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Effect of sulfur on graphite aspect ratio and tensile properties in compacted graphite irons
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1701–1705
journal homepage: www.elsevier.com/locate/jmatprotec
Effect of sulfur on graphite aspect ratio and tensile properties in compacted graphite irons M. Bazdar, H.R. Abbasi ∗ , A.H. Yaghtin, J. Rassizadehghani School of Metallurgy and Materials Engineering, Faculty of Engineering, University of Tehran, P.O. Box 11365-4563, Tehran, Iran
a r t i c l e
i n f o
a b s t r a c t
Article history:
In this research, the effect of sulfur content on graphite aspect ratio and tensile properties of
Received 12 October 2006
compacted graphite iron (CGI) was investigated. Different samples with sulfur levels ranging
Received in revised form
from 0.023 to 0.080% were produced in which the amount of magnesium was the same. Mag-
5 April 2008
nesium was added as FeSiMg by sandwich method and sulfur was added as pyrite powder
Accepted 7 April 2008
in reaction chamber of the mold. In order to study the microstructure and mechanical properties, metallographic examination and mechanical tests were conducted on specimens. The metallographic results showed that increasing of sulfur level from 0.023 to 0.080% in
Keywords:
constant magnesium level of 0.057% increases the graphite aspect ratio from 0.6 to 12.4.
Cast iron
Evaluation of the mechanical test results indicated that increasing of sulfur level, decreases
Compacted graphite
the tensile properties of compacted graphite iron. © 2008 Elsevier B.V. All rights reserved.
Sulfur Aspect ratio
1.
Introduction
Compacted graphite (CG) cast iron is a type of cast iron in which the graphite is present in a short, stubby, wormlike form with rounded edges. With this type of structure CG iron possesses properties that are intermediate between gray and ductile cast iron. Based on the results obtained by Dawson and Schroeder (2004) the strength, ductility and toughness of compacted graphite iron are superior to gray iron and can approach those of ductile iron. Liu and Ding (1985) also concluded that machinability, thermal conductivity and damping capacity of compacted graphite iron are also superior to ductile iron and also produces less dross and is less susceptible to shrinkage porosity and carbide formation. For these reasons, compacted graphite iron has been considered for a number of applications such as ingot molds, brake drums, cylinder heads, valve bodies and engine blocks. Chemical composition and inoculation practice strongly influence the graphite shape in CG irons. Liu and Ding (1985)
∗
showed that some elements even in minor amount, affect the shape of the graphite in CG irons. The elements which are used for nodularizing of graphite are magnesium, cerium and calcium and the elements which act as denodularizer agents are titanium, tellurium, arsenic, antimony, lead, bismuth and aluminum. Elements which have great tendency to react with the above elements and restrict graphite growth are sulfur, oxygen and nitrogen. A number of techniques have been advocated for the production of compacted graphite iron. It has been produced by ineffective magnesium treatment, simultaneous treatment with spheroidizing and anti-spheroidizing elements, and inoculation with certain elements, e.g. rare earth method. The use of magnesium treatment has an extremely narrow range of magnesium contents in which compacted graphite can be produced. The prominent result which distinguished the research by Stefanescu and chairman (1998) from the others is that the presence of anti-spheroidizing elements (Ti, Al) extends the range of magnesium content.
Corresponding author. Tel.: +989177142041. E-mail addresses: hrab [email protected] (H.R. Abbasi), [email protected] (A.H. Yaghtin). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.04.015
1702
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1701–1705
Table 1 – Chemical composition (wt.%) of the base melt C
Si
Mn
S
P
Cu
3.4
2.1
0.25
0.01 0.016 0.065
Mg
Sn
Al
Ti
0.057
0.01
0.012 0.017
However, the use of those elements increases the cost of production and machining and also contaminates the scrap with undesirable elements for gray and ductile iron production. One of the methods for production of compacted graphite iron is addition of sulfur to magnesium treated melt that is the subject of investigation in this research. In this method, nodularizing treatment is performed with FeSiMg alloy and followed by graphite compacting treatment with inoculation by sulfur as mentioned by Chisamera and Riposan (1996). The amount of sulfur for inoculation of magnesium treated melt depends on residual magnesium after nodularizing treatment, holding time prior to pouring, casting section thickness and mold type (metal or sand mold). The higher the sulfur content, the higher the amount of treatment alloys needed. It is possible to have sulfur contents which are at very low levels, i.e. 0.015% and lower, but at these levels the nucleation of graphite is restricted. Based on the research conducted by Riposan et al. (2003), inoculation of the melt with sulfur, after magnesium treatment, changes the nodularity of graphite and the structure transforms from spheroidal graphite (SG) to compacted graphite. Liu and Ding (1985) came to this conclusion that inoculation of magnesium treated melt with sulfur makes it possible to obtain different ratios of SG/CG without using denodularizing elements such as titanium and aluminum. Therefore, sulfur can replace these elements in control of graphite nodularity, and returned melt is less harmful on production and machinability. As shown by Su and Chow (1992), for quantities higher than 0.080%, sulfur acts as a harmful element resulting in magnesium-neutralization and increased dross formation. In contrast, for lower quantities, sulfur is beneficial and essential because it promotes suitable nuclei for graphite precipitation.
2.
Experimental procedure
A medium frequency induction furnace was used to melt the base iron with the chemical composition shown in Table 1. Furnace charge for the above composition consisted of steel scrap, ductile and gray iron returns. The mold was made by CO2 process. Mixture of silica sand and liquid sodium silicate binder, followed by passing of CO2 gas through the mold was used. Tapping temperature was in the range of 1440–1460 ◦ C and the pouring temperature from the ladle to the mold was in the range of 1390–1420 ◦ C. Nodularizing treatment was carried out by sandwich method using FeSiMg alloy with magnesium content of 5% and sizing about 1–4 mm. Sulfur was added in the form of iron pyrite in different amounts in the reaction chamber of the mold. Iron pyrite with 28% sulfur content and a size distribution of about less than 200 m was used. Seven standard Y-blocks were cast from the melt. Final Mg content was about 0.057% in all the samples taken from the melt just
Table 2 – Chemical compositions (wt.%) with different sulfur levels C
Si
Mn
S
P
Cu
Mg
Sn
Al
3.4 3.4 3.4 3.4 3.4 3.4 3.4
2.1 2.1 2.1 2.1 2.1 2.1 2.1
0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.023 0.035 0.045 0.055 0.065 0.075 0.080
0.016 0.016 0.016 0.016 0.016 0.016 0.016
0.065 0.065 0.065 0.065 0.065 0.065 0.065
0.057 0.057 0.057 0.057 0.057 0.057 0.057
0.01 0.01 0.01 0.01 0.01 0.01 0.01
0.012 0.012 0.012 0.012 0.012 0.012 0.012
Ti 0.017 0.017 0.017 0.017 0.017 0.017 0.017
before pouring. The amount of sulfur added varied from 0.023 to 0.080%. Chemical compositions of the melts with different levels of sulfur are shown in Table 2. Test specimens were obtained from ASTM A536 Y-blocks. Two tensile test specimens were prepared for each sample according to ASTM E8 M standard and were tested using MTS universal machine with tension rate of 1 mm/min. Metallographic samples were sectioned, ground and polished with 0.3 m alumina powder. The etch solution of 1% Nital was used for investigation of microstructure of samples. The optical microscopy was used to evaluate the graphite shape. In order to measure the graphite aspect ratio, the image analyzer was used. Images from different positions were obtained from polished samples. Fifty graphite nodules from each sample were measured in length and width and the mean value was recorded as graphite aspect ratio as indicated in Eq. (1): graphite aspect ratio =
graphite length graphite width
(1)
Graphite aspect ratio was used as a criterion to investigate the graphite compactness.
3.
Results and discussion
3.1.
Graphite shape
Fig. 1 shows the microstructures that resulted from the addition of sulfur to the base melt with magnesium residual of 0.057% that was held constant for all sulfur levels. The addition of 0.023% sulfur (Fig. 1a) as iron pyrite to the base ductile melt had little or no effect on nodularity of graphite and nodularity was not affected up to 0.030% sulfur. Increased levels of sulfur in the range of 0.035–0.080% changed the graphite shape from spheroidal to compact and eventually to flake-like form as demonstrated in Fig. 1b–g. Photomicrographs presented in these figures indicate that by increasing sulfur level, different amounts of compacted graphite were observed in the structure. 0.035% sulfur in sample no. 2 (Fig. 1b) was the minimum amount necessary for the beginning of the formation of compacted graphite. The most compacted graphite is obtained with samples no. 3–6 (Fig. 1c–f) for the range of 0.045–0.075% sulfur. Sample no. 6 (Fig. 1f) with 0.075% sulfur had about 95% compacted graphite and 5% nodular graphite. The detailed investigation of graphite shape by image analyzer showed that increasing of sulfur at constant magnesium level, changes the graphite aspect ratio from 1 for nodular graphite to about 74 for flake-like graphite
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1701–1705
1703
Fig. 1 – Optical micrographs of the graphite morphology at different sulfur levels and aspect ratios (100×).
as shown in Table 3. Increasing sulfur content lead to a transition from nodular graphite to compacted graphite in sample no. 2 (Fig. 1b) with aspect ratio of 2, and a transition from compacted graphite to flake-like graphite in sample no. 6 (Fig. 1f) with aspect ratio of 25.
The effects of sulfur addition on graphite morphology are as follows: Itofuji et al. (1983) claimed that the nodular graphite is characterized by growth on basal plane with spiral growth mechanism. A detailed study by Subramanian et al. (1982) on
1704
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1701–1705
Table 3 – Graphite aspect ratio and tensile properties Sample number 1 2 3 4 5 6 7
Aspect ratio 1 2 6 10 15 25 74
Sulfur content (wt.%)
Tensile strength (MPa)
0.023 0.035 0.045 0.055 0.065 0.075 0.080
graphite growth under the condition of controlled chemical composition in iron melt confirmed that removal of surface active elements such as sulfur and oxygen results in faceting of prism planes and affects their mobility. On the addition of elements like sulfur the prism face of graphite shows evidence of roughening and changes to nonfaceted, sinusoidally perturbed interface. These junctions act as sources of growth steps. Stefanescu and Loper (1981) revealed that in the absence of impurities, graphite crystal growth is characterized by faceted prism plane and the driving force needed for two-dimensional nucleation of new layer on prism plane exceeds the driving force required for spiral growth normal to basal plane to form nodular graphite. In the presence of impurities such as sulfur and oxygen that are present in untreated cast iron, the mobility of prism plane is dramatically increased. The result is flake-like graphite growth competitively with austenite. The works of Murthy and Seshan (1954) showed that the crystallographic growth direction of compacted graphite in growth tips can be either in c-direction or in a-direction separately or in a- and c-direction simultaneously. However the progressive front of graphite is a-axes. This indicates that crystallographic growth direction of compacted graphite can change from c-axes to a-axes and vice versa. When the concentration of nodularizing elements in growth front is sufficient, graphite growth in a-axes is restricted and grows as spheroidal graphite. On the other hand when concentration of nodularizing elements is not sufficient to restrict growth in a-direction, compacted graphite will grow. In this condition the concentration of nodularizing elements in growing tips vary by growth progress. When the nodularizing elements were absorbed mainly on prism plane of graphite, the growth of graphite in c-direction leads to extension of prism planes and melt interface and therefore the concentration of nodularizing elements on prism planes decreases and therefore the growth of graphite is extended in a-direction clearly. With developing of graphite growth in a-direction, the concentration of nodularizing elements on prism plane is increased, therefore the graphite growth is restricted in a-direction and growth is developed in c-direction as demonstrated by Stefanescu et al. (1979). As previously presented in the results and discussion the best sulfur range to obtain commercial compacted graphite iron with at least 80% compacted graphite with no flake graphite, was 0.045–0.075% in constant magnesium level of 0.057%. In sulfur levels lower than this range, nodular graphite was obtained and in levels higher than that, flake graphite appeared in the microstructure as shown in Fig. 1.
556 450 365 373 375 221 198
3.2.
Yield strength (MPa) 488 339 297 304 292 150 132
Elongation (%) 12.4 7.1 5.8 5.4 5.8 1.1 0.6
Mechanical properties
The examination of tensile test specimens showed that increasing of sulfur decreased the tensile properties. This is clearly evident from the results presented in Table 3. The lower strength values at higher sulfur levels can be attributed to graphite morphology and interface area of graphite with matrix. With increasing of sulfur due to the above mentioned mechanisms, graphite morphology changes from spheroidal to flake type which leads to the following phenomena. The interface of graphite with matrix increases and the area which is loaded in tension decreases which leads to decreasing in tensile strength. Also this change leads to increasing graphite aspect ratio and local stress concentration in graphite tips. The brittle fracture behavior of cast irons under tensile loading is controlled by the crack initiation stage. It can therefore be expected that increasing stress concentration due to reduced roundness of graphite tips, with higher aspect ratio, can act to decrease the tensile stress required to initiate fracture. This relationship between graphite aspect ratio and tensile strength can be clearly observed in Fig. 2 Compacted graphite like flake graphite is interconnected but in contrast to flake type, graphites are thicker, shorter and the ends are rounded. This graphite morphology results in better elongation values than that of gray iron. The mean value of elongation for compacted graphite iron and gray iron is 5.5 and 0.5%, respectively. The results also indicated that the elongation of the samples with compacted graphite was higher than that of flake graphite, although in samples with compacted
Fig. 2 – Effect of graphite aspect ratio on tensile and yield strength.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1701–1705
1705
With increasing graphite aspect ratio, the tensile strength and elongation decrease. The best sulfur range to obtain commercial compacted graphite iron was 0.045–0.075% in constant magnesium level of 0.057%.
Acknowledgement The authors wish to thank Iran Tractor Foundry Company (ITFCo.) that helped us to carry out the research presented in this paper.
references
Fig. 3 – Effect of graphite aspect ratio on elongation.
graphite, higher sulfur levels results in lower elongation, as shown in Fig. 3. In spite of degradation of mechanical properties by increasing of sulfur level, tensile strength and elongation for compacted graphite iron are higher than those of gray iron. Therefore compacted graphite iron can replace with gray iron in applications that need higher tensile strength and elongation.
4.
Conclusions
Based on the results obtained from the effect of in-mold sulfur addition to magnesium treated iron melt the following conclusions can be made: Addition of sulfur made it possible to produce compacted graphite iron from the base ductile iron melt with magnesium level that is suitable for the production of ductile iron. Sulfur has complex effect in magnesium treated irons. Intermediate sulfur levels in the range of 0.045–0.075% changed the graphite morphology to satisfactory compacted form as needed in compacted graphite iron. With increasing amount of sulfur in constant magnesium level, the graphite aspect ratio varied from 1 for nodular graphite to about 74 for flake graphite.
Chisamera, M., Riposan, I., 1996. S-Inoculation of Mg-treated cast iron to obtain CG cast iron and improve graphite nucleation in DI. AFS Transactions, 581–588. Dawson, S., Schroeder, T., 2004. Practical applications for compacted graphite iron. AFS Transactions, 1–9. Itofuji, H., Kawano, Y., Inoyama, N., Yamamoto, S., Chang, B., Nishi, T., 1983. the formation mechanism of compacted/vermicular graphite in cast irons. AFS Transactions, 831–840. Liu, J., Ding, N.X., 1985. Effect of type and amount of treatment alloy on compacted graphite produced by the flotret process. AFS Transactions, 675–688. Murthy, V.S.R., Seshan, S., 1954. Characteristics of compacted graphite cast iron. AFS Transactions, 373–380. Riposan, I., Chisamera, M., Kelly, R., Barstow, M., Naro, R.L., 2003. Magnesium-sulfur relationships in ductile and compacted graphite cast iron as influenced by late sulfur additions. AFS Transactions, 830–844. Stefanescu, D.M., chairman, 1998, ASM Handbook, Volume 15, Casting, Compacted Graphite iron, ninth ed. pp. 667–671. Stefanescu, D.M., Loper, C.R., 1981. Recent progress in the compacted/vermicular graphite cast iron field. Giesserei-Prax, 73–77, No. 5. Stefanescu, D.M., Dinescu, I., Craciun, S., Popescu, M., 1979, Production of vermicular graphite cast irons by operative control and correction of graphite shape. Paper 37 presented at the 46th International Foundry Congress, Madrid. Su, J.Y., Chow, C.T., 1992. Solidification behavior of compacted graphite. AFS Transactions, 565–574. Subramanian, S.V., Kay, D.R., Purdy, G.R., 1982. Compacted graphite morphology control. AFS Transactions, 589–603.
journal homepage: www.elsevier.com/locate/jmatprotec
Effect of sulfur on graphite aspect ratio and tensile properties in compacted graphite irons M. Bazdar, H.R. Abbasi ∗ , A.H. Yaghtin, J. Rassizadehghani School of Metallurgy and Materials Engineering, Faculty of Engineering, University of Tehran, P.O. Box 11365-4563, Tehran, Iran
a r t i c l e
i n f o
a b s t r a c t
Article history:
In this research, the effect of sulfur content on graphite aspect ratio and tensile properties of
Received 12 October 2006
compacted graphite iron (CGI) was investigated. Different samples with sulfur levels ranging
Received in revised form
from 0.023 to 0.080% were produced in which the amount of magnesium was the same. Mag-
5 April 2008
nesium was added as FeSiMg by sandwich method and sulfur was added as pyrite powder
Accepted 7 April 2008
in reaction chamber of the mold. In order to study the microstructure and mechanical properties, metallographic examination and mechanical tests were conducted on specimens. The metallographic results showed that increasing of sulfur level from 0.023 to 0.080% in
Keywords:
constant magnesium level of 0.057% increases the graphite aspect ratio from 0.6 to 12.4.
Cast iron
Evaluation of the mechanical test results indicated that increasing of sulfur level, decreases
Compacted graphite
the tensile properties of compacted graphite iron. © 2008 Elsevier B.V. All rights reserved.
Sulfur Aspect ratio
1.
Introduction
Compacted graphite (CG) cast iron is a type of cast iron in which the graphite is present in a short, stubby, wormlike form with rounded edges. With this type of structure CG iron possesses properties that are intermediate between gray and ductile cast iron. Based on the results obtained by Dawson and Schroeder (2004) the strength, ductility and toughness of compacted graphite iron are superior to gray iron and can approach those of ductile iron. Liu and Ding (1985) also concluded that machinability, thermal conductivity and damping capacity of compacted graphite iron are also superior to ductile iron and also produces less dross and is less susceptible to shrinkage porosity and carbide formation. For these reasons, compacted graphite iron has been considered for a number of applications such as ingot molds, brake drums, cylinder heads, valve bodies and engine blocks. Chemical composition and inoculation practice strongly influence the graphite shape in CG irons. Liu and Ding (1985)
∗
showed that some elements even in minor amount, affect the shape of the graphite in CG irons. The elements which are used for nodularizing of graphite are magnesium, cerium and calcium and the elements which act as denodularizer agents are titanium, tellurium, arsenic, antimony, lead, bismuth and aluminum. Elements which have great tendency to react with the above elements and restrict graphite growth are sulfur, oxygen and nitrogen. A number of techniques have been advocated for the production of compacted graphite iron. It has been produced by ineffective magnesium treatment, simultaneous treatment with spheroidizing and anti-spheroidizing elements, and inoculation with certain elements, e.g. rare earth method. The use of magnesium treatment has an extremely narrow range of magnesium contents in which compacted graphite can be produced. The prominent result which distinguished the research by Stefanescu and chairman (1998) from the others is that the presence of anti-spheroidizing elements (Ti, Al) extends the range of magnesium content.
Corresponding author. Tel.: +989177142041. E-mail addresses: hrab [email protected] (H.R. Abbasi), [email protected] (A.H. Yaghtin). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.04.015
1702
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1701–1705
Table 1 – Chemical composition (wt.%) of the base melt C
Si
Mn
S
P
Cu
3.4
2.1
0.25
0.01 0.016 0.065
Mg
Sn
Al
Ti
0.057
0.01
0.012 0.017
However, the use of those elements increases the cost of production and machining and also contaminates the scrap with undesirable elements for gray and ductile iron production. One of the methods for production of compacted graphite iron is addition of sulfur to magnesium treated melt that is the subject of investigation in this research. In this method, nodularizing treatment is performed with FeSiMg alloy and followed by graphite compacting treatment with inoculation by sulfur as mentioned by Chisamera and Riposan (1996). The amount of sulfur for inoculation of magnesium treated melt depends on residual magnesium after nodularizing treatment, holding time prior to pouring, casting section thickness and mold type (metal or sand mold). The higher the sulfur content, the higher the amount of treatment alloys needed. It is possible to have sulfur contents which are at very low levels, i.e. 0.015% and lower, but at these levels the nucleation of graphite is restricted. Based on the research conducted by Riposan et al. (2003), inoculation of the melt with sulfur, after magnesium treatment, changes the nodularity of graphite and the structure transforms from spheroidal graphite (SG) to compacted graphite. Liu and Ding (1985) came to this conclusion that inoculation of magnesium treated melt with sulfur makes it possible to obtain different ratios of SG/CG without using denodularizing elements such as titanium and aluminum. Therefore, sulfur can replace these elements in control of graphite nodularity, and returned melt is less harmful on production and machinability. As shown by Su and Chow (1992), for quantities higher than 0.080%, sulfur acts as a harmful element resulting in magnesium-neutralization and increased dross formation. In contrast, for lower quantities, sulfur is beneficial and essential because it promotes suitable nuclei for graphite precipitation.
2.
Experimental procedure
A medium frequency induction furnace was used to melt the base iron with the chemical composition shown in Table 1. Furnace charge for the above composition consisted of steel scrap, ductile and gray iron returns. The mold was made by CO2 process. Mixture of silica sand and liquid sodium silicate binder, followed by passing of CO2 gas through the mold was used. Tapping temperature was in the range of 1440–1460 ◦ C and the pouring temperature from the ladle to the mold was in the range of 1390–1420 ◦ C. Nodularizing treatment was carried out by sandwich method using FeSiMg alloy with magnesium content of 5% and sizing about 1–4 mm. Sulfur was added in the form of iron pyrite in different amounts in the reaction chamber of the mold. Iron pyrite with 28% sulfur content and a size distribution of about less than 200 m was used. Seven standard Y-blocks were cast from the melt. Final Mg content was about 0.057% in all the samples taken from the melt just
Table 2 – Chemical compositions (wt.%) with different sulfur levels C
Si
Mn
S
P
Cu
Mg
Sn
Al
3.4 3.4 3.4 3.4 3.4 3.4 3.4
2.1 2.1 2.1 2.1 2.1 2.1 2.1
0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.023 0.035 0.045 0.055 0.065 0.075 0.080
0.016 0.016 0.016 0.016 0.016 0.016 0.016
0.065 0.065 0.065 0.065 0.065 0.065 0.065
0.057 0.057 0.057 0.057 0.057 0.057 0.057
0.01 0.01 0.01 0.01 0.01 0.01 0.01
0.012 0.012 0.012 0.012 0.012 0.012 0.012
Ti 0.017 0.017 0.017 0.017 0.017 0.017 0.017
before pouring. The amount of sulfur added varied from 0.023 to 0.080%. Chemical compositions of the melts with different levels of sulfur are shown in Table 2. Test specimens were obtained from ASTM A536 Y-blocks. Two tensile test specimens were prepared for each sample according to ASTM E8 M standard and were tested using MTS universal machine with tension rate of 1 mm/min. Metallographic samples were sectioned, ground and polished with 0.3 m alumina powder. The etch solution of 1% Nital was used for investigation of microstructure of samples. The optical microscopy was used to evaluate the graphite shape. In order to measure the graphite aspect ratio, the image analyzer was used. Images from different positions were obtained from polished samples. Fifty graphite nodules from each sample were measured in length and width and the mean value was recorded as graphite aspect ratio as indicated in Eq. (1): graphite aspect ratio =
graphite length graphite width
(1)
Graphite aspect ratio was used as a criterion to investigate the graphite compactness.
3.
Results and discussion
3.1.
Graphite shape
Fig. 1 shows the microstructures that resulted from the addition of sulfur to the base melt with magnesium residual of 0.057% that was held constant for all sulfur levels. The addition of 0.023% sulfur (Fig. 1a) as iron pyrite to the base ductile melt had little or no effect on nodularity of graphite and nodularity was not affected up to 0.030% sulfur. Increased levels of sulfur in the range of 0.035–0.080% changed the graphite shape from spheroidal to compact and eventually to flake-like form as demonstrated in Fig. 1b–g. Photomicrographs presented in these figures indicate that by increasing sulfur level, different amounts of compacted graphite were observed in the structure. 0.035% sulfur in sample no. 2 (Fig. 1b) was the minimum amount necessary for the beginning of the formation of compacted graphite. The most compacted graphite is obtained with samples no. 3–6 (Fig. 1c–f) for the range of 0.045–0.075% sulfur. Sample no. 6 (Fig. 1f) with 0.075% sulfur had about 95% compacted graphite and 5% nodular graphite. The detailed investigation of graphite shape by image analyzer showed that increasing of sulfur at constant magnesium level, changes the graphite aspect ratio from 1 for nodular graphite to about 74 for flake-like graphite
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1701–1705
1703
Fig. 1 – Optical micrographs of the graphite morphology at different sulfur levels and aspect ratios (100×).
as shown in Table 3. Increasing sulfur content lead to a transition from nodular graphite to compacted graphite in sample no. 2 (Fig. 1b) with aspect ratio of 2, and a transition from compacted graphite to flake-like graphite in sample no. 6 (Fig. 1f) with aspect ratio of 25.
The effects of sulfur addition on graphite morphology are as follows: Itofuji et al. (1983) claimed that the nodular graphite is characterized by growth on basal plane with spiral growth mechanism. A detailed study by Subramanian et al. (1982) on
1704
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1701–1705
Table 3 – Graphite aspect ratio and tensile properties Sample number 1 2 3 4 5 6 7
Aspect ratio 1 2 6 10 15 25 74
Sulfur content (wt.%)
Tensile strength (MPa)
0.023 0.035 0.045 0.055 0.065 0.075 0.080
graphite growth under the condition of controlled chemical composition in iron melt confirmed that removal of surface active elements such as sulfur and oxygen results in faceting of prism planes and affects their mobility. On the addition of elements like sulfur the prism face of graphite shows evidence of roughening and changes to nonfaceted, sinusoidally perturbed interface. These junctions act as sources of growth steps. Stefanescu and Loper (1981) revealed that in the absence of impurities, graphite crystal growth is characterized by faceted prism plane and the driving force needed for two-dimensional nucleation of new layer on prism plane exceeds the driving force required for spiral growth normal to basal plane to form nodular graphite. In the presence of impurities such as sulfur and oxygen that are present in untreated cast iron, the mobility of prism plane is dramatically increased. The result is flake-like graphite growth competitively with austenite. The works of Murthy and Seshan (1954) showed that the crystallographic growth direction of compacted graphite in growth tips can be either in c-direction or in a-direction separately or in a- and c-direction simultaneously. However the progressive front of graphite is a-axes. This indicates that crystallographic growth direction of compacted graphite can change from c-axes to a-axes and vice versa. When the concentration of nodularizing elements in growth front is sufficient, graphite growth in a-axes is restricted and grows as spheroidal graphite. On the other hand when concentration of nodularizing elements is not sufficient to restrict growth in a-direction, compacted graphite will grow. In this condition the concentration of nodularizing elements in growing tips vary by growth progress. When the nodularizing elements were absorbed mainly on prism plane of graphite, the growth of graphite in c-direction leads to extension of prism planes and melt interface and therefore the concentration of nodularizing elements on prism planes decreases and therefore the growth of graphite is extended in a-direction clearly. With developing of graphite growth in a-direction, the concentration of nodularizing elements on prism plane is increased, therefore the graphite growth is restricted in a-direction and growth is developed in c-direction as demonstrated by Stefanescu et al. (1979). As previously presented in the results and discussion the best sulfur range to obtain commercial compacted graphite iron with at least 80% compacted graphite with no flake graphite, was 0.045–0.075% in constant magnesium level of 0.057%. In sulfur levels lower than this range, nodular graphite was obtained and in levels higher than that, flake graphite appeared in the microstructure as shown in Fig. 1.
556 450 365 373 375 221 198
3.2.
Yield strength (MPa) 488 339 297 304 292 150 132
Elongation (%) 12.4 7.1 5.8 5.4 5.8 1.1 0.6
Mechanical properties
The examination of tensile test specimens showed that increasing of sulfur decreased the tensile properties. This is clearly evident from the results presented in Table 3. The lower strength values at higher sulfur levels can be attributed to graphite morphology and interface area of graphite with matrix. With increasing of sulfur due to the above mentioned mechanisms, graphite morphology changes from spheroidal to flake type which leads to the following phenomena. The interface of graphite with matrix increases and the area which is loaded in tension decreases which leads to decreasing in tensile strength. Also this change leads to increasing graphite aspect ratio and local stress concentration in graphite tips. The brittle fracture behavior of cast irons under tensile loading is controlled by the crack initiation stage. It can therefore be expected that increasing stress concentration due to reduced roundness of graphite tips, with higher aspect ratio, can act to decrease the tensile stress required to initiate fracture. This relationship between graphite aspect ratio and tensile strength can be clearly observed in Fig. 2 Compacted graphite like flake graphite is interconnected but in contrast to flake type, graphites are thicker, shorter and the ends are rounded. This graphite morphology results in better elongation values than that of gray iron. The mean value of elongation for compacted graphite iron and gray iron is 5.5 and 0.5%, respectively. The results also indicated that the elongation of the samples with compacted graphite was higher than that of flake graphite, although in samples with compacted
Fig. 2 – Effect of graphite aspect ratio on tensile and yield strength.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1701–1705
1705
With increasing graphite aspect ratio, the tensile strength and elongation decrease. The best sulfur range to obtain commercial compacted graphite iron was 0.045–0.075% in constant magnesium level of 0.057%.
Acknowledgement The authors wish to thank Iran Tractor Foundry Company (ITFCo.) that helped us to carry out the research presented in this paper.
references
Fig. 3 – Effect of graphite aspect ratio on elongation.
graphite, higher sulfur levels results in lower elongation, as shown in Fig. 3. In spite of degradation of mechanical properties by increasing of sulfur level, tensile strength and elongation for compacted graphite iron are higher than those of gray iron. Therefore compacted graphite iron can replace with gray iron in applications that need higher tensile strength and elongation.
4.
Conclusions
Based on the results obtained from the effect of in-mold sulfur addition to magnesium treated iron melt the following conclusions can be made: Addition of sulfur made it possible to produce compacted graphite iron from the base ductile iron melt with magnesium level that is suitable for the production of ductile iron. Sulfur has complex effect in magnesium treated irons. Intermediate sulfur levels in the range of 0.045–0.075% changed the graphite morphology to satisfactory compacted form as needed in compacted graphite iron. With increasing amount of sulfur in constant magnesium level, the graphite aspect ratio varied from 1 for nodular graphite to about 74 for flake graphite.
Chisamera, M., Riposan, I., 1996. S-Inoculation of Mg-treated cast iron to obtain CG cast iron and improve graphite nucleation in DI. AFS Transactions, 581–588. Dawson, S., Schroeder, T., 2004. Practical applications for compacted graphite iron. AFS Transactions, 1–9. Itofuji, H., Kawano, Y., Inoyama, N., Yamamoto, S., Chang, B., Nishi, T., 1983. the formation mechanism of compacted/vermicular graphite in cast irons. AFS Transactions, 831–840. Liu, J., Ding, N.X., 1985. Effect of type and amount of treatment alloy on compacted graphite produced by the flotret process. AFS Transactions, 675–688. Murthy, V.S.R., Seshan, S., 1954. Characteristics of compacted graphite cast iron. AFS Transactions, 373–380. Riposan, I., Chisamera, M., Kelly, R., Barstow, M., Naro, R.L., 2003. Magnesium-sulfur relationships in ductile and compacted graphite cast iron as influenced by late sulfur additions. AFS Transactions, 830–844. Stefanescu, D.M., chairman, 1998, ASM Handbook, Volume 15, Casting, Compacted Graphite iron, ninth ed. pp. 667–671. Stefanescu, D.M., Loper, C.R., 1981. Recent progress in the compacted/vermicular graphite cast iron field. Giesserei-Prax, 73–77, No. 5. Stefanescu, D.M., Dinescu, I., Craciun, S., Popescu, M., 1979, Production of vermicular graphite cast irons by operative control and correction of graphite shape. Paper 37 presented at the 46th International Foundry Congress, Madrid. Su, J.Y., Chow, C.T., 1992. Solidification behavior of compacted graphite. AFS Transactions, 565–574. Subramanian, S.V., Kay, D.R., Purdy, G.R., 1982. Compacted graphite morphology control. AFS Transactions, 589–603.