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In uence of Si, Ce, Sb and Sn onchunky graphite formation

Male, born in 1941, Dr. Eng., Professor. He was a senior researcher inMechanical Eng. Res. Lab. Hitachi Ltd. from 1971 to 1983, and Director of KAGAMI Memorial Lab. for Materials Sci. and Tech., Waseda Univ. from2003 to 2006. He has nearly 20 publications. His research interests mainly

focus on the interface between solid and liquid and cast iron. He receivedsix Best Paper Awards from Japan Foundry Eng., Soc. and Japan Instituteof Metals. He was Past President of Japan Foundry Engineering Society.

E-mail: h.nakae@waseda.jpReceived: 2010-07-06; Accepted: 2010-08-20

*Hideo Nakae

*Hideo Nakae1, Masayuki Fukami

2, Takayuki Kitazawa

3and Ying Zou

4

(1. Lab. for Mater. Sci. and Tech., Waseda Univ., Tokyo , Japan; 2. JFE Steel Corp., Japan; 3. Nippon Steel Corp., Japan; 4. Appl.

Mech. and Eng., Waseda Univ., Tokyo , Japan)

S pheroidal graphite (abbreviated SG) cast iron is widelyused in industry due to its excellent mechanical properties

and good castability. Nevertheless, it is well-known that thegraphite morphology changes from SG to chunky graphite(abbreviated CHG) in heavy SG castings [1,2] . The motivationof this study is to elucidate the CHG formation mechanism.There are many reports that have described the alloyingelements, such as Si, Ni, Ce and Ca, as the CHG formationelements and Sn, Sb and Te as the preventing elements [1-3] .

The Si and Ni are graphitizing elements, the Ca and Ce are thespheroidizing elements and the Sn, Sb and Te are the elementsthat prevent spheroidizing. It is well known that the Sb andTe can neutralize the in uence of Ce on the CHG formation.Tsumura [4] reported the in uence of the Sb addition on the Ce-treated SG iron. The in uence of Ca was reported by Church [5].

Abstract: The thirteen mother alloys, C%+1/3Si% = 4.45%, differing in their Si, Ce, Sb and Sn contents, wereprepared. Seventy grams of these alloys was remelted in a high purity alumina crucible at 1,450ºC under an Ar atmosphere, and then cooled at 30 K/min for obtaining their cooling curves. Their graphite morphologies wereobserved using an optical microscope and an SEM. Their three-dimensional graphite shapes were observed by theSEM using the samples whose matrices were etched off with an acid-aqua solution, to con rm the chunky graphite.

For discussing the influence of the Si and Ce contents on the chunky graphite formation, two experimentswere carried out. In the rst one, the Si contents were changed from 0 to 4% in the 0.15%Ce alloys, and for thesecond one, the 3.5%Si and 4%Si samples that differed in the Ce contents of 0.1 and 0.2% were used. In the thirdexperiment, the in uence of Sb and Sn on the chunky graphite formation was investigated by using the 4%Si and0.1%Ce samples. The results showed that with the increase of the Si content, the volume fraction of the chunkygraphite increases, while the volume fraction of the ledeburite decreases, and the chunky graphite volume fractionin the 0.2%Ce samples is higher than that of the 0.1%Ce samples. The effect of the Sb and Sn additions on theprevention of chunky graphite formation cannot be con rmed due to their high Si contents. Therefore, further studieswill be needed in this eld.

Key words: chunky graphite; spheroidal graphite; graphite morphology; solidi cation of cast ironCLC number: TG143.5 Document code: A Article ID: 1672-6421(2011)01-096-05

Nevertheless, nobody has yet explained the formationmechanism of the CHG, therefore, we prepared Fe-C-Si-Cesamples for discussing the CHG formation mechanism usingconstant cooling rate experiments [3].

1 Experimental procedure

We prepared eight kinds of Fe-C-Si-Ce alloys with differentSi and Ce contents for discussing the in uence of Si and Ce on

the CHG formation. Another ve samples, using the 4 mass%(abbreviated %) Si and 0.1%Ce alloys, the Sb-samples and Sn-samples, were prepared in order to study the in uence of Sband Sn on the CHG formation. To produce these samples, weused electrolytic iron, high purity graphite (>99.99%), pure Si(>99.999%), pure Ce (>99.9%), pure Sb (>99.9%) and pure Sn(>99.9%). These alloys were melted using an Al 2O3-lined 7 kghigh-frequency induction furnace under owing Ar. The meltswere cast into ceramic molds that were 15 mm in diameterand 300 mm in length. The chemical compositions of thesesamples are shown in Table 1. Their sulfur contents were lessthan 0.005%.

Seventy grams of these samples was remelted in a highpurity aluminum crucible under an Ar atmosphere at 1,450 ℃ using an electric furnace, as shown in Fig. 1, and the sampleswere then cooled at the rate of 30 K/min at 1,200 ℃ . The

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cooling curves were measured by a B-type thermocouplelocated at the center of the sample in order to discuss thesolidification mechanism. All of the graphite morphologieswere observed using an optical microscope and a SEM. Theirthree-dimensional graphite shapes were observed by the SEM

using the samples whose matrices were etched off with ahydrochloric aqua solution.

The volume fractions of the CHG portion were measuredusing thirty microstructure photos of fty magni cation. Thecooling curves were differentiated to determine the transitionpoints, namely the onset and end points of the SG and CHGformations.

Table 1: Chemical composition of samples (mass%)

Sample No. C Si Ce Sb Sn

0 Si 4.40 0.005 0.16 - -

2 Si 3.79 2.00 0.16 - -

3 Si 3.50 3.00 0.17 - -

4 Si 3.18 3.98 0.17 - -

3.5Si-0.15Ce 3.49 3.58 0.12 - -

3.5Si-0.25Ce 3.48 3.58 0.21 - -

4Si-0.15Ce 3.31 4.08 0.13 - -

4Si-0.25Ce 3.37 4.10 0.24 - -

4Si-0.1Ce 3.49 3.93 0.09 - -

0.02Sb 3.23 4.11 0.08 0.02 -

0.14Sb 3.24 4.06 0.10 0.14 -

0.05Sn 3.25 4.19 0.11 - 0.05

0.10Sn 3.26 4.10 0.10 - 0.10

Fig. 1: Schematic of constant cooling rate experiment

2 Results and discussion

2.1 In uence of Si on CHG formation

The in uences of the Si contents on the graphite morphology,observed by an optical microscope and their fractions in eachgraphite portion, are shown in Fig. 2. In this gure, SG, CHGand Led mean the volume fractions of SG portion, CHGportion and ledeburite portion, respectively. There is no CHGand 95% of the matrix is ledeburite in the 0 Si sample andthe volume fraction of CHG in the 4 Si sample is 92%. TheCHG fraction increases with the increasing of the Si contentsand that of the ledeburite decreases. Their three-dimensionalshapes of the CHG, observed by SEM are shown in Fig. 3. Ascan be clearly seen, the graphites are highly continued eachother; therefore, we con rmed that they are CHG.

Fig. 2: In uence of Si on CHG graphite formation

Fig. 3: Three dimensional graphite shapes of 2% to 4%Si samples

SG: 5%, CHG: 0%, Led: 95% SG: 9%, CHG: 69%, Led: 22% SG: 6%, CHG: 82%, Led: 12% SG: 8%, CHG: 92%, Led: 0%

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2.2 Interaction of Si and Ce on CHG formationThe interaction of Si and Ce for the 3.5%Si and 4%Si is shownin Fig. 6. As can be clearly seen, the difference in the CHGfraction is small; nevertheless, in the 0.25%Ce samples thevolume fraction is slightly greater than that of the 0.10%Ce

samples. Moreover, a very small amount of ledeburiteformation in the 4%Si-0.25%Ce sample is con rmed by theoptical microscope.

The cooling curves of these samples are shown in Fig. 4. Thecooling curves are slightly different from that of the sand moldcastings [6]. Namely, the eutectic solidi cation temperature of the SG gradually decreases, but for the sand mold castings, itstays constant as Sertuucha et al. [7] reported. Nevertheless, the

CHG solidification temperature stays nearly constant with asigni cant recalescence, the same as the solidi cation of theflake graphite. This means that the solidification rate of theCHG iron is much higher than that of the SG iron due to thedifference in the solidi cation mode as shown in Fig. 5.

Fig. 4: In uences of Si contents on cooling curves

Fig. 5: Schematic solidi cation models of akegraphite, CHG and SG irons

Fig. 6: In uence of Si and Ce contents on graphitemorphology

SG is directly crystallized from the melt and covered withan austenite shell at the onset of the eutectic solidi cation [8] asshown in Fig. 5 [8,9] . Moreover, the thickness of the austeniteshell for the SG increases with time up to 1.4 times of the

graphite radius, r , for the pure Fe-C alloy. On the other hand,the thickness of the austenite layer at the tip of the CHG staysnearly constant during the eutectic solidification. Therefore,the solidi cation rate of SG is lower than that of the CHG dueto the dif culty in carbon diffusion.

If we look at the cooling curves of these samples, shown inFigs. 7 and 8, the formation of ledeburite in the 4%Si-0.25%Cesample can be con rmed by the differentiated curve during the

nal stage of the solidi cation. Moreover, the differentiated valuesin the SG formation period are less than zero while that in theCHG formation stage, the value is more than zero. This can beexplained by the solidi cation mode of CHG mentioned in Fig. 5.

We show the volume fractions of the SG and CHG,measured by the optical microscope, and by the solidi cationtime, in these gures. They are very similar to each other. Thismeans that the solidi cation time agrees with the volume of the solidi cation due to the constant cooling rate.

Fig. 7: Cooling curve and the differentiatedcurve of 3.5Si-0.25Ce sample

Led: 6%

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2.3 In uence of Sb and Sn on CHG formationThe microstructures of these five samples, the Sb-samplesand the 3.5C-4Si samples, are shown in Fig. 9. The Sb andSn are well known elements that prevent the CHG formation;nevertheless, we cannot confirm the effect based on the

gure and Table 2. The three dimensional shape of the CHGobserved by the SEM shows that these elements affect thegraphite size by making it much ner.

Table 2: In uence of Sb and Sn addition onCHG formation

Sample No. Δ T (K) CHG (%)

3.5C-4Si 7.6 81

0.02 Sb 16.1 88

0.14 Sb 12.0 91

0.05 Sn 13.9 83

0.10 Sn 13.2 85

Fig. 9: In uence of Sb and Sn on graphite morphology for 4%Si alloys

Fig. 10: Cooling curves and CHG and SG fraction of 3.5C-4.0Si (a) and0.05Sn (b) samples

If we look at the cooling curves of thesesamples in Fig. 10, their addition producesnot only an increase in the CHG fraction, buta signi cant recalescence for the formation of CHG.

The recalescence, Δ T , was measured from theonset of the CHG formation and their maximumeutectic temperature for the morphologicaltransition from SG to CHG. The easiness of solidification is the main reason for the CHGformation. Nevertheless, the solidi cation mode

transfers directly from SG to ledeburite only inthe case of the pure Fe-C alloy or low siliconalloys, as shown in Fig. 2, due to the dif cultyof graphitization. All of these results showthat the significant undercooling during the

Fig. 8: Cooling curve and the differentiatedcurve of 4.0Si-0.25Ce sample

(a) (b)

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Fig. 11: Graphite morphological transitionmechanism of SG to CHG and ledeburite

SG formation, due to the increase in the dif culty of carbondiffusion, produces the CHG.

We then propose the morphological transition model in Fig.11 [10] based on the difference in the solidi cation mode of SGand CHG, shown in Fig. 5. Nevertheless, these solidi cation

rates are only reference values.

3 Conclusions

We discussed the influence of alloying elements on thechunky graphite formation in view of the solidi cation modeof the spheroidal graphite cast iron and that of the chunkygraphite cast iron. In this study, a number of experimentswere conducted for clarifying the chunky graphite formationmechanism. We can nally conclude that the signi cant under-cooling during the spheroidal graphite eutectic solidi cation

produces the morphological transition from SG to CHGand ledeburite due to the increase in the dif culty of carbondiffusion. This is the main reason for the CHG formation.

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Corp. , 1966.[2] Basutkar P K and Loper C R. Predicting graphite nodularity in

heavy section ductile iron by thermal analysis. AFS report of research project, 1971: 1 - 17.

[3] Nakae H, Jung S and Shin H C. Formation mechanism of chunky graphite and its preventive measure. J. Mater. Sci. Tech.,2008, 24: 289 - 295.

[4] Tsumura O, Ichinomiya Y, Narita H, Miyamoto T and TakenouchiT. Effects of rare earth elements and antimony on morphologyof spheroidal graphite in heavy-walled ductile cast iron. Imono,1995, 67(8): 540 - 545.

[5] Church N L and Schelleg R D. Detrimental effect of calcium on

graphite structure in heavy section ductile iron. Modern Casting,1970, (1): 5 - 8.

[6] Bäckerud L, Nilsson K and Steen H. Study of nucleationand growth of graphite in magnesium-treated cast iron. TheMetallurgy of Cast Iron, B. Lux, I. Minkoff, F. Mollars (eds.), St.Saphorin (Switzerland): Georgi Pub. Co., 1975: 625 - 637.

[7] Sertucha J, et al. Thermal analysis of the formation of chunkygraphite during solidification of heavy-section spheroidalgraphite iron parts. ISIJ International 2009, 49: 220 - 228.

[8] Nakae H and Yamauchi T. Effect of sulphur on growthmorphology of unidirectional solidified Fe-C alloys. J. Inst.Metals, 1994, 58: 30 - 36.

[9] Tatsuzawa Y, Jung S and Nakae H. Cooling curve and graphitemorphology in Ni-C alloys. Intl. J. Cast Metals Research, 2008,21: 17 - 22.

[10] Nakae H, Kitazawa T and Fukami M. In uence of solidi cationrate on graphite morphological change in cast iron. In:Proceedings of the 3rd International Conference on ProcessingMaterials for Properties, 2009: 1085 - 1090.

The paper was presented at the 69th World Foundry Congress, Hangzhou China 2010, republished in China Foundry with the

authors' kind permission.