- Email: [email protected]
Optimization of the Chemical Composition of Cast Iron Used for Casting Ball Bearing Grinding Disks
TSINGHUA SCIENCE AND TECHNOLOGY ISSN 1007-0214 09/20 pp164-169 Volume 13, Number 2, April 2008
Optimization of the Chemical Composition of Cast Iron Used for Casting Ball Bearing Grinding Disks Aurel Crisan**, Sorin Ion Munteanu, Ioan Ciobanu, Iulian Riposan† Department of Materials Science and Engineering, Transilvania University, Brasov 500036, Romania; † Department of Materials Science and Engineering, Politehnica University, Bucharest 060042, Romania Abstract: The chemical composition of cast iron used for casting ball bearing machining disks was varied to optimize the properties such as castability, hardenability, and durability in ball machining. The cast iron characteristics were most strongly dependent on the Ni content and the carbon saturation degree, Sc. This paper describes the types of test specimens, the working conditions, and the experimental results. The increase of the degree of carbon saturation reduces the tendency to form shrinkholes in the castings. The decrease in the Ni content negatively affects the final hardening treatment. A way to control solidification defects in cast iron, by reducing the Ni content, has been verified on cast disks. Key words: cast iron; micro-shrinkhole; shrinkage; shape factor; solidification
Introduction Ball bearing polishing/filing disks are cast parts with forms and dimensions established by internal norms of manufacturers and users[1]. The disks are medium size parts with masses of 100-1000 kg and a relatively simple geometric configuration, with a rim and a single cut out, with an optional girder. From a technical viewpoint, the manufacturing of these disks raises a number of specific problems for plate type parts with relatively thick walls (40-80 mm) related to the solidification as well as the heat treatment that determines their final characteristics. As end products, the disks have two surfaces with different utilities, as shown in Fig. 1. For disks with H≥70 mm, the usable area Hu = H−40 mm. The working surface has cut channels to ensure that the ball bearings circulate during polishing. After some time of using, when the channel reaches the maximum allowable value, a cutting operation is carried out and a new channel is cut. This operation can be repeated Received: 2007-10-07; revised: 2007-10-31
﹡﹡To whom correspondence should be addressed. E-mail: [email protected]
can be repeated until the usable disk thickness is completely used (see Fig. 1). The disks must have a hardness of 440-520 HB on the working surface, after heat treatment (with a superficial maximum variance of 30 HB) and a minimum 400 HB at the limit of the usable disk thickness. Further, the usable zone of the disk must have no casting defects detectable by nondestructive methods (such as contraction pores, blowholes, or inclusions).
Fig. 1 Using zones and surfaces of the disks
1
Experimental Methodology
A set of samples was made to investigate the relationship between the chemical composition of the cast iron
Aurel Crisan et al：Optimization of the Chemical Composition of Cast Irons Used for …
used to cast the disks and the tendency to form certain specific defects during solidification using the mold shown in Fig. 2[2,3]. Samples 1, 2, and 3 were created to study the casting properties of the cast iron, while Sample 4 was used to study the material behavior after the final heat treatment[4].
1, Lower semi-mould; 2, Upper semi-mould; 3, Separation core; 4, Sample 1; 5, Sample 2; 6, Sample 3; 7, Distribution channel; 8, Feeder for sample 1; 9, Sample 4
Fig. 2 Separation plane of the mould and section A-A through mould (unit: mm)
The placement of the cast iron samples in the mould allowed analysis of the concentrated shrinkhole volume (for the type 1 samples with vertically upwards directional solidification under the influence of the type 2 samples), of the shrinkhole volume for inversed direction solidification (for the type 2 samples with its solidification influenced by the type 1 samples), and of the volume and aspect ratios of shrinkholes in the absence of directional solidification (for the type 3 samples with solidification characteristics close to those of a spherical body). The feed system to these samples was designed to avoid shrinkage compensation with
165
liquid metal during solidification or cooling after casting. The moulds were made of raw sand with a separation core of sodium silicate based sand hardened with carbon dioxide. The cast iron was smelted in an electric arc furnace with similar conditions to those applied in the casting of the disks. Table 1 shows the chemical composition of the 15 charges of hypoeutectic cast iron. The Ni composition was varied, which essentially determined the variation of the carbon saturation degree, Sc, and hence affected the tendency to create certain solidification defects and the hardenability of the cast iron[5-7]. The main casting property monitored during the present study was the tendency to create shrinkholes during solidification, in the form of macro-or microshrinkholes, since this type of defect significantly affects the service life of disks and may even cause them to be scrap (depending on the “healthy” thickness of the cast disk as shown in Fig. 1)[4]. The volume of an open shrinkhole Vret was determined by direct measurement. For a closed shrinkhole, within the context of sample solidification considered to be microshrinkholes, the volume was determined by weighing the samples in air and water and calculating the porosity P using Eq. (1). ρ (G − Ga ) − ρ a GR P= f R × 100% (1) ρf (GR − Ga )
where ρf is the theoretical density of the cast iron (which is a function of the chemical composition), ρa = 1 g / cm3 is the density of water, GR is the mass of the samples in air, and Ga is the mass of the samples in water. To as correctly as possible express the link between the chemical composition and the tendency of shrinkhole occurrence, both the micro-shrinkhole and open shrinkhole volumes were expressed (by direct measurements and computations) in cm3 as well as in %. The total shrinkhole volume, Vtret, was found by adding the micro- and open shrinkhole volumes. For the percentage expressions, the volume of each type of shrinkhole was related to the entire volume for the sample Vret Vret (2) Φret = = Vsample + Vtret Vsample + Vporosity + Vret where Φ ret represents the proportion of the open
Tsinghua Science and Technology, April 2008, 13(2): 164-169
166
Table 1 Chemical composition of the cast iron after casting No.
C
Si
Mn
P
S
Cu
Cr
Ni
Sc
1
3.111
1.69
0.67
0.053
0.0086
0.43
0.64
0.56
0.837
2
3.074
1.74
0.65
0.057
0.0270
0.43
0.63
0.80
0.831
3
3.063
1.80
0.65
0.053
0.0170
0.43
0.63
0.54
0.832
4
3.175
1.75
0.71
0.056
0.0128
0.42
0.65
0.56
0.859
5
3.009
1.82
0.69
0.131
0.0313
0.33
0.41
1.73
0.824
6
2.760
2.10
0.66
0.128
0.0384
0.33
0.41
1.10
0.776
7
2.788
2.23
0.66
0.115
0.0211
0.33
0.41
0.74
0.792
8
2.685
2.02
0.65
0.131
0.0310
0.32
0.41
3.18
0.749
9
2.696
2.03
0.64
0.137
0.0316
0.32
0.40
3.21
0.753
10
2.816
1.68
0.64
0.130
0.0335
0.32
0.40
3.29
0.783
11
2.823
1.84
0.63
0.125
0.0328
0.35
0.51
3.26
0.774
12
2.792
1.90
0.63
0.134
0.0429
0.35
0.51
4.78
0.770
13
2.896
1.95
0.63
0.139
0.0400
0.35
0.52
5.07
0.804
14
2.672
1.89
0.63
0.110
0.0208
0.35
0.51
4.40
0.735
15
2.674
2.03
0.63
0.120
0.0356
0.34
0.52
7.26
0.746
shrinkhole volume to the entire volume, Vret is the volume of the open shrinkholes (cm3), Vporosity is the volume of the micro-shrinkholes (cm3), and Vsample is the theoretical volume of the sample (cm3) (calculated as the ratio of the weight of the sample in air and its theoretical density). The possibility of directional solidification with the specified conditions for each casting sample was quantified by a directional index of solidification, Id, expressed as the ratio of Vret and Vtret measured in cm3 V I d = ret × 100% (3) Vtret
2
Experimental Results
The dependence between the chemical composition of the cast iron and the tendency to form solidification contraction defects was fit to a second degree polynomial regression curve, thus facilitating the identification and selection of intervals with the optimum characteristics. 2.1
(%)
Correlation between the chemical composition of cast iron and the open shrinkhole volume
The dependence of the open shrinkhole volume, Vret, on Sc (shown in Fig. 3) can be explained for each sample type by their solidification mechanism. An increase of Sc generates more shrinkholes. Therefore,
the open shrinkhole volume may increase. This effect is augmented by directional solidification, as seen in the type 1 samples with the vertically directed solidification in opposition to type 3 samples where the solidification was not influenced by external factors. The effect of the directional solidification can also be observed in the type 2 samples where the directional solidification was vertically down (so the solidification was in the opposite direction to gravity). Therefore, an increase of Sc can create concentrated shrinkholes. The experiments show that in this case (type 2 samples) there was not an open shrinkhole at the upper side of the samples. The results in Fig. 3b show that the Ni content, wNi had a strong influence on the open shrinkhole volumes. In absence of directional solidification in type 3 samples the increase of wNi reduces the tendency to form open shrinkholes relative to the results for the type 1 and 2 samples. With strong directional solidification (as in the type 1 samples) a minimum open shrinkhole volume occurs at around wNi = 4%. For the reversed directional solidification (type 2 samples) a maximum open shrinkhole volume occurs around wNi = 3%. These dependencies were also observed for the microshrinkhole volume, the total shrinkhole volume, and the index of directional solidification, as will be presented later.
Aurel Crisan et al：Optimization of the Chemical Composition of Cast Irons Used for …
167
by the fact that in the absence of sufficient directional solidification, the micro-shrinkhole volume is not sufficiently localized, so that only a part of the resulting volumes will be micro-shrinkholes, based on the specific considerations of the present study (volumes isolated from the exterior). Therefore, the increase of Sc eventually has an effect of reducing the tendency to form micro-shrinkholes.
Fig. 3 Influence of (a) the carbon saturation degree, Sc, and (b) the Ni content on the concentrated shrinkhole volume, Φret
2.2
Influence of chemical composition on the micro-shrinkhole volumes (porosity)
In the present study the micro-shrinkhole volume was assumed to be equal to the total volume of closed holes in the cast samples. Although a small number of these holes may be due to gas in the cast alloy, considering the similar casting conditions for the samples, it was assumed that the effect of gas microporosity does not affect the study results. The data in Fig. 4a shows that for the type 1 samples, an increase of Sc tends to reduce the occurrence of micro-shrinkhole. This reduction is enhanced at higher values of Sc. For the type 2 and 3 samples, the microporosity increases with increasing Sc. This increase, however, becomes smaller as Sc increases and tends to stop entirely for Sc>0.84. The explanation for this type of dependency can be found in the evolution of the solidification mechanism of the cast iron as the chemical composition changed[8]. The specific growth pattern of the type 2 and 3 samples can be accounted for largely
Fig. 4 Influence of (a) the carbon saturation degree, Sc, and (b) the Ni content on the micro-shrinkholes volume (Φporosity)
As the directional solidification intensifies, the effect of wNi on the occurrence of micro-shrinkholes also changes. For the type 1 samples with directional solidification, the increase in wNi increases the microshrinkhole volume. For the reversed directional solidification, the increase in wNi slowly reduces the tendency to form micro-shrinkholes. The strongest reduction of the micro-shrinkholes volume was observed in the type 3 samples. This effect of wNi can be explained by its balancing influence on the structure of the liquid state immediately prior to solidification, thus enabling expansion of the two-phase domain during
168
solidification. With more intensive directional solidification with a developed two-phase area, the microshrinkholes are more likely to develop. Thus, to avoid the widespread occurrence of microshrinkholes, the effect of wNi in the cast iron must be linked to the intensity of the directional solidification. 2.3
Influence of chemical composition on the total shrinkhole volume
The data in Fig. 5a shows that for all sample types, the total shrinkhole volume tends to increase with increasing carbon saturation of the cast iron. This tendency is very low in the type 1 samples, higher in the type 2 samples, and highest in the type 3 samples. In each case, the differences were determined by the different solidification effect in the three sample types. Thus as the directional solidification intensifies, the total shrinkhole volume is less influenced by the increase in the degree of carbon saturation.
Fig. 5 Influence of (a) the carbon saturation degree, Sc, and (b) the Ni content on the total shrinkhole volume
The influence of wNi on the total shrinkhole volume (Fig. 5b) also depends on the cast iron solidification
Tsinghua Science and Technology, April 2008, 13(2): 164-169
conditions. The type 1 samples have a minimum around wNi = 3%. The type 2 and 3 samples have a maximum at around the same Ni content, with a significant decrease in the total shrinkhole volume beyond this value, especially for the type 3 samples. A plausible explanation is that the liquid structure is such that during solidification the interface is no longer fed liquid metal. 2.4
Influence of chemical composition on the directional solidification index
The directional solidification index, Id, is influenced by the carbon saturation degree Sc due to the solidification mechanism. Thus, for the type 1 samples (with strong directional solidification), the increase in the carbon saturation degree increases the directional solidification effect (Fig. 6a). In the type 2 and 3 samples, the increase of the carbon saturation degree results in a slight decrease of the directional solidification capacity. In the type 3 samples, this effect occurs only for lower
Fig. 6 Influence of (a) the carbon saturation degree, Sc, and (b) the Ni content on the directional solidification index, Id
Aurel Crisan et al：Optimization of the Chemical Composition of Cast Irons Used for …
values of the carbon saturation. For carbon saturations exceeding 0.82, the directional solidification growth rate increases. This type of dependence is typical. The relationship for the type 2 samples results from the manner in which the volumes of the macro- and microshrinkholes are defined. The increase of wNi has no significant influence on the directional solidification index. In the type 1 samples (Fig. 6b) the increase in wNi causes a slight decrease in the directional solidification capacity, while the type 2 samples exhibit a maximum, the type 3 samples exhibit a minimum. For wNi less then 3%, the largest directional solidification indices are found in the type 1 samples, so the directional solidification is not affected.
3
Conclusions
Increasing of the degree of carbon saturation, Sc, reduces the tendency to form shrinkholes in the castings. This effect is augmented by the directional solidification, as observed in the type 1 samples, where the micro-shrinkhole volume decreases for higher Sc. Higher Sc, means the solidification is more easily directed for the same cooling conditions, which could be used to reduce the microporosity in the utilisable zone of the cast disks. To achieve adequate directional solidification for the type 1 samples, wNi should be lower than approximately 3.5%. Although Table 1 shows that for the samples with various combinations of Sc and wNi for their composition, a decrease in wNi negatively affects the final hardening treatment. Reduced wNi provides a good way to control solidification defects in
169
cast iron, which has been verified on cast disks. The properties of the cast iron after the final heat treatment have also been investigated[5]. References [1]
Crisan A, Masnita M, Munteanu S I, Ciobanu I. Study on the characteristics of cast irons used for the casting of bearing ball grinding disks. In: Proceedings of International Conference on Materials Science and Engineering. Bramat, Romania, 2003, 1: 244 -249.
[2]
Kurz W, Fischer D J. Fundamentals of Solidification. Switzerland-Germany-UK-USA: Trans. Tech. Publication, 1986.
[3]
Stefanescu D M. Science and Engineering of Casting Solidification. New York: Kluwer Academic/ Plenum, 2002.
[4]
Crisan A, Milosan I, Neagu A, Moasa D, Iuga I, Orlandea L. Study about the correlation between cast irons chemical composition and compactness in casting ball bearings grinding disks. Revista de Turnatorie, 2002, (4): 21-24. (in Romanian)
[5]
Crisan A, Munteanu S I, Ciobanu I. Chemical composition effect on cast irons hardening behavior. Recent Review, Brasov, Romania, 2004, (12): 24-28. (in Romanian)
[6]
Barton R. Porosity of ductile iron. Foundry Trade Journal, 1984, 160: 235-239.
[7]
Yu S K, Loper C R, Cornell H H. The Effect of molybdenum, copper and nickel on the microstructure, hardness and hardenability of ductile cast irons. Transactions of American Foundrymen’s Society, 1986, 94: 557-576.
[8]
Hecht M. Tendency to create porosity in flaked graphite cast irons. Fonderie, 1980, (2): 43-46. (in French)
Optimization of the Chemical Composition of Cast Iron Used for Casting Ball Bearing Grinding Disks Aurel Crisan**, Sorin Ion Munteanu, Ioan Ciobanu, Iulian Riposan† Department of Materials Science and Engineering, Transilvania University, Brasov 500036, Romania; † Department of Materials Science and Engineering, Politehnica University, Bucharest 060042, Romania Abstract: The chemical composition of cast iron used for casting ball bearing machining disks was varied to optimize the properties such as castability, hardenability, and durability in ball machining. The cast iron characteristics were most strongly dependent on the Ni content and the carbon saturation degree, Sc. This paper describes the types of test specimens, the working conditions, and the experimental results. The increase of the degree of carbon saturation reduces the tendency to form shrinkholes in the castings. The decrease in the Ni content negatively affects the final hardening treatment. A way to control solidification defects in cast iron, by reducing the Ni content, has been verified on cast disks. Key words: cast iron; micro-shrinkhole; shrinkage; shape factor; solidification
Introduction Ball bearing polishing/filing disks are cast parts with forms and dimensions established by internal norms of manufacturers and users[1]. The disks are medium size parts with masses of 100-1000 kg and a relatively simple geometric configuration, with a rim and a single cut out, with an optional girder. From a technical viewpoint, the manufacturing of these disks raises a number of specific problems for plate type parts with relatively thick walls (40-80 mm) related to the solidification as well as the heat treatment that determines their final characteristics. As end products, the disks have two surfaces with different utilities, as shown in Fig. 1. For disks with H≥70 mm, the usable area Hu = H−40 mm. The working surface has cut channels to ensure that the ball bearings circulate during polishing. After some time of using, when the channel reaches the maximum allowable value, a cutting operation is carried out and a new channel is cut. This operation can be repeated Received: 2007-10-07; revised: 2007-10-31
﹡﹡To whom correspondence should be addressed. E-mail: [email protected]
can be repeated until the usable disk thickness is completely used (see Fig. 1). The disks must have a hardness of 440-520 HB on the working surface, after heat treatment (with a superficial maximum variance of 30 HB) and a minimum 400 HB at the limit of the usable disk thickness. Further, the usable zone of the disk must have no casting defects detectable by nondestructive methods (such as contraction pores, blowholes, or inclusions).
Fig. 1 Using zones and surfaces of the disks
1
Experimental Methodology
A set of samples was made to investigate the relationship between the chemical composition of the cast iron
Aurel Crisan et al：Optimization of the Chemical Composition of Cast Irons Used for …
used to cast the disks and the tendency to form certain specific defects during solidification using the mold shown in Fig. 2[2,3]. Samples 1, 2, and 3 were created to study the casting properties of the cast iron, while Sample 4 was used to study the material behavior after the final heat treatment[4].
1, Lower semi-mould; 2, Upper semi-mould; 3, Separation core; 4, Sample 1; 5, Sample 2; 6, Sample 3; 7, Distribution channel; 8, Feeder for sample 1; 9, Sample 4
Fig. 2 Separation plane of the mould and section A-A through mould (unit: mm)
The placement of the cast iron samples in the mould allowed analysis of the concentrated shrinkhole volume (for the type 1 samples with vertically upwards directional solidification under the influence of the type 2 samples), of the shrinkhole volume for inversed direction solidification (for the type 2 samples with its solidification influenced by the type 1 samples), and of the volume and aspect ratios of shrinkholes in the absence of directional solidification (for the type 3 samples with solidification characteristics close to those of a spherical body). The feed system to these samples was designed to avoid shrinkage compensation with
165
liquid metal during solidification or cooling after casting. The moulds were made of raw sand with a separation core of sodium silicate based sand hardened with carbon dioxide. The cast iron was smelted in an electric arc furnace with similar conditions to those applied in the casting of the disks. Table 1 shows the chemical composition of the 15 charges of hypoeutectic cast iron. The Ni composition was varied, which essentially determined the variation of the carbon saturation degree, Sc, and hence affected the tendency to create certain solidification defects and the hardenability of the cast iron[5-7]. The main casting property monitored during the present study was the tendency to create shrinkholes during solidification, in the form of macro-or microshrinkholes, since this type of defect significantly affects the service life of disks and may even cause them to be scrap (depending on the “healthy” thickness of the cast disk as shown in Fig. 1)[4]. The volume of an open shrinkhole Vret was determined by direct measurement. For a closed shrinkhole, within the context of sample solidification considered to be microshrinkholes, the volume was determined by weighing the samples in air and water and calculating the porosity P using Eq. (1). ρ (G − Ga ) − ρ a GR P= f R × 100% (1) ρf (GR − Ga )
where ρf is the theoretical density of the cast iron (which is a function of the chemical composition), ρa = 1 g / cm3 is the density of water, GR is the mass of the samples in air, and Ga is the mass of the samples in water. To as correctly as possible express the link between the chemical composition and the tendency of shrinkhole occurrence, both the micro-shrinkhole and open shrinkhole volumes were expressed (by direct measurements and computations) in cm3 as well as in %. The total shrinkhole volume, Vtret, was found by adding the micro- and open shrinkhole volumes. For the percentage expressions, the volume of each type of shrinkhole was related to the entire volume for the sample Vret Vret (2) Φret = = Vsample + Vtret Vsample + Vporosity + Vret where Φ ret represents the proportion of the open
Tsinghua Science and Technology, April 2008, 13(2): 164-169
166
Table 1 Chemical composition of the cast iron after casting No.
C
Si
Mn
P
S
Cu
Cr
Ni
Sc
1
3.111
1.69
0.67
0.053
0.0086
0.43
0.64
0.56
0.837
2
3.074
1.74
0.65
0.057
0.0270
0.43
0.63
0.80
0.831
3
3.063
1.80
0.65
0.053
0.0170
0.43
0.63
0.54
0.832
4
3.175
1.75
0.71
0.056
0.0128
0.42
0.65
0.56
0.859
5
3.009
1.82
0.69
0.131
0.0313
0.33
0.41
1.73
0.824
6
2.760
2.10
0.66
0.128
0.0384
0.33
0.41
1.10
0.776
7
2.788
2.23
0.66
0.115
0.0211
0.33
0.41
0.74
0.792
8
2.685
2.02
0.65
0.131
0.0310
0.32
0.41
3.18
0.749
9
2.696
2.03
0.64
0.137
0.0316
0.32
0.40
3.21
0.753
10
2.816
1.68
0.64
0.130
0.0335
0.32
0.40
3.29
0.783
11
2.823
1.84
0.63
0.125
0.0328
0.35
0.51
3.26
0.774
12
2.792
1.90
0.63
0.134
0.0429
0.35
0.51
4.78
0.770
13
2.896
1.95
0.63
0.139
0.0400
0.35
0.52
5.07
0.804
14
2.672
1.89
0.63
0.110
0.0208
0.35
0.51
4.40
0.735
15
2.674
2.03
0.63
0.120
0.0356
0.34
0.52
7.26
0.746
shrinkhole volume to the entire volume, Vret is the volume of the open shrinkholes (cm3), Vporosity is the volume of the micro-shrinkholes (cm3), and Vsample is the theoretical volume of the sample (cm3) (calculated as the ratio of the weight of the sample in air and its theoretical density). The possibility of directional solidification with the specified conditions for each casting sample was quantified by a directional index of solidification, Id, expressed as the ratio of Vret and Vtret measured in cm3 V I d = ret × 100% (3) Vtret
2
Experimental Results
The dependence between the chemical composition of the cast iron and the tendency to form solidification contraction defects was fit to a second degree polynomial regression curve, thus facilitating the identification and selection of intervals with the optimum characteristics. 2.1
(%)
Correlation between the chemical composition of cast iron and the open shrinkhole volume
The dependence of the open shrinkhole volume, Vret, on Sc (shown in Fig. 3) can be explained for each sample type by their solidification mechanism. An increase of Sc generates more shrinkholes. Therefore,
the open shrinkhole volume may increase. This effect is augmented by directional solidification, as seen in the type 1 samples with the vertically directed solidification in opposition to type 3 samples where the solidification was not influenced by external factors. The effect of the directional solidification can also be observed in the type 2 samples where the directional solidification was vertically down (so the solidification was in the opposite direction to gravity). Therefore, an increase of Sc can create concentrated shrinkholes. The experiments show that in this case (type 2 samples) there was not an open shrinkhole at the upper side of the samples. The results in Fig. 3b show that the Ni content, wNi had a strong influence on the open shrinkhole volumes. In absence of directional solidification in type 3 samples the increase of wNi reduces the tendency to form open shrinkholes relative to the results for the type 1 and 2 samples. With strong directional solidification (as in the type 1 samples) a minimum open shrinkhole volume occurs at around wNi = 4%. For the reversed directional solidification (type 2 samples) a maximum open shrinkhole volume occurs around wNi = 3%. These dependencies were also observed for the microshrinkhole volume, the total shrinkhole volume, and the index of directional solidification, as will be presented later.
Aurel Crisan et al：Optimization of the Chemical Composition of Cast Irons Used for …
167
by the fact that in the absence of sufficient directional solidification, the micro-shrinkhole volume is not sufficiently localized, so that only a part of the resulting volumes will be micro-shrinkholes, based on the specific considerations of the present study (volumes isolated from the exterior). Therefore, the increase of Sc eventually has an effect of reducing the tendency to form micro-shrinkholes.
Fig. 3 Influence of (a) the carbon saturation degree, Sc, and (b) the Ni content on the concentrated shrinkhole volume, Φret
2.2
Influence of chemical composition on the micro-shrinkhole volumes (porosity)
In the present study the micro-shrinkhole volume was assumed to be equal to the total volume of closed holes in the cast samples. Although a small number of these holes may be due to gas in the cast alloy, considering the similar casting conditions for the samples, it was assumed that the effect of gas microporosity does not affect the study results. The data in Fig. 4a shows that for the type 1 samples, an increase of Sc tends to reduce the occurrence of micro-shrinkhole. This reduction is enhanced at higher values of Sc. For the type 2 and 3 samples, the microporosity increases with increasing Sc. This increase, however, becomes smaller as Sc increases and tends to stop entirely for Sc>0.84. The explanation for this type of dependency can be found in the evolution of the solidification mechanism of the cast iron as the chemical composition changed[8]. The specific growth pattern of the type 2 and 3 samples can be accounted for largely
Fig. 4 Influence of (a) the carbon saturation degree, Sc, and (b) the Ni content on the micro-shrinkholes volume (Φporosity)
As the directional solidification intensifies, the effect of wNi on the occurrence of micro-shrinkholes also changes. For the type 1 samples with directional solidification, the increase in wNi increases the microshrinkhole volume. For the reversed directional solidification, the increase in wNi slowly reduces the tendency to form micro-shrinkholes. The strongest reduction of the micro-shrinkholes volume was observed in the type 3 samples. This effect of wNi can be explained by its balancing influence on the structure of the liquid state immediately prior to solidification, thus enabling expansion of the two-phase domain during
168
solidification. With more intensive directional solidification with a developed two-phase area, the microshrinkholes are more likely to develop. Thus, to avoid the widespread occurrence of microshrinkholes, the effect of wNi in the cast iron must be linked to the intensity of the directional solidification. 2.3
Influence of chemical composition on the total shrinkhole volume
The data in Fig. 5a shows that for all sample types, the total shrinkhole volume tends to increase with increasing carbon saturation of the cast iron. This tendency is very low in the type 1 samples, higher in the type 2 samples, and highest in the type 3 samples. In each case, the differences were determined by the different solidification effect in the three sample types. Thus as the directional solidification intensifies, the total shrinkhole volume is less influenced by the increase in the degree of carbon saturation.
Fig. 5 Influence of (a) the carbon saturation degree, Sc, and (b) the Ni content on the total shrinkhole volume
The influence of wNi on the total shrinkhole volume (Fig. 5b) also depends on the cast iron solidification
Tsinghua Science and Technology, April 2008, 13(2): 164-169
conditions. The type 1 samples have a minimum around wNi = 3%. The type 2 and 3 samples have a maximum at around the same Ni content, with a significant decrease in the total shrinkhole volume beyond this value, especially for the type 3 samples. A plausible explanation is that the liquid structure is such that during solidification the interface is no longer fed liquid metal. 2.4
Influence of chemical composition on the directional solidification index
The directional solidification index, Id, is influenced by the carbon saturation degree Sc due to the solidification mechanism. Thus, for the type 1 samples (with strong directional solidification), the increase in the carbon saturation degree increases the directional solidification effect (Fig. 6a). In the type 2 and 3 samples, the increase of the carbon saturation degree results in a slight decrease of the directional solidification capacity. In the type 3 samples, this effect occurs only for lower
Fig. 6 Influence of (a) the carbon saturation degree, Sc, and (b) the Ni content on the directional solidification index, Id
Aurel Crisan et al：Optimization of the Chemical Composition of Cast Irons Used for …
values of the carbon saturation. For carbon saturations exceeding 0.82, the directional solidification growth rate increases. This type of dependence is typical. The relationship for the type 2 samples results from the manner in which the volumes of the macro- and microshrinkholes are defined. The increase of wNi has no significant influence on the directional solidification index. In the type 1 samples (Fig. 6b) the increase in wNi causes a slight decrease in the directional solidification capacity, while the type 2 samples exhibit a maximum, the type 3 samples exhibit a minimum. For wNi less then 3%, the largest directional solidification indices are found in the type 1 samples, so the directional solidification is not affected.
3
Conclusions
Increasing of the degree of carbon saturation, Sc, reduces the tendency to form shrinkholes in the castings. This effect is augmented by the directional solidification, as observed in the type 1 samples, where the micro-shrinkhole volume decreases for higher Sc. Higher Sc, means the solidification is more easily directed for the same cooling conditions, which could be used to reduce the microporosity in the utilisable zone of the cast disks. To achieve adequate directional solidification for the type 1 samples, wNi should be lower than approximately 3.5%. Although Table 1 shows that for the samples with various combinations of Sc and wNi for their composition, a decrease in wNi negatively affects the final hardening treatment. Reduced wNi provides a good way to control solidification defects in
169
cast iron, which has been verified on cast disks. The properties of the cast iron after the final heat treatment have also been investigated[5]. References [1]
Crisan A, Masnita M, Munteanu S I, Ciobanu I. Study on the characteristics of cast irons used for the casting of bearing ball grinding disks. In: Proceedings of International Conference on Materials Science and Engineering. Bramat, Romania, 2003, 1: 244 -249.
[2]
Kurz W, Fischer D J. Fundamentals of Solidification. Switzerland-Germany-UK-USA: Trans. Tech. Publication, 1986.
[3]
Stefanescu D M. Science and Engineering of Casting Solidification. New York: Kluwer Academic/ Plenum, 2002.
[4]
Crisan A, Milosan I, Neagu A, Moasa D, Iuga I, Orlandea L. Study about the correlation between cast irons chemical composition and compactness in casting ball bearings grinding disks. Revista de Turnatorie, 2002, (4): 21-24. (in Romanian)
[5]
Crisan A, Munteanu S I, Ciobanu I. Chemical composition effect on cast irons hardening behavior. Recent Review, Brasov, Romania, 2004, (12): 24-28. (in Romanian)
[6]
Barton R. Porosity of ductile iron. Foundry Trade Journal, 1984, 160: 235-239.
[7]
Yu S K, Loper C R, Cornell H H. The Effect of molybdenum, copper and nickel on the microstructure, hardness and hardenability of ductile cast irons. Transactions of American Foundrymen’s Society, 1986, 94: 557-576.
[8]
Hecht M. Tendency to create porosity in flaked graphite cast irons. Fonderie, 1980, (2): 43-46. (in French)