Chilling Tendency and Chill of Cast Iron

TSINGHUA SCIENCE AND TECHNOLOGY ISSN 1007-0214 11/20 pp177-183 Volume 13, Number 2, April 2008

Chilling Tendency and Chill of Cast Iron E. Fraś**, M. Górny, W. Kapturkiewicz, H. López† AGH University of Science and Technology, Reymonta 23, 30-059 Cracow, Poland; † University of Wisconsin-Milwaukee, P.O. Box 784, Milwaukee, WI 53201, USA Abstract: An analytical expression is presented for the susceptibility of liquid cast iron to solidify according to the Fe-C-X metastable system (also known as the chilling tendency of cast iron, CT). The analysis incorporates the nucleation and growth processes associated with the eutectic transformation. The CT is related to the physicochemical state of the liquid, the eutectic cells in the flake graphite, and the number of nodules in nodular cast iron. In particular, the CT can be related to the critical wall thickness, scr, or the chill width, Wcr, in wedge shaped castings. Finally, this work serves as a guide for understanding the effect of technical factors such as the melt chemistry, the spheroidizing and inoculation practice, and the holding time and temperature on the resultant CT and chill of the cast iron. Theoretical calculations of scr and Wcr compare well with experimental data for flake graphite and nodular cast iron. Key words: chill; chilling tendency; gray cast iron; nodular cast iron

Introduction The susceptibility of liquid cast iron to solidify according to the Fe-C-X metastable system (i.e. the chilling tendency，CT) of cast iron dictates its subsequent performance in many applications. In particular, cast irons possessing a high chilling tendency are prone to develop zones of white or mottled iron. Considering that these regions can be extremely hard, their machinability can be severely impaired. Alternatively, if white iron is the desired structure, a relatively small chilling tendency f1avours the formation of grey iron which in turn leads to poor hardness and wear properties for the cast components. Hence, considerable effort has been made to correlate the various factors affecting the chill of cast iron such as chemical composition[1-4], pouring temperature[2], spheroidization and inoculation treatment[2,3,5,6], casting geometry[7], plate thickness[2,3,7], mold material[8], and nodule count[1]. Received: 2007-06-06

﹡ ﹡To whom correspondence should be addressed. E-mail: [email protected]

Furthermore, some works[4,5] have given qualitative descriptions of the influence of the chemical composition and spheroidization and inoculation practice on the CT. These experimental relationships are very useful but are limited in their physical meaning. Accordingly, this work presents analytical expressions that explain the chill formation mechanism. For the sake of simplicity, this paper focuses on phenomena and results which still hold when one ignores the segregation effects of alloying elements, such as silicon, manganese, including the so-called inverse chill.

1

Analysis

A theoretical analysis of the solidification of cast iron[9,10] showed that the critical wall thickness, scr, (below which, the chill is formed) and the width of the total chill, W (ASTM A367-55T Standard) can be given by scr = 2 pCT (1)

W=

4np CT cos (α /2 )

(2)

Tsinghua Science and Technology, April 2008, 13(2): 177-183

178

where for flake graphite cast iron, p=

a π

B = ln

1/ 6

⎛ 2 T ⎞ ⎜ 3 2 ⎟ L c φ ef ⎠ ⎝ e 5

3 s

(3)

⎡ ⎛ b 1 CT = ⎢ exp ⎜ 3 8 ⎢⎣ N s (1 − f γ ) µ ∆Tsc ⎝ ∆Tsc

⎞⎤ ⎟⎥ ⎠ ⎦⎥

1/ 6

(4)

⎤ ⎥ ⎥⎦

1/ 6

(5)

and for ductile cast iron, 1/ 6

⎛ ⎞ Ts3 p=a ⎜ 5 2 2 2 4 ⎟ ⎝ 4 π β B Le z c ⎠ 1 CT = 1/ 2 D

⎡ ⎛ b ⎞⎤ 1 exp ⎜ ⎢ ⎟⎥ 2 ⎝ ∆Tsc ⎠ ⎥⎦ ⎣⎢ N s β ∆Tsc

(6) 1/ 3

(7)

or

⎤ 1 ⎡ 1 CT = 1/ 2 ⎢ 2 ⎥ D ⎣ N cr β ∆Tsc ⎦

1/ 3

(8)

∆Tsc = Ts − Tc

(9)

φ = cef B1 + cB2

(10) Table 1

Tl , Ts

B1 = ln

cef = c +

or

⎡ 1 CT = ⎢ 3 8 ⎢⎣ N cr (1 − f γ ) µ ∆Tsc

Ti , T1

B2 = ln

Ti T1

Lγ Tlγ − Ts

z =0.41+0.93B

(11) (12)

(13)

In these equations CT is the chilling tendency of the cast iron; Ns and b are the nucleation coefficients for flake graphite or nodular cast iron; fγ is the proeutectic austenite volume fraction; a is the material mould ability to absorb heat; cef is the effective specific heat of the proeutectic austenite; Ti is the initial metal temperature just after filling the mould; φ is the heat transfer coefficient; α is the wedge angle; ∆Tsc = Ts − Tc and c, D, Le , Lγ , Tl , Tlγ , Ts , Tc , µ , and β are defined in Table 1; Ncr is the critical cells or nodule count at temperature T≈Tc; and n is the wedge size coefficient which expresses the influence of wedge size on the chill width. The ProductLog [y]=x is the Lambert function, also known as the omega function which can be easily calculated by means of the instruction ProductLog [y] in Mathematica™.

Selected thermophysical data

Parameter

Value

Latent heat of graphite eutectic

Le=2028.8 J/cm3

Latent heat of austenite

Lγ=1904.4 J/cm3

Specific heat of cast iron

c=5.95 J/(cm3·℃)

Growth coefficient of graphite eutectic

µ= ( 9.2 − 6.3Si 0.25 ) × 10−6 cm/(℃2·s)

Material mould ability to absorb heat

a=0.10 J/(cm2·s1/2·℃)

The diffusion coefficient of carbon in austenite

D = 3.9 10−6 cm2/s

Coefficient related with the slopes of the solubility lines JE’, E’S’, and BC’

β = 0.001 55℃–1

in Fe-C system Liquidus temperature for austenite

Tl=[1636 –113(C+0.25Si+0.5P)]℃

Formation temperature for cementite eutectic

Tc=[1130.56+4.06(C–3.33Si–12.58P)]℃

Graphite eutectic equilibrium temperature

Ts=[1154.0+5.25Si–14.88P]℃

Carbon content in graphite eutectic

Ce=(4.26–0.30Si–0.36P)%

Maximum carbon content in austenite at Ts

Cγ=(2.08–0.11Si–0.35P)%

Liquidus temperature of austenite for austenite composition Cγ

Tlγ=[1636–113(2.08+0.15Si+0.14P)]℃

Weight fraction of austenite

gγ=(Ce–C)/(Ce–Cγ)

Austenite density

ργ=7.51 g/cm3

Melt density

ρm=7.1 g/cm3

Volume fraction of proeutectic austenite

fγ=ρm gγ/[ργ+gγ(ρm–ργ)]

Note: C, Si, and P indicate content of carbon, silicon, and phosphorus in the cast iron, %.

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E. Fraś et al：Chilling Tendency and Chill of Cast Iron

The theoretical predictions agree qualitatively with the available data in the literature. In particular, it is well known that the chill of cast iron decreases with increasing eutectic cell or nodule count N (and in consequence Ncr) as a result of inoculation, as well as decreased heating times and decreased bath superheating temperature[11]. It is also known that increasing C and Si in the cast iron reduces the chill[11]. An increase in the carbon (1) increases the eutectic cell count in uninoculated cast iron or the number of nodules (Ncr)[11], (2) decreases the proeutectic austenite fraction fγ (Table 1), (3) slightly narrows the ∆Tsc range (Eq. (9) and Table 1) and (4) decreases the austenite liquidus temperature Tl. Points 1 and 2 are the main reason for the reduction in the chill by carbon (Eqs. (1) and (2)). An increase in the silicon (1) reduces the growth coefficient for graphite eutectic µ (Table 1), (2) increases the cell or the number of nodules (Ncr)[11], (3) decreases the proeutectic austenite fraction fγ (Table 1), (4)

Fig. 1

widens the ∆Tsc range (Eq. (9) and Table 1), and (5) decreases the austenite liquidus temperature Tl. The effects of increases in the number of cells or nodules and of the proeutectic austenite fraction are the most important and as a result silicon reduces the chill (Eqs. (1) and (2)). In addition, the critical plate wall thicknesses, scr, and the wedge chill widths, W, according to Eqs. (1), (2), (3), and (6) increase as the ability of the mold to absorb heat, a, increases. scr and W also depend on B, B1, and B2 (Eq. (11)) and, hence, on the initial temperature, Ti, of the cast iron just after pouring into the mould. Higher pouring temperatures, Tp, will result in higher Ti. Therefore, decreasing Tp reduces φ parameter (Eq. (10)), thus increasing scr and W (Eqs. (1) and (2)). A schematic diagram showing the role of the various factors on the chilling tendency and the chill of cast iron is given in Fig. 1.

Schematic representation of the effect of various factors on the chilling tendency

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180

2

Experimental Procedure

2.1

Flake graphite cast iron

Experimental melts were made in a 15-kg capacity crucible of an electric induction furnace. The raw materials were pig iron, steel scrap, commercially pure silicon, and Fe-P and Fe-S alloys. Melting was followed by liquid iron superheating up to 1420℃ and inoculation using FOUNDRYSIL with a 0.2-0.5-cm granulation and added as 0.5 % of the total charge weight. After various time intervals (1.5, 5, 10, 15, 20, and 25 min) from the instant of inoculation, the cast iron was poured into plate shaped molds of s=0.6, 1.0, 1.6, 2.2, and 3.0 cm in thickness. The average chemical composition of cast iron was 3.18% C, 1.91% Si, 0.13% Mn, 0.092% P, and 0.064% S. In all cases, the plates had a common gating system. The foundry molds were prepared using conventional molding sand. In addition, they were instrumented with Pt/PtRh10 thermocouples enclosed in quartz sleeves. The thermocouple tips were located in the geometric center of each mold cavity. An Agilent 34970A electronic module was used to record the cooling curves which were used to determine the initial metal temperature Ti just after the mold filling and the maximum undercooling,

∆Tm, at the onset of eutectic solidification. After cooling, specimens were taken for metallographic examination from the geometric centers of the plates. Metallographic examinations were made on polished and etched (stead reagent) specimens to show the graphite eutectic cell boundaries. The planar microstructure was characterized by the average number of eutectic cells NF per unit area (cell count)[12] N + 0.5 N w + 1 NF = i (14) F

where Ni is the number of eutectic cells inside a rectangle S, Nw is the number of eutectic cells that intersect the sides of S but not their corners and F is the surface area of S. The average number of eutectic cells N per unit volume (volumetric cell count) was given by[13] N ≅ 0.568 N F3 / 2

(15) o

Wedges with Bw=1.25 cm and α=28.5 and samples for chemical composition measurement were also cast simultaneously with the plates. Figure 2 shows typical planar microstructures of the wedges. The width, W, of the total chill and the critical cell count, Ncr, were measured at the junction of the gray structure with the first spot of cementite as shown in Fig. 2.

Fig. 2 Exhibited microstructure in wedge-shaped of flake graphite castings

2.2

Ductile cast iron

The test melts were made in an electric induction furnace having 8000 kg capacity. The raw materials were iron scrap, steel scrap, and commercially pure silicon. After melting and preheating at 1485℃, the cast iron was poured into a casting ladle where it was spheroidized using the cored wired injection method. Different inoculants in various amounts were used. The aim

of using different inoculants and inoculation processes was to induce different maximum undercoolings, ∆Tm, and various nodule counts N. The average chemical composition of the nodular iron was 3.69% C, 2.63% Si, 0.42% Mn, 0.02% P, 0.02% S, and 0.04% Mg. The nodular cast iron was poured into the same molds as for the gray cast iron. The cooling curves and metallographic structure were examined in the same way as for the gray cast iron. In the nodular cast iron the

181

E. Fraś et al：Chilling Tendency and Chill of Cast Iron

graphite nodules were characterized by Raleigh distributions[14] so the volumetric nodule count, N, could be related to the planar nodule count, NnF, using the Wiencek equation[15] N =

N F3 f gr

(16)

where fgr is the volume fraction of graphite at room temperature, fgr≈0.11-0.14.

3

Results and Discussion

3.1

Flake graphite cast iron

to determine the nucleation coefficients Ns and b by statistical methods (shown in Table 2). In all cases, the correlation coefficients were high (0.86-0.99) so the relationship between N and ∆Tm described by Eq. (17) is in good agreement with the experimental evidence. The nucleation coefficients Ns and b can be related to a dimensionless time td = t / tr , where t is the time calcu-

lated from the instant in which the inoculant was introduced into the melt and tr is the time when the observed changes in the cell count were negligible (tr=25 min). N s = ( 6.5 − 0.8 td − 5.3 td 2 ) × 106 cm −3 ,

The experimentally determined ∆Tm and N were used with[16,17] ⎛ b ⎞ N = N s exp ⎜ − ⎟ ⎝ ∆Tm ⎠

b = (96.9 + 122.6 td − 59.2 td 2 )℃

(18)

(17) Table 2 Experimental and calculated results

Case No. I/1

td

∆Tsc/℃

Nucleation coefficients −3

Ns/cm —

Measured

Calculated

total width of

total width of

CT/( s1/2·℃–1/3)

b/℃

chill (mm)

chill (mm)

35.2

5

1.6×10

76.8

7.9

8.8

1.10

I/2

0.06

51.1

6.1×106

104

3.3

3.6

0.43

I/3

0.20

52.3

6.8×106

119

3.6

3.6

0.44

I/4

0.40

52.3

4.4×106

135

4.0

4.1

0.50

I/5

0.60

51.5

5.4×106

154

4.5

5.4

0.65

152

4.8

5.2

0.63

162

5.5

5.8

0.71

I/6

0.80

53.0

1.3×106

I/7

1.00

52.9

8.4×105

Note: Mean temperature (just after pouring) of wedge Ti ≈ 1270℃, n=0.877

The chilling tendency, CT, and the total width of the chill, W, can be estimated from Eqs. (2) and (4) as functions of the chemical composition of the cast iron, the nucleation coefficients, the mean initial temperature of wedge, Ti, the wedge size coefficients n in the note of Table 2, and thermophysical data in Table 1. The results in Table 2 show that the theoretical predictions agree well with the experimental results. 3.2

Ductile cast iron

The experimental data for ductile iron ∆Tm and N can be used with Eq. (17) to calcutate the melt nucleation coefficients, Ns and b, listed in Table 3. The correlation coefficients for all the melts are quite high (0.84-0.98). It is not surprising that the nucleation coefficients Ns and b for each melt differ. However, in each case, N

and ∆Tm are related as in Eq. (17), thus confirming that the theory is in good agreement with the experimental results. Table 3 Nucleation coefficients, temperature ranges, ∆Tsc, and chilling tendencies, CT, for graphite Melt

Nucleation coefficients

No.

Ns/cm−3

1

4.13×107 8

b/℃ 58

∆Tsc/ ℃

CT/(s1/2·℃–1/3)

59.7

1.14

2

2.95×10

100

51.9

0.90

3

5.69×107

60

56.9

1.09

4

4.01×107

42

58.9

1.07

5

4.24×107

20

61.2

0.90

6

7

5.16×10

20

60.9

0.84

7

4.85×107

43

59.2

1.00

Note: CT is calculated by Eq. (7).

Tsinghua Science and Technology, April 2008, 13(2): 177-183

182

The chilling tendency CT can be calculated from Eq. (7) as a function of the chemical composition of the cast iron, Ns and b, listed in Table 3, the mean temperature of the metal just after pouring, Ti≈1260℃, and any relevant thermophysical data (shown in Table 1). Table 4 shows results for plate-shape castings reported in the literature[4] with various chemical compositions and wall thicknesses, as well as the number of nodules and the cementite fraction. In these cases, the nucleation coefficients b and Ns are not known, so CT is calulated using Eq. (8). The results in Table 4 show that in melts I and II, chill starts to occur in walls with thicknesses between 3 and 6 mm, while in melt III chill starts to occur for wall thicknesses between 1.5 and 2 mm. scr must be calculated using Eq. (1) to compare these results with the theoretical predictions. These calculations assumed that a=0.11 J/(cm2 · s1/2 · ℃) and Ti=1250℃ with other information taken from Table 1. The results in Table 4 show that in melt I the

change of the wall thickness, s, from 6 mm to 3 mm is closely linked to the change in the number of nodules from 588 to 1039 mm−2. As a result, an average nodule count of NF,cr=813 mm−2 was used in this work. Similar determinations were made for melts II and III with NF,cr values of 945 and 1959 mm−2, respectively. scr for melts I, II, and III were then 3.4 mm, 3.1 mm, and 1.8 mm as shown in Table 4. In addition, comparison of s and scr in Table 4 indicates that the predictions from the theoretical analysis are in good agreement with the experimental determinations of the wall thickness at which chill occurs. Table 4 shows the CT for the three melts calculated using Eq. (8) and data in Table 1. The results in Table 4 show that as the chilling tendency increases from 0.34 to 0.68 s1/2·℃–1/3, the critical wall thickness, scr, increases from 1.8 to 3.4 mm.

Table 4 Chemical composition, wall thicknesses, nodule count, cementite fraction, and chilling tendency for three ductile cast iron melts Melt No.

C/%

Si/%

P/%

Wall thickness (mm)

Nodule count

Fraction of

(NF/mm−2)

cementite (%)

588

0.0

0.68

1039

9.0

—

2.0

1380

24.0

—

1.5

1311

34.0

—

854

0.0

0.60

Experimental s 6.0

I

3.40

2.70

0.046

3.0

Calculated scr 3.4

6.0 II

III

3.45

3.31

2.91

4.42

0.044

0.051

1037

7.3

—

2.0

1100

24.0

—

6.0

1127

0.0

—

3.0

1726

0.0

—

1890

0.0

0.34

2027

9.3

—

3.0

2.0 1.5

4

3.1

CT/( s1/2·℃–1/3)

Conclusions

(1) A simple theoretical analysis is presented which predicts the chilling tendency and chill in flake graphite and ductile cast iron based on The chemical composition of the casting which for flake graphite cast iron is characterized by ∆Tsc, fγ, T, µ, Ncr or Ns, and b, and for ductile cast iron is characterized by ∆Tsc, D, Tl, Ncr or Ns and b; The inoculation and spheroidization method, the superheating temperature, and bath holding

1.8

times as characterized by Ncr or Ns and b; Selected thermophysical data of the metal and mold material as well as the metal temperature just after pouring into the mold. (2) Theoretical calculations of the width of the total chill in wedges for flake graphite and of the critical wall thickness for nodular cast iron compare well with experimental data. (3) The absolute values of the chilling tendency of cast iron are calculated to range from 0.3 to 1.1 s1/2·℃–1/3.

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E. Fraś et al：Chilling Tendency and Chill of Cast Iron

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