role of bisomth and silicon on sgi

8/16/2019 Role of Bisomth and Silicon on SGI

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Effect of Silicon and Bismuth on Solidification Structure

of Thin Wall Spheroidal Graphite Cast Iron

Hiromitsu Takeda, Hiroyuki Yoneda and Kazunori Asano

Department of Mechanical Engineering, School of Science and Engineering, Kinki University, Higashiosaka 577-8502, Japan

Although the thinning of spheroidal graphite cast iron castings has been promoted to reduce the weight of the castings, the thinning tends to

cause chilling. Due to the chilling, the required mechanical properties can not be obtained. The addition of certain elements is a way to solve this

problem. In this study, the spheroidal graphite cast iron melt containing minor Bi, 3.3 to 3.7 mass%C and 2.0 to 3.2 mass%Si was poured into a

stepped plate mold to obtain the thin wall castings, and observation of their graphite and matrix microstructure, thermal analysis during the

solidification process of the melt in the mold and the qualitative analysis of elements inside the spheroidal graphite by FE-EPMA were carried

out.

It was found thatan increase in the Si/C mass ratio in the spheroidal graphite cast iron was effective for decreasing the amount of cementite

(chill) in the matrix, and the chill was further inhibited by adding 0.01 mass% Bi even for the thin wall castings of 2 mm. Amounts up to

0.01 mass%Bi promoted refinement of the graphite, increased the graphite nodule, and promoted ferritizing of the matrix. It was also found that a

high Si/C mass ratio in the spheroidal graphite cast iron promoted the effects of Bi. The temperature of the eutectic start and that of the eutectic

solidification end increased due to the 0.01 mass%Bi. The temperature of the eutectoid transformation start increased and the stability eutectoid

transformation of the thin wall castings was promoted by containing a minor amount of Bi. It was confirmed that substances including Bi and Mg

existed in the graphite containing Bi. These results lead to the conclusion that the Bi compound and the Mg compound acted as heterogeneous

nuclei of the graphite, and the nuclei promoted the crystallization of the graphite, and then the graphite nodule increased.[doi:10.2320/matertrans.M2009255]

(Received July 23, 2009; Accepted October 22, 2009; Published December 9, 2009)

Keywords: spheroidal graphite cast iron, thin wall, bismuth, chill, graphite nodule, matrix microstructure

1. Introduction

Cast iron has been produced in quantity as castings for

automobile parts and industrial machines, because it has an

excellent castability, good wear resistance and damping

capacity. In recent years, the reduction in weight and size of the machine products has been promoted to reduce the energy

consumption, use of raw materials and emitting of green-

house gas. This trend leads to the promotion of thinning of

the spheroidal graphite cast iron castings. However, the cast

iron melt in a thin wall is exposed to rapid cooling, and

cementite (chill) tends to increase in the matrix. The chilling

causes a decrease in the mechanical properties of the

castings. Generally, a ferrosilicon (Fe-Si) alloy containing a

small amount of elements, such as aluminum, calcium, and

barium, is added to the cast iron melt as a graphitizer to

prevent the chill. For the spheroidal graphite cast iron, it is

reported that the critical graphite nodule count for preventingthe chill exists for each cooling rate.1) This indicates that an

increase in the graphite nodules in the matrix is effective for

preventing the chilling. It is reported that the addition of a

small amount of bismuth (Bi) is effective for increasing the

graphite nodule count.2–5) Based on this finding, the inoculant

containing Bi6,7) or pure Bi8) is sometimes added to the

spheroidal graphite cast iron melt. However, Bi is classified

as a graphite spheroidization inhibition element because the

spheroidization is inhibited by including excessive Bi in the

cast iron. Some researchers have reported the critical content

of Bi to inhibit the graphite spheroidization. Morrogh9)

reported that the graphite spheroidization starts to interfere

when the Bi content exceeds 0.003 mass%, and the spher-

oidization is completely inhibited when the Bi content is

0.006 mass%. Donoho10) reported that the graphite spheroid-

ization is inhibited by 0.005 mass%Bi or more in the melt,

and Cole11) reported that the spheroidization is inhibited

by 0.006 mass%Bi or more. On the other hand, for the cast

iron containing titanium,12,13) it has been reported that the

graphite spheroidization is interfered by containing about

0.001 mass%Bi. These reports indicate that a small amount of

Bi inhibits the spheroidization and the critical Bi contentchanges due to the content of the main element and the

existence of other elements. Silicon (Si), the main element of

cast iron, is a graphitizing element. Horie et al.1) reported that

the graphite nodule count in the spheroidal graphite cast iron

increases by increasing the Si content or carbon equivalent

(CE). However, there are few reports on the effect of the C

and Si contents for a constant CE of the graphite and matrix

structure of the thin wall spheroidal graphite cast iron

containing Bi. Moreover, there are no reports which examine

the effect of Bi on the solidification process of the thin wall

spheroidal graphite cast iron by a thermal analysis.

In this study, the spheroidal graphite cast iron melt withvarious Bi, C and Si contents were poured into a stepped plate

mold to obtain the thin wall specimens, and the relationship

between the thickness of the specimens, the graphite

spheroidization rate, graphite nodule count, and area fraction

of ferrite and pearlite was examined, and then the effect of the

Si and Bi contents on the graphite and matrix structure was

examined (experiment A). Subsequently, the thermal analy-

sis of the spheroidal graphite cast iron melt was carried out,

and the effect of Bi on the eutectic solidification and the

eutectoid transformation reaction was examined (experiment

B). Moreover, structure of the spheroidal graphite in the

specimens containing Bi was analyzed in order to examine

the Bi distribution. Based on these results, the effects of Si

and Bi on the microstructure of the thin wall spheroidal

graphite cast iron and the graphite refinement mechanism by

Bi were examined.

Materials Transactions, Vol. 51, No. 1 (2010) pp. 176 to 185#2010 The Japan Institute of Metals EXPRESS R EGULAR A RTICLE

http://dx.doi.org/10.2320/matertrans.M2009255http://dx.doi.org/10.2320/matertrans.M2009255


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2. Experimental Procedure

Raw materials with the chemical composition shown in

Table 1 were used to fabricate the cast iron specimens. They

were placed in a graphite crucible and melted in a small high

frequency induction furnace (10 kWh, 3 kHz) under an Ar gas

atmosphere, followed by graphite spheroidizing, inoculation

and Bi addition at 1773 K. The graphite spheroidizing,

inoculation and Bi addition were simultaneously carried

out by adding the spheroidizing agent (45.92 mass%Si, 4.93

mass%Mg, 2.37 mass%Ca, 0.66 mass%Al, 1.84 mass%RE),

inoculant (75.77 mass%Si, 1.28 mass%Ca, 2.16 mass%Al),

and pure Bi (99.9 mass%Bi), respectively. As a result of apreliminary experiment, it was found that the yield of Bi in

the spheroidal graphite cast iron was 8% in this experiment

condition. The addition of the inoculant was 0.3 mass%.

Table 2 shows the chemical composition, CE and Si/C

mass ratio of the specimens. Specimens No. 1 to No. 4

were used in experiment A. In experiment A, the target

chemical composition of the specimens was as follows: 3.3,

3.4, 3.5 and 3.7 mass%C, 2.0, 2.4, 2.8, and 3.2 mass%Si. The

CE(=C+Si/3+P/3) was set to 4.4 (constant). The target

contents of Mn, P, S and Mg were 0.04, 0.02, 0.01

and 0.04 mass%, respectively. As a result of a preliminary

experiment, it was found that the graphite spheroidization

was insufficient when the Bi content exceeded 0.01 mass%.

Therefore, the Bi content was set to 0.005 and 0.01 mass%.

The ratio of the Si content to C content (Si/C mass ratio) was

used as a parameter, showing an increase in the Si content for

the same CE values. It is known that the Si/C mass ratio has

a correlation with the tensile strength and hardness.14,15)

Specimens No. 5 and No. 6 shown in Table 2 were used

for experiment B. In experiment B, the carbon content was

set to 3.4 mass% (constant), the Si content was set to 2.2 and

3.2 mass%; the CE was 4.1 (hypoeutectic composition) and

4.4 (hypereutectic composition).Melt was poured into the CO2 mold shown in Fig. 1 at

1673 K to obtain the stepped specimens plates with 2, 3, 5 and

10 mm thicknesses (50 mm width 150 mm length). In

experiment B, R thermocouples were inserted in the center

of the cavity and the thermal analysis of the melt was carried

out. To obtain a high heat sensitivity, the tip of the

thermocouple was exposed.

The microstructure in the center part of the specimen

(in the vicinity of the tip of the thermocouple) was observed.

The graphite spheroidization ratio, the graphite particle

diameter, the graphite nodule count and area fractions of the

graphite, ferrite and pearlite were measured by an image

analyzer. To obtain the mean values of these parameters, 10

optical micrographs were used for the measurement. These

values of the specimens generating chill were excluded.

Graphite particles with a diameter less than 1 mm were also

Table 1 Chemical composition of raw materials (mass%).

C Si Mn P S Cr Cu Zn

Pig iron 4.22 0.099 0.027 0.029 0.015 0.032 — —

Electro lytic ir on 0 .02


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excluded because it is difficult to distinguish the graphite.

The distribution of elements in the spheroidal graphite in

the specimen containing Bi was examined by FE-EPMA.

In the thermal analysis of experiment B, the changes in the

eutectic solidification of the melt and eutectoid transfor-

mation process by the addition of Bi were examined. The

measurement points in the thermal analysis are shown in

Fig. 2.

3. Results and Discussion

3.1 Effect of Bi and Si on microstructure (experiment A)

3.1.1 Microstructure

Figure 3 shows the microstructure of the specimens

without Bi. The microstructure in the center part and the

surface part of each thickness of specimens were almost

the same. The Si/C mass ratio is also shown in the figure.

When the Si/C mass ratio was 0.56, the specimen with a

10 mm thickness had a bull’s eye type structure without

chill (Fig. 3(d)). The chill was partially observed in the

specimen with a 5 mm thickness (Fig. 3(c)) and the matrix

of the specimen with a 2 mm thickness was completely

chill. A similar structure was observed when the Si/C massratio was 0.67 (Fig. 3(e)–(h)). When the Si/C mass ratio

was 0.79, the chill was not observed in the specimens

with the thicknesses of 3, 5 and 10 mm (Fig. 3(j)–(l)),

although the chill was partially observed in the specimen

with a 2 mm thickness (Fig. 3(i)). When the Si/C mass

ratio was 0.97, the no chill was observed even in the

specimen with a 2 mm thickness (Fig. 3(m)) and every

specimen contained fine nodular graphite particles. The

ferrite in the matrix increased as the Si/C mass ratio

TP

TES

TEM

TEU

TEE

ET

TEDE

TEDS

Fig. 2 Measurement point on cooling curve. Temperature of primary

crystallization (TP) ` Temperature of eutectic start (TES) ´ Temperatureof eutectic undercooling (TEU) ˆ Temperature of eutectic maximum

(TEM) ˜ End of eutectic solidification (TEE) ¯ Eutectic solidification

time (ET) ˘ Temperature of eutectoid start (TEDS) ˙ End of eutectoid

transformation (TEDE).

32 5 10

Thicknesst /mm

3.63 mass%C

2.45 mass%Si

Si/C =0.56

Si/C =0.97

Si/C =0.79

Si/C =0.67

3.75 mass%C

2.10 mass%Si

3.54 mass%C

2.81 mass%Si

3.31 mass%C

3.21 mass%Si

Specimen

No.4-1

No.3-1

No.1-1

No.2-1

(a) (c) (d)

(e) (f) (g) (h)

(i) (j) (k) (l)

100 µ m

(b)

(m) (n) (o) (p)

Chill

Fig. 3 Microstructure of specimens without Bi (0 mass%Bi) and various C and Si contents at CE4.4 (Nital etched).

178 H. Takeda, H. Yoneda and K. Asano


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increased. These results indicated that the chilling does notoccur and an increase in the ferrite structure in the matrix

is pronounced in the thin wall cast iron castings when the

Si/C mass ratio is high (high Si content). The specimens

containing 0.005 mass%Bi have almost the same micro-

structure as the specimens without Bi.

Figure 4 shows the microstructure of the specimens

containing 0.01 mass% Bi. When the Si/C mass ratios were

0.55 and 0.66, no chill was observed in the specimens with

the thicknesses of 3, 5 and 10 mm (Fig. 4(b)–(d), (f)–(h)).

When the Si/C mass ratio was high (0.79 and 0.97), no chill

was observed even in the specimen with a 2 mm thickness

(Fig. 4(i), (m)).These results show that an increase in the Si content is

effective for decreasing the chill in the thin wall spheroidal

graphite cast iron, and the chilling is further inhibited by a

0.01 mass%Bi content.

3.1.2 Relation among Bi content, Si/C mass ratio,

thickness and microstructure

For all the specimens, the graphite spheroidization ratio

was 80% or more. When the Si/C mass ratio was 0.5, a

0.01 mass%Bi content slightly reduced the spheroidization

ratio in the specimens.

Figure 5 shows the relation between the Bi content, Si/C

mass ratio, thickness, and the graphite particle diameter

of the specimens. It can be seen that the graphite particle

diameter decreased as the specimen thickness decreased.

This is due to the fact that the cooling rate of the melt

increased as the thickness decreased. For the same Si/C mass

ratio, the graphite particle diameter tends to decrease as theBi content increases. This tendency is pronounced when the

Si/C mass ratio is small.

Figure 6 shows the relation between the Bi content, Si/C

mass ratio, thickness, and the graphite nodule count of the

specimens. The graphite nodule count increased by thinning

of the specimen. This is due to the high cooling rate of the

melt by the thinning as well as the graphite particle diameter.

The tendency that the graphite nodule count increased along

with the Bi content was observed for every thickness. This

tendency was also reported by Horie et al.2,3) and Sato et al.4)

The graphite nodule count increased as the Si/C mass ratio

increased. Generally, the addition of Si decreases the graphiteparticle diameter and increases the nodule count.7,8) Also for

the thin wall spheroidal graphite cast iron castings used in

this study, an increase in the Si/C mass ratio increased the

graphite nodule count.

Figure 7 shows the effect of the Bi content and Si/C mass

ratio on the matrix structure. The graphite area fraction was

constant (approximately 10%) regardless of the Bi content or

Si/C mass ratio. The area fraction of ferrite increased and the

area fraction of pearlite decreased as the Si/C mass ratio

increased. For example, when the Si/C mass ratio was 0.97

and the thickness of the specimen was 10 mm, the area

fraction of ferrite increased about 20% and that of pearlite

decreased about 20% by a 0.01 mass%Bi content. When the

thickness of the specimen was 3 mm, the area fraction of

ferrite increased about 10% and the that of pearlite decreased

about 10% by a 0.01 mass%Bi content.

32 5 10

Thicknesst /mm

3.62 mass%C

2.38 mass%Si

Si/C =0.55

Si/C =0.97

Si/C =0.79

Si/C =0.66

3.76 mass%C

2.06 mass%Si

3.52 mass%C

2.78 mass%Si

3.33 mass%C

3.22 mass%Si

Specimen

No.4-3

No.3-3

No.1-3

No.2-3

(a) (c) (d)

(e) (f) (g) (h)

100 µ m

(b)

(m) (n) (o) (p)

(i) (j) (k) (l)

Fig. 4 Microstructure of specimens with 0.01mass%Bi and various C and Si contents at CE4.4 (Nital etched).

Effect of Silicon and Bismuth on Solidification Structure of Thin Wall Spheroidal Graphite Cast Iron 179


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(a) No.5-1 Bi 0mass%

100 µ m

(b) No.5-2 Bi 0.01mass%

(c) No.6-1 Bi 0mass%

(d) No.6-2 Bi 0.01mass%

No.5 (CE 4.1) No.6 (CE 4.4)

Fig. 8 Microstructure of specimens No. 5 and No. 6 (thickness (t ) = 10 mm) (Nital etched).

Table 3 The graphite spheroidization rate, the graphite particle diameter, the graphite nodule count and the area fraction of the matrix

structure of the specimens No. 5 and No. 6.

No. 5 (CE 4.1) No. 6 (CE 4.4)

Specimen No. 5-1 No. 5-2 No. 6-1 No. 6-2

0 mass%Bi 0.01 mass%Bi 0 mass%Bi 0.01 mass%Bi

Thickness mm 2 3 5 10 2 3 5 10 2 3 5 10 2 3 5 10

Graphite spheroidization rate

% — — 82.3 81.5 — — 80.2 80.1 81.4 82.8 82.4 81.8 80.3 82.3 81.1 80.4

Average graphite particle diameter

mm — — 15.2 18.4 — — 13.8 16.7 10.0 12.1 13.6 16.5 9.2 10.4 12.1 13.8

Graphite nodule count

mm2 — — 344 187 — — 426 227 1025 883 482 298 1179 1028 639 346

Graphite — — 10.6 11.6 — — 11.6 10.4 12.2 12.3 11.1 11.4 12.4 11.5 10.7 10.2

Area fraction % Pearlite — — 57.8 54.1 — — 53.9 52.9 19.5 10.3 11.2 10.6 18.9 10.9 9.4 8.9

Ferrite — — 31.6 34.3 — — 34.5 36.7 68.3 77.4 77.7 78.0 68.7 77.6 79.9 80.9

1500

1400

1300

1200

1100

1000

(CE 4.4, Si/C 0.9)

(b) No.6

Time, T /s

200 400 6000 300100 500

Bi 0mass%Bi 0.01mass%

eutectoid

TEDS

TEDE

eutecticTES

TEM

TEE

TEU

Time, T /s

600

1600

900

0 200 400

(CE 4.1, Si/C 0.6)

800

500

T e m p e r a t u r e , T e m / K

300100

Bi 0mass%Bi 0.01mass%

eutectoid

(a) No.5eutecticTPTES

TEM

TEE

TEU

TEDSTEDE

Fig. 9 Cooling curves of specimens No. 5 and No. 6 (thickness (t ) = 10mm).



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composition which generates the solidification latent heat

by crystallization of the primary crystal austenite whereas

specimen No. 6 has a hypereutectic composition in whichthe graphite crystallizes as the primary crystal. A steep

increase in the temperature from the temperature of the

eutectic undercooling (TEU) was observed by adding Bi.

This indicates that many or much amount of graphite

particles are simultaneously crystallized out, followed by

the rapid solidification of the melt. In addition, the tem-

perature of the eutectic start (TES) and temperature of

the eutectoid start (TEDS) were high for the specimen

containing Bi.

Table 4 shows the average cooling rate from TP to TES for

each thickness of specimens No. 5 and No. 6. It can be seen

that the cooling rate was increased by containing Bi for everyspecimen. This is probably due to the fact that the vapor

pressure of Bi is high16) and the yield of Bi in the melt is

thought to be bad. It is thought that the evaporation of a

quantity of Bi deprives the heat of the melt.

Moreover, the cooling rate of the specimen No. 6 was

faster than that of the specimen No. 5. This reason is thought

as follows: In the hypoeutectic composition (specimen No. 5),

the reduction in melt temperature was thought to be sup-

pressed due to the solidification latent heat generated by the

primary austenite crystallization. On the other hand, in the

hypereutectic composition (specimen No. 6), it is thought

that the reduction in melt temperature is not suppressed

because little heat is generated when the primary graphite

crystallizes out.

Figure 10 shows the effect of the Bi content and cooling

rate on the temperature of the eutectic start (TES) and end

of the eutectic solidification (TEE). For every cooling rate,the TES of specimen No. 6 is higher than that of specimen

No. 5. This is due to the fact that the CE of specimen No. 6 is

higher than that of specimen No. 5. Regardless of the cooling

rate of the specimens, the TES and TEE of the specimens

containing 0.01 mass%Bi were higher than those of the

specimens without Bi.

Figure 11 shows the effect of the Bi content and eutectic

solidification time (ET) for specimens No. 5 and No. 6.

Although the change in the ET by containing Bi was not seen

for specimen No. 5 (hypoeutectic composition), the ET was

decreased in specimen No. 6 (hypereutectic composition)

by containing Bi. As previously described, the graphite

nodule count of specimen No. 6-2 containing 0.01 mass%Biwas more than that of specimen No. 6-1 without Bi. Ohi

et al.17) have researched the influence of graphite nodule

count on the eutectic solidification of hypereutectic spher-

oidal graphite cast iron and reported that the eutectic

solidification time (ET) shortened as the graphite nodule

count increased. From this result, it is thought that ET is

shortened because an increase in the graphite nodule count

reduced the time required for graphite growth. Furthermore,

it is thought that an increase in the cooling rate of hyper-

eutectic spheroidal graphite cast iron melt by Bi addition

shorten ET.

3.2.3 Effect of Bi on eutectoid transformationFigure 12 shows the effect of the Bi content, thickness of

the specimen on the temperature of the eutectoid start

(TEDS) and end of the eutectoid transformation (TEDE)

for the specimens without chill. For specimen No. 5, the

TEDS increased, while the TEDE did not change by adding

Bi (Fig. 12(a)). For specimen No. 6 with the hypereutectic

composition containing 0.01 mass%Bi, the TEDS increased

and the TEDE decreased (Fig. 12(b)). As previously

described, the graphite nodule count increased and the

ferritizing was promoted by the Bi. The increase in the

graphite nodule count leads to a decrease in the distance

among the graphite nodules in the matrix. This promotes

the diffusion of C in the matrix into the previously

crystallized graphite during the cooling after the eutectic

solidification. This probably leads to the phenomenon that

the eutectoid transformation reaction starts at a compara-

Table 4 The average cooling rate (K /s) to temperature of the eutectic start

vs. specimen thickness.

Specimen Bi content Thickness

mass% 2 mm 3 mm 5 mm 10 mm

No. 50 35.2 10.7 5.0 2.0

0.01 37.8 15.9 5.1 2.1

No. 60 37.5 17.9 9.6 4.7

0.01 40.0 18.8 11.7 5.7

T e m p e r a t

u r e ,

T e m / K

Cooling rate, K /s

0 10 20 30 401320

1360

1400

1420

1340

1380

1440

0 10 20 30

(a) No.5 (CE 4.1,Si/C 0.6) (b)

40

No.6 (CE 4.4,Si/C 0.9)

0.01

TES

0

TEEBi,mass%

0.01

TES

0

TEEBi,mass%

Cooling rate, K /s

Fig. 10 Effect of Bi content and cooling rate on temperature of eutectic start (TES) and end of eutectic solidification (TEE).



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tively high temperature and the ferritizing of the matrix is

promoted.

These results show that containing 0.01 mass%Bi in-creases the temperature of the eutectoid start and promotes

a steady eutectoid transformation of the thin wall spheroidal

graphite cast iron.

3.3 EPMA analysis of spheroidal graphite

Since the refinement of the graphite nodule and the

increase in the graphite nodule count was recognized by

adding Bi, the inside of the spheroidal graphite in the cast

iron containing Bi was analyzed by FE-EPMA in order to

examine the effect of Bi on the graphite refinement.

Figure 13 shows the SEI and the X-ray images of C and

Bi in the spheroidal graphite of a specimen containing

0.01 mass%Bi (specimen No. 5-2). It can be seen that Bi

exists in the vicinity of the center of the graphite.

Figure 14 shows the magnified observations of where

Bi was detected. It indicates that the center of the graphite

consists of the Bi oxide and the Bi sulfide, because Bi, O

and S were detected from almost the same area. Moreover,

it is thought that a compound of Mg and Si existsbecause Mg and Si were also detected from the same

area. Ce and La were also distributed in almost the same

position as S, although their detection brightness is low.

Ce and La are probably from RE in the spheroidization

agent. Igarashi et al.18) reported that MgS and MgO are

included in the vicinity of the center of the spheroidal

graphite in the spheroidal graphite cast iron without Bi.

This report supports the fact that MgS and MgO also exist

in the center of the graphite specimen without Bi in this

study.

Subsequently, the formation of compounds in the graphite

at the spheroidization temperature (1773 K) in the present

study was examined from the viewpoint of the standard

free energy of formation. The standard free energies of

formations of the oxide, sulfide and silicide of Bi and Mg

are as follows:19)

0

20

40

60

120

100

80

E u t e c t i c s o l i d i f i c a

t i o n t i m e , E T / s

Thickness, t /mm0 4 6 128 10 0 2 4 6 8 10

(a) No.5 (CE 4.1, Si/C 0.6) (b)No.6 (CE 4.4, Si/C 0.9)

12

0 mass%Bi

0.01 mass%Bi

0 mass%Bi

0.01 mass%Bi

Thickness, t /mm

2

Fig. 11 Effect of Bi content and thickness of specimen on eutectic solidification time.

T e m p e r a t u r e , T e m / K

800

900

1000

1050

850

950

1100

1150

1200

0 4 6 128 0 2 4 6 8 10

Thickness, t /mm

(a) No.5 (CE 4.1, Si/C 0.6) (b)No.6 (CE 4.4, Si/C 0.9)

12

Bi,mass%

0.01Bi

0 Bi

TEDS TEDEBi,mass%

0.01Bi

0 Bi

TEDS TEDE

Thickness, t /mm

102

Fig. 12 Effect of Bi content and thickness of specimen on the temperature of eutectoid start (TEDS) and end of eutectoid transformation

(TEDE).



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2/3Bi2O3 ¼ 109:1 kJ/molO2 ð1Þ

2/3Bi2S3 ¼ 42:9 kJ/molS2 ð2Þ

MgO ¼ 733:7 kJ/molO2 ð3Þ

MgS ¼ 395:2 kJ/molS2 ð4Þ

Mg2Si ¼ 8552:5 kJ/molSi ð5Þ

These values show that these Bi compounds and Mg

compounds are easily formed, and coexist in the melt after

the spheroidization. This fact and the results from the FE-EPMA lead to the conclusion that Bi2O3 and Bi2S3 were

formed from Bi, O and S, and Mg2Si was formed from Mg

and Si. Moreover, the reason why these compounds coexist

was considered. As expressed by eq. (6), the Bi sulfide is

oxidized to form the Bi oxide.

2Bi2S3 þ 9O2 ! 2Bi2O3 þ 6SO2 ð6Þ

When Mg in the spheroidizing agent diffuses into the melt

during the spheroidizing treatment, Mg reduces the Bi oxide

to form MgO. However, under rapid solidification, the Mg

not used for the reduction combines with Si in the melt and

forms the Mg-Si compound in the vicinity of the Bi oxide and

Bi sulfide. This leads to the coexistence of Bi2O3, Bi2S3 and

Mg2Si.

These results lead to the conclusion that the Bi compound

was first formed, then the Mg compound was formed around

the Bi compound to form a nucleus, and then the graphite

crystallized out from the nucleus. The formation of such

many heterogeneous nucleation sites in the melt would lead

to the distribution of many fine graphite particles.

4. Conclusions

The effect of Si and Bi on the microstructure of thin wall

spheroidal graphite cast iron has been investigated by

examining the microstructure and cooling curves of thespheroidal graphite cast irons specimens with 2 to 10 mm

thicknesses. The results obtained are as follows:

(1) For the C and Si contents in the present study, an

increase in the Si/C mass ratio was effective for decreasing

the chill in the thin wall spheroidal graphite cast iron, and the

chilling was inhibited by containing 0.01 mass% Bi even

though the thickness of the specimen was 2 mm.

(2) Amounts up to 0.01 mass%Bi promoted refinement in

the graphite, increased the graphite nodule, and promoted the

ferritizing of the matrix of the spheroidal graphite cast iron.

It was also found that the high Si content promoted these

effects of Bi even though the specimen is thin.

(3) The temperatures of the eutectic start and the eutectic

solidification end increased by 0.01 mass%Bi. Especially,

at the hypereutectic composition, the eutectic solidification

time was shortened by containing Bi. The temperature of the

C BiC BiC BiSEI C Bi

Fig. 13 Secondary electron image and X-ray images of C and Bi in the spheroidal graphite in the specimen containing Bi. (specimen

No. 5-2)

SEI Bi S

Si O

Mg

La Ce

Fig. 14 Secondary electron image and X-ray images of Bi, S, Mg, Si, O, La and Ce in a compound in the center of the spheroidal graphiteshown in Fig. 13.



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eutectoid transformation start increased and the stable

eutectoid transformation was promoted by the Bi.

(4) The substance including the Bi and Mg compounds

existed in the vicinity of the center of the spheroidal graphite

in the cast iron containing Bi. This result indicates that these

compounds act as a nucleus of the graphite. It is thought

that this promotes the graphitzation to increase the graphitenodule count.

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