A R C H I V E S o f

F O U N D R Y E N G I N E E R I N G

Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

ISSN (1897-3310)Volume 8

Issue 4/2008

236 – 240

44/4

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 8 , I s s u e 4 / 2 0 0 8 , 2 3 6 - 2 4 0 236

Crystallization of nodular cast iron

with carbides

S. Pietrowski, G. Gumienny*

Department of Materials Engineering and Production Systems, Technical University of Łódź Stefanowskiego 1/15 St., 90-924 Łódź, Poland

*Corresponding author. E-mail address: grzegorz.gumienny@p.lodz.pl

Received 14.07.2008; accepted in revised form 21.07.2008

Abstract

In this paper a crystallization process of nodular cast iron with carbides having a different chemical composition have been presented. It have been found, that an increase of molybdenum above 0,30% causes the ledeburutic carbides crystallization after (γ+ graphite) eutectic phase crystallization. When Mo content is lower, these carbides crystallize as a pre-eutectic phase. In this article causes of this effect have been given. Keywords: Theory of crystallization, TDA method, Nodular cast iron with carbides

1. Introduction In papers [1÷3] the possibility obtaining carbides in nodular

cast iron with Cr, Mo, Cu and Ni was presented. Phases separate during its crystallization were identified. The possibility of control of carbides amount and volume depending on the chemical composition and a cooling rate was found. The aim of this work was the presentation of the crystallization process of cast iron having a different chemical composition and giving causes of proceed processes.

2. Work methodology

Cast iron was melted in an 0,5Mg induction furnace. Its spheroidization was made using „Sandwich” method. SB5 inoculant was given on the metal stream during cast iron pouring to the casting ladle.

The chemical composition of tested cast iron and its degree of eutectic saturation Sc are showed in Table 1.

Table 1. Chemical composition of tested cast iron and its degree of eutectic saturation Sc

Chemical composition, % Cast

C Si Mn Cr Cu Mo Ni

Sc

1 3,15

÷ 3,29

2,40 ÷

2,52

0,28 ÷

0,55

0,52 ÷

1,23

0,97 ÷

1,50

0,23 ÷

0,25

0,98 ÷

1,05

0,91 ÷

0,97

2 3,21

÷ 3,31

2,35 ÷

2,43

0,20 ÷

0,48

0,50 ÷

1,19

0,95 ÷

1,40

0,20 ÷

0,26

4,68 ÷

5,00

0,97÷ 0,99

3 3,41

÷ 3,55

2,48 ÷

2,59

0,27 ÷

0,55

0,52÷ 1,30

0,92 ÷

1,35

0,50 ÷

0,56

4,79÷ 4,95

1,02÷ 1,08

Impurities content in groups of tested cast iron was amounts to: Pmax = 0,05%, Smax = 0,01%.

A degree of eutectic saturation Sc was calculated as following:

eut

CC C

MoNiCuCrMnSiCS

⋅−⋅+⋅+⋅−⋅−⋅+=

015,0053,0074,0063,0027,031,0 (1)

3. Results In Figure 1 (a, b) the representative microstructure (a) and

TDA curves (b) of pearlitic-ferritic nodular cast iron with carbides from group 1 is presented. a)

b)

microstrusture: spheroidal graphite, pearlite, ferrite,

ledeburitic carbides

AB C D E H K L

0 100 200 300 400 500

τ, s

800

900

1000

1100

1200

1300

t, o C

-4

-3

-2

-1

0

1

dt/d

τ, o C

/sdt/dτ = f'(τ)

t = f(τ)

Point τ, s t, °C dt/dτ, °C/s

A 33 1217 -1,25 B 49 1188 -2,35 C 104 1121 -0,44 D 127 1111 -0,52 E 152 1105 -0,03 H 324 992 -1,56 K 353 948 -1,35 L 367 928 -1,55

Fig. 1. The representative microstructure (a) and TDA curves (b) of nodular cast iron with carbides from group 1 containing: 3,15% C, 2,40% Si, 0,35% Mn, 0,52% Cr, 0,25% Mo, 0,97% Cu, 1,00%

Ni, Sc = 0,92

On the derivative curve there are thermal effects from crystallization of: AB – austenite, BCD – ledeburitic carbides, DEH – (γ + graphite) eutectic phase, HKL – secondary carbides from austenite. It is hypoeutectic cast iron (Sc = 0,92).

In Figure 2 a scheme of the crystallization process of this cast iron is presented. It begins from its over-cooling below liquidus line (line B1-C1, Fig. 2), when phase γ nucleuses create. After exceeding a critical size they transform into dendrites. On the derivative curve the thermal effect from a primary phase crystallization is described by points A and B (Fig. 1b). During its crystallization and growth, there is the highest concentration of carbon, chromium and molybdenum on the γ – liquid phases boundary. The carbon concentration in a liquid surrounding a γ phase increases along B1-C1 line, till C1 value at the eutectic phase temperature. To its start the rest of the liquid cast iron have to be Δt volume over-cooling, when the carbon concentration achieves x1 value (Fig. 2). Despite contents graphitizing elements in cast iron i.e. extending the range of the temperature of the eutectic phase crystallization in the stable (tES) and metastable (tEM) system, addiction of chromium and molybdenum causes a crystallization of part of cast iron according to the metastable system, as a ledeburitic carbides (thermal effect BCD, Fig. 1b). Carbides crystallization causes a decrease of chromium and molybdenum concentration in a residual liquid. It causes increase the range of the eutectic phase crystallization according to the stable (tES) and metastable (tEM) system. Then the liquid cast iron comes over-cooling only below tES temperature. Simultaneously a heat of the carbides crystallization causes an increase of the temperature of the residual liquid cast iron to the extended range between the stable and metastable system. Then the crystallization process of (γ + graphite) eutectic phase begins (thermal effect DEH, Fig. 1b). After the end of the eutectic phase crystallization process, from austenite secondary carbides separate (thermal effect HKL, Fig 1b).

200μm

50μm

Fig. 2. The scheme of the hypoeutectic cast iron crystallization

process

As a result of nodular cast iron crystallization a specified microsegregation of components elements forms. Its example is shown in Figure 3. A parabolic distribution of the graphitizing elements concentration is described as:

y = ax2 + bx + c (2)

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and exponential of elements counteracting graphitization as: y = c exp(ax2 + bx) (3)

Fig. 3. The scheme of hypoeutectic nodular cast iron structure and

a microsegregation of its component elements There is not in every case an elements microsegregation

described by (1) and (2) equations [5]. It depends on the γ dendrites size, graphite diameter and a distance between its separations. The lesser distance between graphite nodules the distribution of Si, Cu and Ni concentration in eutectic grain is more uniform. Nevertheless there is an increased concentration of carbide-forming elements on the austenite dendrite and the eutectic grain boundary. In discussed cast iron carbides crystallize close to austenite dendrites, where there is the highest concentration of C, Cr and Mo (Fig. 1 a, b). After the end of the carbides crystallization process, (γ + graphite) eutectic phase crystallization begins.

In Figure 4 (a, b) the microstructure (a) and component elements distribution maps (b) in discussing cast iron are shown.

a)

microstructure: spheroidal graphite, pearlite, ferrite,

ledeburitic carbides

b)

Fig. 4. The microstructure (a) and elements distribution maps (b)

in group 1 cast iron

Results from it nearly uniform distribution of Ni, Cu and Si and increased concentration of Cr and Mo elements in the ledeburitic carbide. It is caused by large graphite dispersion in tested cast iron; it is for example showed in Figure 5. In this case there is not always possible a component microsegregation in the eutectic grain.

50μm

Fig. 5. Exemplary distances amongst graphite separations in group 1 cast iron

Increase of nickel concentration to 4,75% caused changes

in a microstructure and TDA curves presented in Figure 6. Results from it, that the cast iron crystallization begins after an over-cooling below the liquidus line from dendrite austenite separation. The thermal effect from its crystallization is described by points A and B (Fig. 6b). The carbon content in a liquid increases till hypereutectic composition. Nickel increase to 4,75% caused decrease of the thermal effect from the eutectic phase crystallization according to the metastable system, because this element has an influence on an increase of tendency to the crystallization according to the stable system. This effect is described by points BD (Fig. 6b)

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 8 , I s s u e 4 / 2 0 0 8 , 2 3 6 - 2 4 0 238

a)

b)

microstructure: spheroidal graphite, martensite, pearlite,

ledeburitic carbides, retained austenite

dt/d = f'( )τ τ

τt = f( )

AB D E H I KL

0 100 200 300 400 500

τ, s

800

900

1000

1100

1200

1300

t, o C

-4

-3

-2

-1

0

1

dt/d

τ, o C

/s

Point τ, s t, °C dt/dτ, °C/s A 42 1177 -1,11 B 58 1156 -1,51 D 95 1120 -0,28 E 123 1117 -0,02 H 271 1062 -1,36 I 333 969 -1,54 K 348 947 -1,32 L 361 929 -1,46

Fig. 6. The representative microstructure (a) and TDA curves (b)

of martensitic-pearlitic nodular cast iron with carbides from group 2 containing: 3,25% C, 2,40% Si, 0,35% Mn, 0,52% Cr, 0,25%

Mo, 1,20% Cu, 4,75% Ni (Sc = 0,97)

From Fig. 6a results, that like in group 1 of cast iron, carbides crystallize on phase γ dendrites boundaries. However, they are smaller considerably. After decrease of Cr and Mo concentration in a liquid, it is possible the (γ + graphite) eutectic phase

crystallization (thermal effect DEH, Fig. 6b). Similarly like in group 1 cast iron, after solidification, from austenite separate secondary carbides (effect IKL, Fig. 6b).

An increase of C and Mo content, with a little alteration of another elements, caused changes in a microstructure and TDA curves presented in Figure 7 (a, b) a)

b)

200μm

200μm

50μm

50μm

microstructure: spheroidal graphite, martensite, carbides, retained austenite

E FG H L

0 100 200 300 400 500

τ, s

800

900

1000

1100

1200

1300

t, o C

-4

-3

-2

-1

0

1

dt/d

τ, o C

/sdt/dτ = f'(τ)

t = f(τ)

K

Point τ, s t, °C dt/dτ, °C/s E 77 1098 0,01 F 248 1006 -1,42 G 262 987 -0,98 H 289 952 -1,54 K 301 934 -1,41 L 310 922 -1,50

Fig. 7. The representative microstructure (a) and TDA curves (b) of martensitic nodular cast iron with carbides from group 3

containing: 3,45% C, 2,48% Si, 0,28% Mn, 0,60% Cr, 0,52% Mo, 1,39% Cu, 4,80% Ni, (Sc = 1,05)

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Results from it, that it is eutectic cast iron (Sc = 1,05), and its crystallization begins from a creation of graphite nucleuses in a liquid bath, which grow up in a globular form after exceeding the critical size. The graphite growth causes a decrease of the carbon content in a liquid close to the graphite – liquid phase boundary. After achieving hypoeutectic composition by the liquid, on a graphite an austenite envelop forms, because it is a good catalyst its nucleation. The austenite growth causes an increase of the carbon content in a liquid and its diffusion to the graphite through an austenite envelop, till achieving hypereutectic composition by the liquid. Chromium and molybdenum atoms are „pushed” by the crystallization front to the liquid. An increase of Cr and Mo concentration in the liquid causes a temperature reduction of the rest of the liquid, so carbides crystallize after an eutectic transformation. This process begins at a temperature of 1006°C (point F, Fig. 7b), at a temperature of 987°C there is a maximum thermal effect (point G, Fig. 7b) and at a temperature of 952°C there is the end of the crystallization (point H, Fig. 7b). The map of the component elements distribution of this cast iron is for example presented in Figure 8 (a, b). Results from it the increased Cr and Mo concentration in a carbide located on the eutectic grain boundary. Similarly like for previously described kinds of cast iron, after the end of the crystallization, from austenite secondary carbides separate (thermal effect HKL, Fig. 7b).

4. Conclusions

The results have indicated the following: • ledeburitic carbides (Fe,Cr,Mo)3C type in alloy cast iron can

crystallize before (γ + graphite) eutectic phase or after this process,

• the carbides crystallization temperature depends on the cast iron chemical composition,

• in hypoeutectic cast iron carbides crystallize on austenite dendrite boundaries, before (γ + graphite) eutectic phase crystallization,

• in eutectic cast iron carbides crystallize after (γ + graphite) eutectic phase crystallization process.

a)

b)

Fig. 8. The microstructure (a) and elements distribution (b) in

group 3 cast iron

References [1] S. Pietrowski, G. Gumienny, Ductile cast iron with carbides,

Archives of Foundry, No. 19, (2006), 233-238 (in Polish). [2] S. Pietrowski, G. Gumienny, Crystallization of ductile cast

iron with Mo, Cr, Cu and Ni, Archives of Foundry, No. 22, (2006), 406-413 (in Polish).

[3] S. Pietrowski, G. Gumienny, Carbides in Nodular Cast Iron with Cr and Mo, Archives of Foundry Engineering, Vol. 7, Issue 3, (2007), 223-230.

[4] E. Fraś, Crystallization of metals, WNT, Warsaw, 2003 (in Polish).

[5] K. Sękowski, Foundry Review, 8-9, (1973), 250 (in Polish).

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 8 , I s s u e 4 / 2 0 0 8 , 2 3 6 - 2 4 0 240