure include mechanical
stirring (MS), electromagnetic stirring (ES), strain-in-
duced melt activation (SIMA), spray deposition (SD),
liquidus cast and semi-solid isothermal heat treatment
[1,2]. Among these methods, the semi-solid isothermal
heat treatment is a new way being found in the 1990s,
which omits the special procedure to fabricate the semi-
solid materials but fulfils the semisolid non-dendritic
microstructure during heating prior to thixoforming.
Although there are currently a plenty of new and ad-
vanced materials, cast irons are still the most used cast-
ing alloy for its considerable reduction in their cost of
production. Their popularity stems from an ability to cast
complex shapes at relatively low cost and the wide range
of properties that can be achieved by careful control over
composition and cooling rate. All gray cast irons contain
flake graphite dispersed in iron matrix including silicon.
The properties of the gray cast iron depend on the size,
amount and distribution of the graphite flakes and the
matrix structure [3-6].
This research is aimed to design a novel process for
heat treated gray cast iron in semi-solid state that will
mainly focus on the industrial application of the devel-
oped concept. During the services of gray cast iron spear
parts, it faces a variety of stress, corrosion, temperature
and wear. Improvements of mechanical properties for
wide range of application are the main goals for this re-
search to obtain high performance gray cast iron through
control of their microstructure, especially graphite mor-
phology by semi-solid isothermal heat treatment. This
research is considered to be a basic research that contrib-
utes to the development of new technologies for the proc-
essing of high performance gray cast iron. In addition,
the heat treatment processing industry should have the
means and tools to tailor and optimize alloys for specific
performance.
2. Experimental Work
The alloy was melted in a 100 kg capacity medium fre-
quency induction furnace with a silica lining. The gray
cast iron raw test material was made of pig iron, returned
material and steel scrap at the ratio of 40:30:30. The mi-
nor alloying materials were pure copper, FeSi 75% Si.
Inoculants used for treated the melt was FeSi 75% Si with
size of 1 - 3 mm. The heat of gray cast iron at tempera-
ture of 1400˚C with wt% of 3.53% C, 2.14% Si, 0.67%

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Mn, 0.1% Cu, 0.03% Cr and 0.04% Ni was cast into 50
mm Y blocks made from green sand moulds. The differ-
ential scanning calorimetric analysis (DSC) for the stud-
ied gray cast iron sample of 70.900 mg was conducted
using NETZSCH STA 409 C/CD showing liquidus tem-
perature of 1242˚C and solidus temperature of 1156˚C.
Specimens of approximate dimensions 20 × 20 × 15 mm
were cut from both side of the Y blocks for microstruc-
ture examination and graphite morphology measurements.
All of the test specimens were sampled from the same
position in the Y blocks and the top sections of the
blocks were rejected in order to avoid variations in gra-
phite morphology and porosity. All specimens were heat-
ed to 1163˚C hold for 5, 10, 15, 20 and 25 min, respec-
tively in an electrically heated resistance furnace with
heating rate of 10˚C·min −1. After the semi-solid heat
treatment, the samples were taken out immediately for
both water quenching and still air cooling. Samples iso-
thermally heat treated and water quenched is only con-
sidered for study the change for graphite morphology.
Specimens in either as cast or heat treated condition
were grinded, polished, etched with 4% nital and exam-
ined metallographically using an optical microscope and
photomicrographs were taken. Hardness measurements
were made using a Rockwell standard hardness tester
with a 150 kgf load. Each measurement represents the
average of six indentations. Percent area of graphite,
pearlite, cementite and mean graphite length were auto-
matically calculated by Scentis image analyzer software.
3. Results and Discussion
Semi-solid processing of gray cast iron using cooling
plate method has been reported in the literature [6,7]
where, an improved structure of fine globular primary
particles with a high degree of sphericity and phases
clearly distinct from adjacent one are obtained. However,
the previous literature [6,7] did not include any discus-
sion on the effect of semi-solid isothermal heat treatment
on the distribution and morphology of the graphite flakes
on matrix structure. In the present study show that per-
cent of graphite and cementite in gray cast iron structure
slightly changed due to isothermal heating at 1163˚C for
heating time range from 5 to 15 min and significantly
changed for heating time above 15 min as shown in Fig-
ures 1 and 2.
Comparing the as cast and isothermally heating micro-
structure transformations are shown and plotted in Fig-
ures 2-4, the following general features can be noticed.
The as cast gray iron shows about 15% graphite flacks
dispersed in about 6% ferrite and 79% pearlite matrix
structure. Heat treated air cooled gray iron shows graph-
ite flacks, cementite and fine pearlite matrix structure,
meanwhile, heat treated water quenched gray iron shows
graphite flacks in fine martenisite matrix structure.
At the early stages of heating time (up to 15 min), the
gray irons usually show slightly decrease in amounts of
graphite and slightly increase in amounts of cemintite.
By increasing heating time, above 15 min, the gray irons
usually show significant decrease in amounts of graphite
and significant increase in amounts of cementite. Pervi-
ous study [8] dealing with the microstructure of reheated
semisolid cast ductile iron for different holding time,
indicated that, at the holding time of 5 min at 1165˚C, the
liquid is formed only at the corner of the grain bounda-
ries because of their low melting points. At the reheating
time of 10 min, the most grain boundaries become liquid
but the primary solid nodules remain unchanged. At the
holding time of 15 min, the shape of the primary solids
gradually changes to spherical and the rounded islands
are created. With increasing holdingtime to 20 min and
more, liquid fraction and solid globularity change
slightly.
Pervious study [8] concentrated only on grain size and
primary grain morphology changed by heating time and
as not been mentioned for such effects on graphite mor-
phology. The present study and pervious one [8] are in a
good agreement that increasing heating time increases
significantly the fraction of liquid up to certain heating
time value and above this value the fraction of liquid
slightly increases. This concept, explain the behavior
shown in Figures 2 and 3, the change of graphite and
cementite values are strongly related to carbon diffusion
at high temperature (1163˚C).
Up to 15 min both heating time and the amount found
of liquid fraction lead to a slight increase in the carbon
diffusion from high carbon concentrations (flack graphite)
zones to the matrix structure, meanwhile, above heat
time of 15 min the increasing of carbon diffusion de-
creases the percent of carbon in graphite flack during
holding time and increases the percent of cementite due
to the action of air cooling as well. Figure 3 shows the
relation between hardness values and heating time for air
cooled gray cast iron. It is clear that hardness increases
with increasing heating time due to increasing the
amount of cementite and fine pearlite matrix (see Figure
2).
Figure 4 shows the microstructure of as cast and heat
treated cast iron as a function of heating time at higher
magnifications. It is clear that graphite morphology in-
fluenced generally by the heating time. Carbon diffusion
is the main reason that led to graphite length decreasing
by heating time. In as cast structure, the graphite flack
always does not show a homogeneous thickness, whereas
there are thin sections through the graphite flacks, as
shown in Figures 4(a) and (b). The higher diffusion rate
of carbon in these thin sections due to higher surface area
around, especially with larger iron/graphite interface ar-
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N. FATHY
Open Access JMMCE
328
Figure 1. Microstructure of gray cast iron (a) as cast, and isothermally heat treated at 1163 for (b) 5 min, (c) 10 min, (d) 15
min, (e) 20 min, (f) 25 min, followed by water quenching.
eas that are relatively lower in carbon content, will led to
disconnect a part of graphite flack. Fine graphite flacks
will be found due to this separation action causing
change in mechanical properties of gray cast iron. Previ-
ous researches [5,9] also qualitatively describe the ef-
fects of the graphite morphology and the matrix charac-
teristics on the mechanical properties, where both the
small graphite flakes and the maximum primary austenite
content is known to increase the strength of cast iron.
The crack initiation and propagation in the cast irons are
influenced by the degree of absorbed energy by the ma-
trix for plastic deformation. Increase of the energy ab-

N. FATHY 329
Figure 2. Percent area of graphite and cementite phases as a
function of isothermal heating time.
0510 15 20 25 30
10
20
30
40
50
60
70
80
90
100
110
120
As Cast hardness
As Cast graphite length
Mean graphite length,  m
Hardness, HRG
Heatin
g
Time, min
Mean graphite length,

m
45
50
55
60
65
70
75
80
85
90
95
100
Hardness, HRG
Figure 3. Mean graphite length and hardness as a function
of isothermal heating time for air cooled gray cast iron.
sorption of the plasticized matrix of fracture is yielded in
the case of gray cast iron with finer graphite and with
matrix bridges between graphite segments wider than
those in the ordinary gray cast iron.
For optimization the isothermal heat treatment of gray
cast iron and taking the amount of cementite to be less
than 10% in matrix structure (to keep the mechanical and
physical properties at reasonable range), the range of 15
to 20 min. heating time at 1163˚C should be attend.
4. Conclusions
Investigations carried out to study the influence of semi-
solid isothermal heat treatment on microstructure of gray
cast iron led to the following conclusions:
1) Heat treated air cooled gray cast iron shows graphite
flacks, cementite and fine pearlite matrix structure,
meanwhile, heat treated water quenched gray iron shows



50 m 50 m
50 m 25 m
50

m 25

m
(a) (b)
(c) (d)
(e) (f)
Figure 4. Effect of isothermal heat treatment at 1163 on
graphite morphology of gray cast iron (a), (b) as cast at
zones 1 and 2 respectively, (c) isothermally heat treated for
10 min, (d) higher magnification of selected zone of (c), (e)
isothermally heat treated for 25 min, (f) higher magnifica-
tion of selected zone of (e).
graphite flacks in fine martenisite matrix structure.
2) At the early stages of heating time (up to 15 min),
the gray irons usually show slightly decrease in amounts
of graphite and slightly increase in amounts of cemintite.
By increasing heating time, above 15 min, the gray irons
usually show significant decrease in amounts of graphite
and significant increase in amounts of cementite.
3) Flack graphite morphology was changed significant-
ly by isothermal heating of gray cast iron at 1163˚C for
heating time above 15 min result in fine graphite mor-
phology in matrix structure.
4) Hardness increases with increasing heating time due
to the amount of cementite and fine pearlite matrix for air
cooled gray cast iron.
5) The optimum heating treatment condition was achie-
ved at the temperature of 1163˚C for the range of 15 to 20
mins.
5. Acknowledgements
The author gratefully acknowledges the support of his
research program by Staff of Foundry Technology Lab of
Central Metallurgical R&D Institute, Egypt.
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N. FATHY
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