Materials Transactions, Vol. 50, No. 5 (2009) pp.

1128 to 1134
#2009 The Japan Institute of Metals

Mechanical Properties of Spheroidal Graphite Cast Iron

Made by Reduced Pressure Frozen Mold Casting Process
Kazumichi Shimizu1 , Yaer Xinba1 , Masahito Tanaka2 and Hideki Shudai3
1
Department of Material Science and Engineering, Muroran Institute of Technology, Muroran 050-8585, Japan
2
Sankyo.Co.Ltd, Osaka 555-0001, Japan
3
Mayekawa MFG. Co., Ltd, Tokyo 135-8482, Japan

The reduced pressure frozen mold casting process has been known as a recycling-based casting method with several advantages, such as
improvement of the work environment, reduction of industrial waste and significant improvement of product yield. In this method, only water
and silica sand were used to make mold, which was rapidly frozen at
40 C, then molten metal was poured into it. In the present investigation,
samples were made by the reduced pressure frozen mold casting process and previous processes, and comparisons of their mechanical
properties, especially the fatigue strength, were reported.
As a result, it was clarified that cast iron made by the reduced pressure frozen mold casting process has a sufficient strength; therefore the
reduced pressure frozen mold casting process was expected to be applicable to other castings that have made by previous casting processes.
[doi:10.2320/matertrans.MRA2008357]

(Received October 8, 2008; Accepted February 23, 2009; Published April 8, 2009)
Keywords: reduced pressure, frozen mold, casting, mechanical property, fatigue strength

1. Introduction nology and application of LNG (Liquefied Natural Gas) low

temperature industrial technology, many research institutions
Recently, establishing a recycling-based society with a and enterprises have been studied for the practical use of this
small environmental load has produced a strong demand. In method.3–8) This former type frozen mold was called the F-set
the casting industry, where cast recycling is possible, acting mold, which was a mold making method that froze the
on improving the working environment and environmental moisture in green sand using dry ice or liquid nitrogen to
problems are being regarded as more and more important. strengthen the mold. Compared to this, the reduced pressure
One of the previous casting processes was producing a cast frozen mold (RPFM) casting process aimed at the establish-
part by forming a mold from a sand mixture and pouring ment of an energy saving/non-environmental pollution
molten liquid metal into the cavity of the mold. However, the casting technology, and was developed as part of a new
working environments of the casting process was very poor regional consortium R&D enterprise supported by METI
due to the noise, dust, vibration, stench, etc., and even worse, (Ministry of Economy, Trade and Industry).
a large quantity of industrial waste, such as large amounts The RPFM casting system basically consists of a molding
of sand, was produced. In addition, in the recycling and workshop, freezing equipment, a teeming workshop, and
regenerating process of these sands, the sand and hardener sand disposal equipment. Since the sand can be directly
were finely ground and floated into air, which also polluted reused, the sand recycling equipment and the large-scale
the surrounding environment of the factory, the cost of the environmental purifier equipment are not needed. This
environmental protection, such as a dust collector, increased. RPFM casting process has the following superior features
The domestic production cost of these casting processes rose when compared to the conventional processes:
because of the expense of the hardener and disposal costs of (1) Only water is used as a binder in the sand mixing/
these industrial wastes. Accordingly, in the present day, molding process to obtain a sufficient mold compres-
Japan’s casting industry could not compete with the import- sive strength. No organic binder/curing agents, which
ing of cheap overseas castings and tried to overcome the cause gas generation and the large environmental load
difficulties of new technology studies. Also again the typical during teeming, are needed.
[dirty], [hard] and [dangerous] workshops were also difficult (2) During the shake-out process, the binder (the frozen
to ensure the health of young laborers. water) becomes fluid due to the high temperature metal
As one of the solutions, the frozen mold casting process when the casting is finished. As a result, the bonding
was developed. It was a technology that involved freezing the force of the sand will disappear and the sand mold can
moisture content in the sand that replaced the sand hardening be naturally collapsed. Accordingly no large sand
process in previous casting processes. disposal equipment is needed.
At the beginning of the 1970s, W.H.Booth Co. in England (3) The RPFM casting process can reduce the significant
designed a molding method that used ‘‘Green sand’’, a amount of generated dust, noise, and vibration when
mixture of silica sand, clay, moisture and some other compared to the conventional casting processes.
additives that was frozen by liquid nitrogen to serve as the (4) The metal fluidity is very fine. Even if the amounts and
cast mold, and this method was practically used for frozen sizes of the risers are significantly reduced, casting
mold casting.1,2) In our country, since the 1970s, aimed at defects are controlled to a minimum and the production
energy savings, non-environmental pollution casting tech- yield can be much improved.
 Mechanical Properties of Spheroidal Graphite Cast Iron Made by Reduced Pressure Frozen Mold Casting Process 1129

(a) wooden mold with vent holl (b) flask (c) coating

(d) frozen mold (e) teeming (f) sand removal

Fig. 1 Reduced pressure frozen mold casting process.

(5) It has adaptability to make a core of the casting. Table 1 Ingredient of the silica sand in mass% used in this study.
(6) The RPFM can rapidly and effectively freeze the mold SiO2 Al2 O3 Fe2 O3 MgO CaO
when compared to the mold just placed in a freezing
594:0 52:5 51:0 50:2 50:2
room.
However, there was little information about the mechan-
ical properties of the casting obtained by the RPFM casting mold cavity coated with a heat resistant coating (Fig. 1(c)),
process, especially when compared to the conventional the drag and cope were mated and the pouring cup set to
casting methods. In addition, there has been a concern about complete the frozen mold (Fig. 1(d)). The mold cavity was
the possibility of forming an unexpected chilled structure filled with the molten metal (Fig. 1(e)). The frozen mold was
caused by the rapid solidification by the frozen mold used in heated by the high temperature metal to naturally collapse.
the RPFM casting process. In the view of these points, the After removal of the parts (Fig. 1(f)), the sand would be
mechanical properties of the casting produced by this new returned in to the sand treatment system. These working
molding method using reduced freezing technology was contents are almost the same as the conventional sand
investigated and a comparison with those from the conven- molding casting process except that the mold cavity is
tional methods were reported in this paper. manufactured by the freezing equipment.
It is considered that the strength of the frozen mold differs
2. Casting Process Using Reduced Pressure Frozen with the size of the sand, moisture content, and even the mold
Mold Casting Method captivity when freezing. In this system, the compressive
strength of the frozen mold has a tendency to increase with
In the present study, silica sand with an average diameter the moisture content and reduction of the freezing temper-
of 408 mm was used. Table 1 shows its composition. ature. Therefore the strength of the mold when the moisture
The mold making process, shown as Fig. 1, began with content is 4:05:0 mass%, and the freezing temperature is
filling the sand mixed with a suitable moisture content into a
5
10 C is considered to be the most acceptable.
flask (Fig. 1(b)) that was placed on a wooden mold with vent However, it takes a long time to uniformly freeze the mold
holes (Fig. 1(a)). The mold filled with the sand was rapidly from its surface to center at
10 C or higher, and it is
frozen using a reduced pressure aspirator with a
40 C inefficient from the view point of manufacturing. Using the
freezing equipment. Figure 2 is a schematic diagram of the reduced pressure freezing technology for optimizing the
reduced pressure aspirator. The mold freezing technology mold process, the entire mold can be rapidly and uniformly
makes good use of the certain permeability extent of the sand frozen at
40 C to obtain the desired strength. The method
and lets low temperature air in the freezing room pass also improves the availability of the equipment.
through the sand by force, this enables to freeze the moisture
in the sand rapidly and uniformly. According to a previous 3. Experimental Procedure
study, the RPFM method has 10 times freezing effect when
compared to a mold just placed in freezing room.9) The 3.1 Specimens
pattern was then removed from the flask. After the surface of The specimens investigated in this study were spheroidal
1130 K. Shimizu, Y. Xinba, M. Tanaka and H. Shudai

Room temp. -40°C Air cooler

Cryostat Refrigerator unit

Cold wind

Reduced pressure
aspirator
Frozen mold

Blower
Freezing chamber Refrigerator

Fig. 2 Schematic diagram of the reduced pressure aspirator.

Table 2 Chemical compositions in mass% of FCD made by different

molds.

C Si Mn S P
1 3.7 1.2 0.26 0.014 0.01
RPFM
2 3.6 1.4 0.27 0.015 0.01
Green sand 1 3.6 1.3 0.24 0.017 0.01
Charpy test piece
1 2
mold 2 3.5 1.4 0.24 0.016 0.01
1 2 Tensile test piece
CO2 process 1 3.8 1.2 0.28 0.016 0.01
mold 2 3.8 1.1 0.25 0.017 0.01
Fig. 3 Shape and dimensions in millimeters of Y-shaped block mold and
cutout positions for tensile and Charpy impact test pieces.

a 30  30  15 mm plate. For the Brunel hardness test, the

graphite cast irons (FCD) made by various castings using diameter of the indenter was 10 mm, the load was 29400N
‘‘RPFM’’ which used only silica sand and water, a ‘‘Green with the loading time of 30 seconds, and the number of
sand mold’’ which used green sand that was a mixture of measurement points was 5. For the Rockwell hardness test,
silica sand, clay, moisture, and other additives, and the ‘‘CO2 the B scale was used, the loading time was 30 seconds, and
process mold’’ which employed a mixture of sand and a the number of measurement points was 7.
liquid silicate binder.
These specimens are manufactured by using the Y-shaped 3.3 Impact test
block pattern. The Y-shaped block pattern’s shape and size An instrumented Charpy impact tester was used. The
are shown in Fig. 3. Molten metal is poured under the same pendulum hammer has a load of 243N and the swing arm
conditions (pouring temperature is 15301540 C, pouring length is 0.6385 m. The standard test piece for the Charpy
time is approximately 20 s) for all the molds. impact test was a V-notched specimen with a size of
The specimens cut into two parts from the Y-shaped block, 10 mm  10 mm  55 mm according to JIS Z 2242, shown as
and labeled RPFMs 1 and 2, Green sand molds 1 and 2, and in Fig. 6.
CO2 process molds 1 and 2. Their chemical compositions are
shown in Table 2. It can be clearly seen that their chemical 3.4 Fatigue test
compositions were approximately the same. Each specimen’s The fatigue tests were performed using a plane bending
microstructure, spheroidal graphite ratio and pearlite area fatigue tester (made by TKS Group). For this testing
ratio are also shown in Fig. 4. Spheroidal graphite ratio and machine, the allowable average bending moment was
pearlite area ratio were calculated using the microstructure of 015 Nm, the repeat rate limit was 3001500 c.p.m, and
the specimens according to JIS G 5502. the repeat time can reach 999999  100 times. The cutting
position of the plate test piece from the Y-block is shown in
3.2 Static tests Fig. 7, and the shape and size of the test piece is shown in
The specimens for tensile test were prepared by machin- Fig. 8. The test piece’s minimum width is 20 mm, 3 mm
ing. The tensile test is performed using an Instron universal thick, with a minimum cross-section area of 60 mm2 . A
testing machine (AG-50kNE) at the constant cross head regular sinusoidal stress with the stress ratio of R ¼
1 at the
speed of 1 mm per minute. The test piece had 6 mm diameter loading frequency of 20 Hz was applied. The maximum
and 42 mm parallel portion according to the JIS 14 standard. number of cycles was N ¼ 107 cycles.
The shape is shown in Fig. 5. All tests including tensile test, hardness test, Charpy
For the hardness test, the Brunel hardness test and impact test, and fatigue test were conducted at atmospheric
Rockwell hardness test were preformed. The test piece was pressure and room temperature.
 Mechanical Properties of Spheroidal Graphite Cast Iron Made by Reduced Pressure Frozen Mold Casting Process 1131

Ferrite Pearlite Spheroidal

graphite

RPFM Green sand mold CO2 process mold

Microstructure

Spheroidal
76.5 80.0 73.4
graphite ratio (%)
Pearlite area ratio 40.5 46.7 43.4

Microstructure

Spheroidal
60.5 53.6 53.3
graphite ratio (%)
Pearlite area ratio 34.9 45.7 50.4

Fig. 4 Microstructure, spheroidal graphite ratio and pearlite area ratio of FCD specimens manufactured by the RPFM, Green sand mold,
and CO2 process mold.

55 220
M10 × 1.25 R15 R15 M10 × 1.25
1.6 Cutting line
φ6

20 30 20
140

42
30 5

90
40

Fig. 5 Shape and size in millimeters of sample used for tensile test. 25 90 90
5 5

Fig. 7 Cutting position at Y-shaped block for fatigue test piece (mm).

4. Experimental Results and Discussion

10
8

4.1 Mechanical properties of specimens

45°
Table 3 shows the results of the tensile test, hardness test
and impact test of the FCD made by the various casting
10

methods. From the table, it can be seen that elongation of

some specimens manufactured in this study tended to show
0.83
low values below 10%. In general, the strength of FCD is
27.5
dependent on the base matrix and spheroidal graphite ratio,
55 that is to say, with the decreasing spheroidal graphite ratio,
the tensile strength and elongation decrease. In general, the
Fig. 6 Shape and size in millimeters of sample used for the Charpy impact elongation of the FCD with a spheroidal graphite ratio of
test. less than 80% is less than 10%; accordingly, its specific
1132 K. Shimizu, Y. Xinba, M. Tanaka and H. Shudai

Table 3 Mechanical properties and spheroidal graphite ratio of FCD manufactured by different molds.

0.2%
Charpy
Tensile proof Brinell Rockwell Spheroidal
Elongation impact
strength stress hardness hardness
B /HB graphite
 (%) value

B (MPa)
0:2 HB HRB ratio (%)
C(kJ/m2 )
(MPa)
1 599 330 10.8 168 86.4 87.4 3.57 76.5
RPFM
2 519 307 7.5 163 89.0 71.8 3.18 60.5
Green sand 1 629 345 9.3 171 91.0 75.1 3.68 80.0
mold 2 515 315 6.2 171 86.6 71.8 3.01 53.6
CO2 process 1 574 321 6.2 172 84.7 61.6 3.34 73.4
mold 2 — — — — — — — 53.3

400 RPFM
Green sand mold
350 CO 2 process mold

300
Stress (MPa)

250

200

150

100 4 5 6 7 8
10 10 10 10 10
Fig. 8 Shape and size in millimeters of fatigue test piece. Number of cycles to failure

Fig. 9 S-N curves obtained from the fatigue tests of the FCD specimens
deformation energy will be lost.10) The results of this study made by the RPFM, Green sand mold, and CO2 process mold.
well agree with the common characteristics of the FCD.
Therefore, the effects of the base matrix and spheroidal
graphite ratio of the FCD on mechanical properties are the RPFM casting process have almost the same mechanical
considered in the present investigation. properties from the view points of the tensile strength,
From Table 3, it can also be seen that the tensile strength is hardness, and Charpy impact value.
approximately 3 times its Brunel hardness for each material,
therefore, having a good correlation of
B ¼ 3  HB. In 4.2 Fatigue strength
general, there has a correlation between the hardness and The S-N curves obtained from the fatigue tests of the FCD
tensile strength for the cast iron. In the case of the FCD, the specimens made by various molds such as the RPFM, Green
relationship of
B ¼ 3  HB for the tensile strength (
B ) and sand mold, and CO2 process mold are shown in Fig. 9. As
Brunel hardness (HB) was reported.10) Since the hardness is a consequence, the fatigue limit of the FCD made by the
determined by the base matrix, not or slightly affected by the RPFM, Green sand mold, and CO2 process mold were
spheroidal graphite ratio, the Brunel hardness values as
w ¼ 195 MPa,
w ¼ 200 MPa, and
w ¼ 135 MPa, respec-
shown in Table 3 were almost the same independently of the tively. The fatigue strength of the FCD made by the RPFM
mold-making methods. was approximately the same value as that made by the Green
According to a previous study, the Charpy impact property sand mold, and showed a slightly higher value than that made
of the FCD dramatically differed with the base matrix. The by the CO2 process mold.
Charpy impact value decreased when the pearlite area ratio In order to discuss the results, the microstructures of the
increased.11) In the present investigation, the pearlite area fractured FCD specimens after the highest-cycle fatigue tests
ratios of the specimens made by the various molds including were observed using an optical microscope. The results are
RPFM were approximately the same as shown in Fig. 4. shown in Fig. 10. It can be clearly seen that there were no
Therefore, there were not clear differences in the Charpy significant differences between the FCD made by the RPFM
impact values among them. and by the Green sand mold. Since the pearlite area ratios of
From the results mentioned above, it can be understood these two FCD specimens were approximately the same as
that the FCD specimens made by various methods including shown in Fig. 4, they showed almost the same fatigue
 Mechanical Properties of Spheroidal Graphite Cast Iron Made by Reduced Pressure Frozen Mold Casting Process 1133

RPFM Green sand mold CO2 process mold

Dendrite

Fig. 10 Optical microstructures of FCD specimens made by the RPFM, Green sand mold, and CO2 process mold after fatigue tests.

RPFM Green sand mold CO2 process mold

Graphite
nodule
Graphite
nodule
Graphite
nodule

Beach Beach Beach

mark mark mark

10µm

Fig. 11 SEM observations of the fractured faces of FCD specimens made by the RPFM, Green sand mold, and CO2 process mold after
fatigue tests.

strength. On the other hand, some dendrite structures were is similar to those made by castings using the Green sand
observed in the microstructure of the FCD specimen made by mold and CO2 process mold based on the fatigue fractural
the CO2 process mold. This was caused by the fact that characteristics.
carbon was crystallized to form cementite rather than From the results mentioned above, the FCD specimens
graphite because of the rapid cooling rate during the made by the RPFM casting process have no problems from
solidification. Therefore, the FCD made by the CO2 process the view point of its mechanical properties including the
mold showed a lower fatigue strength value than that made tensile strength, hardness, Charpy impact value, and fatigue
by the others. That is to say, the fatigue strength value strength when compared to conventional casting methods.
significantly depends on the base matrix of the materials. The RPFM casting process is expected to be an alternative
Although there may be a concern about the possibility casting method.
of forming the unexpected chilled structure caused by
the rapid solidification by the frozen mold used in the 5. Conclusions
RPFM casting process, no such structure was observed in
this study. In the present investigation, the FCD specimen was made
The fractured faces of the FCD specimens made by the by the RPFM casting process, and its mechanical properties
RPFM, Green sand mold, and CO2 process mold after the including the fatigue strength, which is indispensable for
highest-cycle fatigue tests were observed using a scanning materials, were evaluated. The conclusions are listed below:
electron microscope (SEM). Figure 11 shows the results of The FCD made by the RPFM casting process has almost
the SEM observations. In these figures, similar structures the same mechanical properties as the FCD made by the
were observed; the black-colored graphite nodules were conventional method. For the FCD made by the RPFM
surrounded by a pearlite base matrix. In addition, the beach casting process, there was no unexpected chilled structure.
marks, which were characteristic of a fatigue structure, were Considering its good working condition and excellent sand
observed in the base matrix. As a consequence, it can be recyclability, the RPFM casting process is expected to be a
considered that the FCD made by the RPFM casting process promising molding method in the future.
1134 K. Shimizu, Y. Xinba, M. Tanaka and H. Shudai

Acknowledgements 4) S. Kita and H. Hino: J. Japan Inst. Metals 50 (1980) 2–8.

5) S. Minowa, H. Ota and M. Minomiya: J. JFS 52 (1980) 530–535.
6) S. Minowa, M. Minomiya and H. Ota: J. JFS 53 (1981) 5–19.
Finally, thanks for MYCOM to variety of cooperation in 7) S. Minowa, M. Minomiya, H. Ota and T. Takayanagi: J. JFS 54 (1982)
the process of the research. 309–313.
8) K. Kobayashi: JACT NEWS, No. 262 (1978) 44–19.
9) H. Shudai and N. Tokunaga: Refrigeration 80 (2005) 930.
REFERENCES 10) S. Harada, T. Kobayashi, T. Noguchi, H. Suzuki and M. Yano: Strength
Assessment of Spheroidal Graphite Cast Iron, (AGUNE Gijutsu
1) C. Moore and D. Beat: Foundry Trade J. May (1979) 1049. Center, 1999) pp. 55–63.
2) K. Kobayashi: JACT NEWS, No. 253 (1978) 14–16. 11) Japan Foundry Engineering Society Ed.: Japan Foundry Engineering’s
3) S. Kita, H. Hino and M. Tominaga: J. JFS 52 (1980) 28–33. Handbook, (Maruzen 2002) pp. 254–281.