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 tocause chilling. Due to the chilling, the required mechanical properties can not be obtained. The addition of certain elements is a way to solve thisproblem. In this study, the spheroidal graphite cast iron melt containing minor Bi, 3.3 to 3.7mass%C and 2.0 to 3.2mass%Si was poured into astepped plate mold to obtain the thin wall castings, and observation of their graphite and matrix microstructure, thermal analysis during thesolidification process of the melt in the mold and the qualitative analysis of elements inside the spheroidal graphite by FE-EPMA were carriedout.

It was found that an increase in the Si/Cmass 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 0.01mass% Bi even for the thin wall castings of 2mm. Amounts up to0.01mass%Bi promoted refinement of the graphite, increased the graphite nodule, and promoted ferritizing of the matrix. It was also found that ahigh 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 eutecticsolidification end increased due to the 0.01mass%Bi. The temperature of the eutectoid transformation start increased and the stability eutectoidtransformation of the thin wall castings was promoted by containing a minor amount of Bi. It was confirmed that substances including Bi andMgexisted in the graphite containing Bi. These results lead to the conclusion that the Bi compound and the Mg compound acted as heterogeneousnuclei 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 forautomobile parts and industrial machines, because it has anexcellent castability, good wear resistance and dampingcapacity. In recent years, the reduction in weight and size ofthe machine products has been promoted to reduce the energyconsumption, use of raw materials and emitting of green-house gas. This trend leads to the promotion of thinning ofthe spheroidal graphite cast iron castings. However, the castiron melt in a thin wall is exposed to rapid cooling, andcementite (chill) tends to increase in the matrix. The chillingcauses a decrease in the mechanical properties of thecastings. Generally, a ferrosilicon (Fe-Si) alloy containing asmall amount of elements, such as aluminum, calcium, andbarium, is added to the cast iron melt as a graphitizer toprevent the chill. For the spheroidal graphite cast iron, it isreported that the critical graphite nodule count for preventingthe chill exists for each cooling rate.1) This indicates that anincrease in the graphite nodules in the matrix is effective forpreventing the chilling. It is reported that the addition of asmall amount of bismuth (Bi) is effective for increasing thegraphite nodule count.2–5) Based on this finding, the inoculantcontaining Bi6,7) or pure Bi8) is sometimes added to thespheroidal graphite cast iron melt. However, Bi is classifiedas a graphite spheroidization inhibition element because thespheroidization is inhibited by including excessive Bi in thecast iron. Some researchers have reported the critical contentof Bi to inhibit the graphite spheroidization. Morrogh9)

reported that the graphite spheroidization starts to interferewhen the Bi content exceeds 0.003mass%, and the spher-oidization is completely inhibited when the Bi content is0.006mass%. Donoho10) reported that the graphite spheroid-ization is inhibited by 0.005mass%Bi or more in the melt,

and Cole11) reported that the spheroidization is inhibitedby 0.006mass%Bi or more. On the other hand, for the castiron containing titanium,12,13) it has been reported that thegraphite spheroidization is interfered by containing about0.001mass%Bi. These reports indicate that a small amount ofBi inhibits the spheroidization and the critical Bi contentchanges due to the content of the main element and theexistence of other elements. Silicon (Si), the main element ofcast iron, is a graphitizing element. Horie et al.1) reported thatthe graphite nodule count in the spheroidal graphite cast ironincreases by increasing the Si content or carbon equivalent(CE). However, there are few reports on the effect of the Cand Si contents for a constant CE of the graphite and matrixstructure of the thin wall spheroidal graphite cast ironcontaining Bi. Moreover, there are no reports which examinethe effect of Bi on the solidification process of the thin wallspheroidal 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 platemold to obtain the thin wall specimens, and the relationshipbetween the thickness of the specimens, the graphitespheroidization rate, graphite nodule count, and area fractionof ferrite and pearlite was examined, and then the effect of theSi and Bi contents on the graphite and matrix structure wasexamined (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 theeutectoid transformation reaction was examined (experimentB). Moreover, structure of the spheroidal graphite in thespecimens containing Bi was analyzed in order to examinethe Bi distribution. Based on these results, the effects of Siand Bi on the microstructure of the thin wall spheroidalgraphite cast iron and the graphite refinement mechanism byBi were examined.

Materials Transactions, Vol. 51, No. 1 (2010) pp. 176 to 185#2010 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

2. Experimental Procedure

Raw materials with the chemical composition shown inTable 1 were used to fabricate the cast iron specimens. Theywere placed in a graphite crucible and melted in a small highfrequency induction furnace (10 kWh, 3 kHz) under an Ar gasatmosphere, followed by graphite spheroidizing, inoculationand Bi addition at 1773K. The graphite spheroidizing,inoculation and Bi addition were simultaneously carriedout by adding the spheroidizing agent (45.92mass%Si, 4.93mass%Mg, 2.37mass%Ca, 0.66mass%Al, 1.84mass%RE),inoculant (75.77mass%Si, 1.28mass%Ca, 2.16mass%Al),and pure Bi (99.9mass%Bi), respectively. As a result of apreliminary experiment, it was found that the yield of Bi inthe spheroidal graphite cast iron was 8% in this experimentcondition. The addition of the inoculant was 0.3mass%.

Table 2 shows the chemical composition, CE and Si/Cmass ratio of the specimens. Specimens No. 1 to No. 4were used in experiment A. In experiment A, the targetchemical composition of the specimens was as follows: 3.3,3.4, 3.5 and 3.7mass%C, 2.0, 2.4, 2.8, and 3.2mass%Si. TheCE(=C+Si/3+P/3) was set to 4.4 (constant). The targetcontents of Mn, P, S and Mg were 0.04, 0.02, 0.01and 0.04mass%, respectively. As a result of a preliminaryexperiment, it was found that the graphite spheroidizationwas insufficient when the Bi content exceeded 0.01mass%.Therefore, the Bi content was set to 0.005 and 0.01mass%.The ratio of the Si content to C content (Si/C mass ratio) wasused as a parameter, showing an increase in the Si content for

the same CE values. It is known that the Si/C mass ratio hasa correlation with the tensile strength and hardness.14,15)

Specimens No. 5 and No. 6 shown in Table 2 were usedfor experiment B. In experiment B, the carbon content wasset to 3.4mass% (constant), the Si content was set to 2.2 and3.2mass%; the CE was 4.1 (hypoeutectic composition) and4.4 (hypereutectic composition).

Melt was poured into the CO2 mold shown in Fig. 1 at1673K to obtain the stepped specimens plates with 2, 3, 5 and10mm thicknesses (50mm width � 150mm length). Inexperiment B, R thermocouples were inserted in the centerof the cavity and the thermal analysis of the melt was carriedout. To obtain a high heat sensitivity, the tip of thethermocouple 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 particlediameter, the graphite nodule count and area fractions of thegraphite, ferrite and pearlite were measured by an imageanalyzer. To obtain the mean values of these parameters, 10optical micrographs were used for the measurement. Thesevalues 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 — —

Electrolytic iron 0.02 <0:005 0.0001 <0:001 0.009 0.002 0.001 0.002

Fe-Si alloy 0.028 75.93 — 0.022 0.010 — — —

Table 2 Chemical composition, CE and Si/C mass ratio of cast iron

specimens (mass%).

No. C Si Mn P S Bi Mg CE� Si/C

1-1 3.75 2.10 0.041 0.022 0.012 0 0.037 4.45 0.56

-2 3.77 2.00 0.043 0.022 0.011 0.005 0.044 4.44 0.53

-3 3.76 2.06 0.046 0.022 0.010 0.010 0.040 4.45 0.55

2-1 3.63 2.45 0.041 0.022 0.010 0 0.043 4.45 0.67

-2 3.61 2.42 0.043 0.023 0.010 0.005 0.039 4.42 0.67

-3 3.62 2.38 0.045 0.021 0.011 0.010 0.042 4.41 0.66

3-1 3.54 2.81 0.042 0.023 0.008 0 0.038 4.48 0.79

-2 3.53 2.80 0.044 0.021 0.009 0.005 0.044 4.46 0.79

-3 3.52 2.78 0.047 0.022 0.010 0.010 0.041 4.45 0.79

4-1 3.31 3.21 0.050 0.020 0.010 0 0.048 4.38 0.97

-2 3.32 3.23 0.042 0.020 0.010 0.005 0.046 4.40 0.97

-3 3.33 3.22 0.045 0.020 0.010 0.010 0.045 4.40 0.97

5-1 3.44 2.10 0.040 0.021 0.009 0 0.037 4.14 0.61

2 3.45 2.20 0.043 0.022 0.011 0.010 0.041 4.18 0.64

6-1 3.40 3.14 0.042 0.023 0.008 0 0.045 4.45 0.92

2 3.41 3.21 0.047 0.022 0.010 0.010 0.042 4.48 0.94

�CE=C+Si/3+P/3

R thermocouple

15075

5 3 2

200

350

50 505050

250

R thermocoupleMeasuring point

75

10

200

Fig. 1 Configuration of CO2 mold equipped with thermocouples.

Effect of Silicon and Bismuth on Solidification Structure of Thin Wall Spheroidal Graphite Cast Iron 177

excluded because it is difficult to distinguish the graphite.The distribution of elements in the spheroidal graphite inthe 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. Themeasurement points in the thermal analysis are shown inFig. 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 specimenswithout Bi. The microstructure in the center part and thesurface part of each thickness of specimens were almostthe same. The Si/C mass ratio is also shown in the figure.When the Si/C mass ratio was 0.56, the specimen with a10mm thickness had a bull’s eye type structure withoutchill (Fig. 3(d)). The chill was partially observed in thespecimen with a 5mm thickness (Fig. 3(c)) and the matrixof the specimen with a 2mm thickness was completelychill. A similar structure was observed when the Si/C massratio was 0.67 (Fig. 3(e)–(h)). When the Si/C mass ratiowas 0.79, the chill was not observed in the specimenswith the thicknesses of 3, 5 and 10mm (Fig. 3(j)–(l)),although the chill was partially observed in the specimenwith a 2mm thickness (Fig. 3(i)). When the Si/C massratio was 0.97, the no chill was observed even in thespecimen with a 2mm thickness (Fig. 3(m)) and everyspecimen contained fine nodular graphite particles. Theferrite 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)´ Temperature

of 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 (0mass%Bi) and various C and Si contents at CE4.4 (Nital etched).

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

increased. These results indicated that the chilling does notoccur and an increase in the ferrite structure in the matrixis pronounced in the thin wall cast iron castings when theSi/C mass ratio is high (high Si content). The specimenscontaining 0.005mass%Bi have almost the same micro-structure as the specimens without Bi.

Figure 4 shows the microstructure of the specimenscontaining 0.01mass% Bi. When the Si/C mass ratios were0.55 and 0.66, no chill was observed in the specimens withthe thicknesses of 3, 5 and 10mm (Fig. 4(b)–(d), (f)–(h)).When the Si/C mass ratio was high (0.79 and 0.97), no chillwas observed even in the specimen with a 2mm thickness(Fig. 4(i), (m)).

These results show that an increase in the Si content iseffective for decreasing the chill in the thin wall spheroidalgraphite cast iron, and the chilling is further inhibited by a0.01mass%Bi content.3.1.2 Relation among Bi content, Si/C mass ratio,

thickness and microstructureFor all the specimens, the graphite spheroidization ratio

was 80% or more. When the Si/C mass ratio was 0.5, a0.01mass%Bi content slightly reduced the spheroidizationratio in the specimens.

Figure 5 shows the relation between the Bi content, Si/Cmass ratio, thickness, and the graphite particle diameterof the specimens. It can be seen that the graphite particlediameter decreased as the specimen thickness decreased.This is due to the fact that the cooling rate of the meltincreased 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 theSi/C mass ratio is small.

Figure 6 shows the relation between the Bi content, Si/Cmass ratio, thickness, and the graphite nodule count of thespecimens. The graphite nodule count increased by thinningof the specimen. This is due to the high cooling rate of themelt by the thinning as well as the graphite particle diameter.The tendency that the graphite nodule count increased alongwith the Bi content was observed for every thickness. Thistendency was also reported by Horie et al.2,3) and Sato et al.4)

The graphite nodule count increased as the Si/C mass ratioincreased. Generally, the addition of Si decreases the graphiteparticle diameter and increases the nodule count.7,8) Also forthe thin wall spheroidal graphite cast iron castings used inthis study, an increase in the Si/C mass ratio increased thegraphite nodule count.

Figure 7 shows the effect of the Bi content and Si/C massratio on the matrix structure. The graphite area fraction wasconstant (approximately 10%) regardless of the Bi content orSi/C mass ratio. The area fraction of ferrite increased and thearea fraction of pearlite decreased as the Si/C mass ratioincreased. For example, when the Si/C mass ratio was 0.97and the thickness of the specimen was 10mm, the areafraction of ferrite increased about 20% and that of pearlitedecreased about 20% by a 0.01mass%Bi content. When thethickness of the specimen was 3mm, the area fraction offerrite increased about 10% and the that of pearlite decreasedabout 10% by a 0.01mass%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)

100µ 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

These results show that the containing up to 0.01mass%Bipromoted the refinement of the graphite, increased thegraphite nodule, and promoted the ferritizing of the matrixof the spheroidal graphite cast iron. It was also found that theincrease in the Si content promoted the effects of Bi eventhough the specimen is thin; i.e., the cooling rate is high.

Therefore, the cooling curves of the thin wall spheroidalgraphite cast irons with the hypoeutectic and hypereutecticcompositions with different Si contents were subsequentlyanalyzed and the effect of Bi on the microstructure wasinvestigated.

3.2 Thermal analysis of thin wall spheroidal graphitecast iron with Bi (experiment B)

3.2.1 MicrostructureFigure 8 shows the microstructure of the specimens with

the thickness of 10mm in No. 5 and No. 6. Table 3 showsthe graphite spheroidization rate, the graphite particlediameter, the graphite nodule count and the area faction ofthe matrix structure of the specimens. Specimen No. 5 with 2and 3mm thickness contained the chill in the matrix. Thegraphite spheroidizing ratio of the specimens without chillwas 80% or more, regardless of the Bi content. The meandiameter of the graphite decreased and graphite nodule countincreased as the thickness decreased and the Bi contentincreased, as well as the specimens from experiment A. Thearea fraction of ferrite increased and that of pearlitedecreased somewhat as the Bi content increased.3.2.2 Effect of Bi on eutectic solidification

Figure 9 shows the cooling curves of specimens No. 5and No. 6. Although the primary crystallization start pointcan be seen in No. 5, it can not be seen in No. 6. This isdue to the fact that specimen No. 5 has a hypoeutectic

NG

2G

raph

ite n

odul

e co

unt,

/mm

400

200

600

800(b) 0.005mass%Bi

040 6 8 10

Si/C

0.97

0.79

0.67

0.56

Thickness, t/mm Thickness, t/mm Thickness, t/mm40 6 8 10

Si/C

0.97

0.79

0.67

0.53

40 6 8 10 12

Si/C

0.97

0.79

0.66

0.55

(c) 0.01mass% Bi(a) 0mass% Bi

2 22

Fig. 6 Relationship between the graphite nodule count and the thickness of the specimens containing various amounts of Bi, C and Si.

(((((((((((((((((((

15

20

25

(((((((((((((((((((

0.67

5

10

µG

raph

ite p

artic

le d

iam

eter

, d/

m(a) 0mass%Bi (b) 0.005mass%Bi (c) 0.01mass%Bi

40 6 8 1210 40 6 8 10 40 6 8 10

Si/C

0.97

0.79

0.97

0.79

0.67

0.53

0.97

0.79

0.66

0.55

Thickness, t /mm Thickness, t /mm

Si/C Si/C

Thickness, t /mm2 2 2

Fig. 5 Relationship between the graphite particle diameter and the thickness of the specimens containing various amounts of Bi, C and Si.

Are

a fr

actio

n (%

)

0.5 0.6 0.7 0.8 0.9 0.5 0.6 0.7 0.8 0.9 1.00

20

40

60

80

0

20

40

60

80

100

Si/C

1.0

Si/C

(c)

Graphite

Ferrite

Perlite

t =10mm (d) t =10mm

(a) t =3mm (b) t =3mm0mass%Bi0.01mass%Bi

0mass%Bi0.01mass%Bi

Fig. 7 Effect of Bi content and Si/C mass ratio on area fraction of graphite,

ferrite and pearlite (thickness (t); 3 and 10mm).

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

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

100µm

(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) = 10mm) (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

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

mm�2 — — 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/s200 400 6000 300100 500

Bi 0mass%Bi 0.01mass%

eutectoid

TEDS

TEDE

eutecticTES

TEMTEE

TEU

Time, T/s600

1600

900

0 200 400

(CE 4.1, Si/C 0.6)

800500

Tem

pera

ture

, Tem

/K

300100

Bi 0mass%Bi 0.01mass%

eutectoid

(a) No.5eutecticTPTES

TEMTEE

TEU

TEDSTEDE

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

Effect of Silicon and Bismuth on Solidification Structure of Thin Wall Spheroidal Graphite Cast Iron 181

composition which generates the solidification latent heatby crystallization of the primary crystal austenite whereasspecimen No. 6 has a hypereutectic composition in whichthe graphite crystallizes as the primary crystal. A steepincrease in the temperature from the temperature of theeutectic undercooling (TEU) was observed by adding Bi.This indicates that many or much amount of graphiteparticles are simultaneously crystallized out, followed bythe rapid solidification of the melt. In addition, the tem-perature of the eutectic start (TES) and temperature ofthe eutectoid start (TEDS) were high for the specimencontaining Bi.

Table 4 shows the average cooling rate from TP to TES foreach thickness of specimens No. 5 and No. 6. It can be seenthat the cooling rate was increased by containing Bi for everyspecimen. This is probably due to the fact that the vaporpressure of Bi is high16) and the yield of Bi in the melt isthought to be bad. It is thought that the evaporation of aquantity of Bi deprives the heat of the melt.

Moreover, the cooling rate of the specimen No. 6 wasfaster than that of the specimen No. 5. This reason is thoughtas 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 theprimary austenite crystallization. On the other hand, in thehypereutectic composition (specimen No. 6), it is thoughtthat the reduction in melt temperature is not suppressedbecause little heat is generated when the primary graphitecrystallizes out.

Figure 10 shows the effect of the Bi content and coolingrate 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 specimenNo. 5. This is due to the fact that the CE of specimen No. 6 ishigher than that of specimen No. 5. Regardless of the coolingrate of the specimens, the TES and TEE of the specimenscontaining 0.01mass%Bi were higher than those of thespecimens without Bi.

Figure 11 shows the effect of the Bi content and eutecticsolidification time (ET) for specimens No. 5 and No. 6.Although the change in the ET by containing Bi was not seenfor specimen No. 5 (hypoeutectic composition), the ET wasdecreased in specimen No. 6 (hypereutectic composition)by containing Bi. As previously described, the graphitenodule count of specimen No. 6-2 containing 0.01mass%Biwas more than that of specimen No. 6-1 without Bi. Ohiet al.17) have researched the influence of graphite nodulecount on the eutectic solidification of hypereutectic spher-oidal graphite cast iron and reported that the eutecticsolidification time (ET) shortened as the graphite nodulecount increased. From this result, it is thought that ET isshortened because an increase in the graphite nodule countreduced 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 additionshorten ET.3.2.3 Effect of Bi on eutectoid transformation

Figure 12 shows the effect of the Bi content, thickness ofthe 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, theTEDS increased, while the TEDE did not change by addingBi (Fig. 12(a)). For specimen No. 6 with the hypereutecticcomposition containing 0.01mass%Bi, the TEDS increasedand the TEDE decreased (Fig. 12(b)). As previouslydescribed, the graphite nodule count increased and theferritizing was promoted by the Bi. The increase in thegraphite nodule count leads to a decrease in the distanceamong the graphite nodules in the matrix. This promotesthe diffusion of C in the matrix into the previouslycrystallized graphite during the cooling after the eutecticsolidification. This probably leads to the phenomenon thatthe eutectoid transformation reaction starts at a compara-

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

vs. specimen thickness.

SpecimenBi content Thickness

mass% 2mm 3mm 5mm 10mm

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

Tem

pera

ture

, Tem

/K

Cooling rate, K/s0 10 20 30 40

1320

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).

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

tively high temperature and the ferritizing of the matrix ispromoted.

These results show that containing 0.01mass%Bi in-creases the temperature of the eutectoid start and promotesa steady eutectoid transformation of the thin wall spheroidalgraphite cast iron.

3.3 EPMA analysis of spheroidal graphiteSince the refinement of the graphite nodule and the

increase in the graphite nodule count was recognized byadding Bi, the inside of the spheroidal graphite in the castiron containing Bi was analyzed by FE-EPMA in order toexamine the effect of Bi on the graphite refinement.

Figure 13 shows the SEI and the X-ray images of C andBi in the spheroidal graphite of a specimen containing0.01mass%Bi (specimen No. 5-2). It can be seen that Biexists in the vicinity of the center of the graphite.

Figure 14 shows the magnified observations of whereBi was detected. It indicates that the center of the graphite

consists of the Bi oxide and the Bi sulfide, because Bi, Oand 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 samearea. Ce and La were also distributed in almost the sameposition as S, although their detection brightness is low.Ce and La are probably from RE in the spheroidizationagent. Igarashi et al.18) reported that MgS and MgO areincluded in the vicinity of the center of the spheroidalgraphite in the spheroidal graphite cast iron without Bi.This report supports the fact that MgS and MgO also existin the center of the graphite specimen without Bi in thisstudy.

Subsequently, the formation of compounds in the graphiteat the spheroidization temperature (1773K) in the presentstudy was examined from the viewpoint of the standardfree energy of formation. The standard free energies offormations of the oxide, sulfide and silicide of Bi and Mgare as follows:19)

0

20

40

60

120

100

80E

utec

tic s

olid

ific

atio

n tim

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/mm2

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

Tem

pera

ture

, Tem

/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).

Effect of Silicon and Bismuth on Solidification Structure of Thin Wall Spheroidal Graphite Cast Iron 183

2/3Bi2O3 ¼ �109:1 kJ/molO2 ð1Þ2/3Bi2S3 ¼ �42:9 kJ/molS2 ð2ÞMgO ¼ �733:7 kJ/molO2 ð3ÞMgS ¼ �395:2 kJ/molS2 ð4ÞMg2Si ¼ �8552:5 kJ/molSi ð5Þ

These values show that these Bi compounds and Mgcompounds are easily formed, and coexist in the melt afterthe spheroidization. This fact and the results from the FE-EPMA lead to the conclusion that Bi2O3 and Bi2S3 wereformed from Bi, O and S, and Mg2Si was formed from Mgand Si. Moreover, the reason why these compounds coexistwas considered. As expressed by eq. (6), the Bi sulfide isoxidized to form the Bi oxide.

2Bi2S3 þ 9O2 ! 2Bi2O3 þ 6SO2 ð6Þ

When Mg in the spheroidizing agent diffuses into the meltduring the spheroidizing treatment, Mg reduces the Bi oxideto form MgO. However, under rapid solidification, the Mgnot used for the reduction combines with Si in the melt andforms the Mg-Si compound in the vicinity of the Bi oxide andBi sulfide. This leads to the coexistence of Bi2O3, Bi2S3 andMg2Si.

These results lead to the conclusion that the Bi compoundwas first formed, then the Mg compound was formed around

the Bi compound to form a nucleus, and then the graphitecrystallized out from the nucleus. The formation of suchmany heterogeneous nucleation sites in the melt would leadto the distribution of many fine graphite particles.

4. Conclusions

The effect of Si and Bi on the microstructure of thin wallspheroidal graphite cast iron has been investigated byexamining the microstructure and cooling curves of thespheroidal graphite cast irons specimens with 2 to 10mmthicknesses. The results obtained are as follows:

(1) For the C and Si contents in the present study, anincrease in the Si/C mass ratio was effective for decreasingthe chill in the thin wall spheroidal graphite cast iron, and thechilling was inhibited by containing 0.01mass% Bi eventhough the thickness of the specimen was 2mm.

(2) Amounts up to 0.01mass%Bi promoted refinement inthe graphite, increased the graphite nodule, and promoted theferritizing of the matrix of the spheroidal graphite cast iron.It was also found that the high Si content promoted theseeffects of Bi even though the specimen is thin.

(3) The temperatures of the eutectic start and the eutecticsolidification end increased by 0.01mass%Bi. Especially,at the hypereutectic composition, the eutectic solidificationtime 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 graphite

shown in Fig. 13.

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

eutectoid transformation start increased and the stableeutectoid transformation was promoted by the Bi.

(4) The substance including the Bi and Mg compoundsexisted in the vicinity of the center of the spheroidal graphitein the cast iron containing Bi. This result indicates that thesecompounds act as a nucleus of the graphite. It is thoughtthat this promotes the graphitzation to increase the graphitenodule count.

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Effect of Silicon and Bismuth on Solidification Structure of Thin Wall Spheroidal Graphite Cast Iron 185