Formation of microstructures in the spheroidal graphite cast iron

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2012 IOP Conf. Ser.: Mater. Sci. Eng. 27 012064

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Formation of microstructures in the spheroidal graphite cast iron

S. Wang1, D. Daloz1, F. Bruneseaux2 and G. Lesoult1 1IJL-SI2M, UMR7198-CNRS, Nancy-Université, UPV-Metz, Ecole des Mines de Nancy, Parc de Saurupt, CS 14234, F-54502 NANCY, Cedex, France 2Centre de Recherche et Développement de St Gobain, Maidières-lès-Pont-à-Mousson, 54704 PONT-A-MOUSSON, Cedex, France

E-mail: dominique.daloz@mines.inpl-nancy.fr

Abstract. Pipeline systems for hydraulic networks are obtained via centrifugal casting of spheroidal graphite cast iron. The very high cooling rate that is achieved in the skin of the product can sometimes lead to carbide instead of graphite in cast iron. An experimental device has been built in the laboratory that allows reproducing the extreme thermal conditions encountered during formation of skin of centrifugally cast pipes. Liquid metal droplets fall on a cold substrate. Rapid directional solidification occurs. The temperature evolution of the lower surface of the droplet is recorded during the very first moment of the solidification (t < 200 ms) thanks to a photodiode, which is located below the substrate. The microstructures that are obtained in laboratory are characterised in both the as-cast state and the heat-treated state. They are compared to the centrifugally cast ones. A model of directional solidification of cast iron under a very large temperature gradient has been built. It allows explaining the transition from stable to metastable microstructure that was observed in the products and reproduced in the laboratory samples.

1. Introduction The thermal conditions and the treatment of the liquid metal for centrifugal casting of pipes lead to the solidification of the melt in the form of spheroidal graphite (SG) iron throughout the thickness. However, sometimes there are zones to be observed that are solidified within the white mode (eutectic austenite / cementite) mainly in the skin of the product. These areas lead to differences in the metallurgical response, which are problematic during the subsequent heat treatment.

The obtention of SG iron is controlled by the preparation of the liquid metal (magnesium treatment + inoculation treatment) and by the cooling mode. Under moderate cooling conditions, the influence and effectiveness of these treatments is known and various models are available in the literature to describe the solidification [1]. When the cooling rate increases as in the first skin of centrifugally cast pipe, one can observe a microstructural transition leading to the formation of white iron. This transition is traditionally explained in terms of maximum undercooling achieved for the nucleation of the cementite (competition of nucleation) and / or a growth competition between graphite and cementite. While the transition is well documented in the literature [2-7], most of the articles deal with the influence of cooling rate as the natural variable controlling the transition. This variable, of common practice does not satisfactorily account for describing into details the origin of the

The 3rd International Conference on Advances in Solidification Processes IOP PublishingIOP Conf. Series: Materials Science and Engineering 27 (2011) 012064 doi:10.1088/1757-899X/27/1/012064

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microstructures for a given composition: for this it is necessary to decompose the cooling rate in terms of the product of thermal gradient and solidification rate. The aim of this study is to characterise and understand the mechanisms responsible for the formation of the structure in the first skin of the centrifugal casting of SG iron. We describe in this article the experimental technique developed in order to capture the phenomena linked to the very beginning of solidification. We present the experimental results and the basis of a solidification model developed in order to interpret the results.

2. Experimental Technique

2.1. Falling droplet experiment. The falling droplet experiment allows the study of the first moments of the solidification of an alloy cast onto a metallic substrate that is instrumented and representative of the centrifugal die [8]. Its principle is to project a small mass of liquid iron, between 5 and 7 g on a substrate cooled by water circulation. The surface temperature of the melt that solidifies in contact with the substrate is measured using a silicon photodiode. The optical sight of the metal surface is carried out through a hole 0.3 mm in diameter drilled in the substrate. The main advantage of the device is its high acquisition speed (between 2.5 and 10 kHz) and its low dead time (less than 300 μs), allowing accurate monitoring of changes in cooling conditions in solidification. A schematic of the experimental device used and developed in the laboratory is shown in figure 1.

Figure 1. Falling droplet experiment

2.2. Metal preparation Industrially obtained spheroidal graphite cast iron in centrifugal casting as a part requires processing of liquid metal by adding magnesium just before pouring it together with inoculation of molten metal during casting by adding an inoculants onto the surface of the centrifugal shell. The device developed in the laboratory does not allow, in a reproducible way, to add the magnesium in the liquid metal just before it is ejected. To overcome this obstacle it was decided to overdose magnesium in the mother alloy. To determine the operating conditions similar to industrial conditions we determined the loss of magnesium associated with the superheat temperature above the liquidus and the holding time corresponding to this temperature. All the tests presented below were made from an hypereutectic cast iron of composition Fe-3,9%C-2.37%Si- 0.096%Mg (wt%). The overheating of the drop before ejection was set to 50° C for a time of 30s leading to a residual Mg content of 0.06% which is representative of industrial conditions. Inoculation of the cast iron was realised by addition of Spherix® in the same manner as the industrial practice.

The 3rd International Conference on Advances in Solidification Processes IOP PublishingIOP Conf. Series: Materials Science and Engineering 27 (2011) 012064 doi:10.1088/1757-899X/27/1/012064

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2.3. Metallographic preparation The samples were characterised metallographically by optical microscopy and scanning electron microscopy. The samples were cut, coated, and polished mechanically to finish with a diamond suspension particle (0.5 microns). The determination of the density of graphite nodules and the volume fraction of graphite was performed by image analysis. Each data represents the average of measurements on three different fields. A heat treatment similar to the industrial processing was performed on the as cast droplet.

3. Experimental results The evolution of the microstructure solidified from the substrate in the case of droplet or shell in the case of cast pipe after the heat treatment is shown in figure 2. The volume fraction of the graphite measured on the surface is plotted parallel. One can noticed that, after the heat treatment, the microstructures are similar in terms of spatial distribution (density of nodules) as well as the volume fraction of graphite. The volume fraction of graphite being in the range of 11% corresponds to the equilibrium volume fraction and is constant along the thickness. Figure 3 shows the microstructure of a centrifugally cast pipe in the as cast state (before heat treatment) and in the case of a droplet. The evolution of the volume fraction of graphite is reported vis-à-vis along the distance to the surface (product thickness). On the surface of the droplet carbides (metastable eutectic) are found. In both cases one observes an increase in the density of nodules along the thickness of the product correlated with changes in the volume fraction of graphite measured: 4 to 10% in the case of the pipe and 0,2 -3% in the case of droplet.

3.1. Structural evolution of the centrifugally cast pipe. In the as cast state the volume fraction of graphite along the product thickness increases form 4 to 10%. During the heat treatment, the volume fraction of graphite increases and becomes homogeneous along the thickness to a value of 11% and a constant density nodule (not reported in this study). The value of 11% after heat treatment is closer to 10% measured in the as cast state to 3 mm from the surface. The small difference reflects the fact that at this point the as cast pipe structure is already practically the equilibrium one.

3.2. Structural evolution of the droplet In the case of droplets one observes an area virtually devoid of graphite nodule with a thickness of about 200 μm, with a volume fraction of graphite measured in the range of 0.2%. Although initially greatly different, during the heat treatment the microstructure of droplets evolve similarly to that of the centrifugally cast pipe in terms of density and in volume fraction of graphite nodule: the as cast structure of solidification of the droplet can be considered as a precursor of the cast pipe microstructure. In that way these observations indicate the development of first skin microstructure of centrifugally cast pipe. The microstructural evolution during similar heat treatment indicates the presence and efficiency of nucleation sites of graphite in the droplet from the first moment of solidification. In the case of the pipe, these sites are also present but one can suppose that in this case, the thermal inertia of the product produced a first heat treatment in the as cast state leading to the difference in graphite volume fraction measured between the two as cast states.

4. Physical modelling

4.1. Description of the model The observation of the microstructure shows that even under very high thermal gradient the competition between the nucleation of the stable eutectic (graphite + austenite) and the growth of the metastable one occurs since the very beginning of solidification. To take this competition into account we built a simple physical model assuming that all the heat transfer proceeds by conduction within the

The 3rd International Conference on Advances in Solidification Processes IOP PublishingIOP Conf. Series: Materials Science and Engineering 27 (2011) 012064 doi:10.1088/1757-899X/27/1/012064

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substrate and solidification taking place directionally (1D modeling). A front tracking method is used based on [9] from which the position of the interface and the temperature profile within the droplet is determined at each time step.

Figure 3. Micrograph and volume fraction of graphite measured in the as cast state in the case of a droplet or a pipe. The bottom of the photograph corresponds to the chilling surface.

The heat equation is solved with finite difference method using an implicit scheme. A constant temperature at the outer surface of the product and a constant cooling rate at the liquid/substrate interface gives the boundary conditions. The following hypotheses/conditions has been used:

i) graphite nodules nucleate and grow since the liquid temperature fall below the graphite liquidus. For the sake of simplicity no thermal or chemical effect are taken into account associated with the growth of the nodule. The following equations (1-2) [3] are used to determine the nodule density.

dN = An (ΔTLg )n −1 d(ΔTL

g )

dtdt if 2

+*>

Δ

dt

Td gL (1)

2=dN if 2+*

<Δ

dt

Td gL (2)

Figure 2. Micrograph and volume fraction of graphite nodules measured in the heat-treated state in the case of a droplet or a pipe. The bottom of the photograph corresponds to the chilling surface.

Surface (shell)

Surface (Substrate)

Surface (Shell)

Surface (substrate)

The 3rd International Conference on Advances in Solidification Processes IOP PublishingIOP Conf. Series: Materials Science and Engineering 27 (2011) 012064 doi:10.1088/1757-899X/27/1/012064

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With N the number of graphite nodules at the time t. An and n, constant of the germination

law and g

LTΔ the local undercooling of the liquid relative to the graphite liquidus.

ii) The carbide/austenite eutectic grows once the liquid temperature fall below the metastable eutectic temperature, entrapping the graphite nodules. The relation between the interface temperature and the growth rate is determined from the equation (3) [3] :

DI

Ems KK

Tv

⋅⋅

Δ= βα6

+* 4

(3)

With ΔTEm the growth undercooling, KIα β a capillarity term and KD a solutal term. The

interlamellar spacing dL of the eutectic is then determined from (4):

sD

IL vK

Kd

⋅=

βα

(4)

In front of the metastable eutectic, graphite nodules nucleates and grow continuously before being captured, their density and size being controlled by the local undercooling determined by the thermal gradient in front of the metastable eutectic front.

4.2. Results Figure 4 shows the calculated evolution of the temperature in the droplet as a function of its thickness for a cooling rate of 350 K/s. We also state in this figure the graphite liquidus temperature (1513 K) and the metastable eutectic temperature (1384 K). One observes along the thickness the decrease in the growth undercooling of the metastable eutectic. This leads to a decrease in the solidification rate that consequently increase the time for the graphite nodules to nucleate and grow. In the figure 5 the calculated interlamellar spacing is shown together with the experimental measurements on the droplet and on the as cast pipe. One observes the increase of the calculated lamellar spacing with the solidified thickness in agreement with the decrease of the growth undercooling. The experimental data show a rather good correlation with the calculated data at the very beginning. Once 100 μm are solidified, the measured interlamellar spacing is higher than the predicted one, those concerning the droplets being smaller than the one concerning the as cast pipe.

Figure 4. Temperature evolution as a function of the distance to the surface

Figure 5. Lamellar spacing as a function of the distance to the surface.

The difference between measurements and calculated data indicates a decrease in the heat extraction rate while solidification proceeds, greater in the case of the as cast pipe than in the case of the droplet. This could be attributed to an increase of the heat transfer resistance that may be due to an air gap formation once a certain amount of the skin is formed, or/and a decrease of the radiative transfer

1200

1250

1300

1350

1400

1450

1500

1550

1600

0 500 1000 1500 2000 2500 3000 3500 4000

Tem

pera

ture

(K)

distance to the surface (µm)

5 ms 20 ms 80 ms 300 ms T*

TLiquidus Graphite

TE metastable

The 3rd International Conference on Advances in Solidification Processes IOP PublishingIOP Conf. Series: Materials Science and Engineering 27 (2011) 012064 doi:10.1088/1757-899X/27/1/012064

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(which is not taken into account in the model) or to an extra reheating by hot liquid in the case of the as cast pipe (which is much larger than the droplet).

Concerning the nodules density, preliminary results indicates a good agreement with the measured density, provided that the An term of the nucleation law (equation (1)) is fixed to An = 4000.

5. Conclusions The aim of this work is to get a deeper understanding on the structure formation of the nodular cast iron at the first instant of solidification. An experimental setup has been built together with a simple physical model. We have shown that:

- the falling droplet technique was able to froze the solidified microstructure in an earlier stage of formation than observed in the as cast pipe. Since the very first moment, graphite nodules are already present even in the skin of the product. Due to their small size they represent a low volume fraction (<0,2%) in the droplet and (4%) in the as cast pipe.

- The physical model allows determining the thermal evolution during solidification, the interface temperature, velocity and structural parameters like the lamellar spacing.

- A good agreement between the experimental measurement and the lamellar spacing is observed in the very beginning of solidification. While solidification proceeds, the difference between measurements and calculated data increases. We suggest that this is correlated to a combination of an air gap formation/ radiative effect. A further difference exist between the droplet and the as cast pipe that leads to larger lamellar spacing in the as cast pipe. This may be correlated to the “in situ” heat treatment leading to the higher volume fraction of graphite measured in the as cast pipe.

A good agreement is obtained concerning the experimental measurement of the nodule density and the calculated one, provided the nucleation law parameters are fixed to a rather “high” value.

References [1] Lesoult G - Traité des matériaux 5 : Thermodynamique des matériaux - Presses

polytechniques et université romandes 2010, ISBN 978-2-88074-690-2, 873 [2] Lacaze J, Castro M, Aichoun N, Lesoult G 1989 - Revue de Métallurgie 2 81 [3] Lacaze J, Castro M, Lesoult G 1998 Acta Metallurgica 46 997 [4] Lesoult M, Castro M, Lacaze J 1998 Acta Metallurgica 46 983 [5] Magnin P, Kurz W 1988 Met. Mat. Trans. A 19 1955 [6] Favreau J R, Lesoult G, Turpin M 1974 Mem. Scien. Rev. Met. 4 [7] Stefanescu D M 2007 Met. Mat. Trans. 38 1433 [8] Mizukami H, Suzuki T, Umeda T 1991 Testu-to-Hagané, 77 134 [9] Rangel R H, Bian X 1995 Numerical Heat Trans. Part A: Applications 28, No. 5, 589

The 3rd International Conference on Advances in Solidification Processes IOP PublishingIOP Conf. Series: Materials Science and Engineering 27 (2011) 012064 doi:10.1088/1757-899X/27/1/012064

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