091215 A study on Ductile Iron Production without the use

091215

A study on Ductile Iron Production without the use of Feeders

Vasilios Fourlakidis

Swerea SWECAST AB Box 2033, 550 02 Jönköping

Telefon 036 - 30 12 00 Telefax 036 - 16 68 66

[email protected] http://www.swereaswecast.se

©2009, Swerea SWECAST AB

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CONTENTS Page TILLKOMST 1 SAMMANFATTNING 1 ORIGIN 2 ABSTRACT 2 1 INTRODUCTION 2 2 LITERATURE SURVEY 3 2.1 Solidification of ductile iron 3 2.2 Shrinkage porosities 4 2.3 Variables influencing the formation of shrinkages 5 2.3.1 Pouring Temperature 5 2.3.2 Magnesium Content 6 2.3.3 Inoculation 6 2.3.4 Carbon Content 7 2.4 Thermal analysis 8 2.5 Casting simulation 9 3 DESIGN OF EXPERIMENTAL HEATS AND SIMULATION 10 4 EXPERIMENT 11 5 RESULTS AND DISCUSSION 13 5.1 Porosities 13 5.2 Thermal analysis 18 6 CONCLUSIONS 22 7 REFERENCES 23

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TILLKOMST

Denna publikation utgör slutrapport för projektet G790J. Detta projekt behandlar ämnet stelningskrympning i segjärn. Rapporten har sammanställts av Vasilios Fourlakidis, metallurgi och materialteknik, Swerea SWECAST AB, Jönköping. Gjutningsexperi-ment har utförts av Sune Jansson och Vasilios Fourlakidis i Nya Arvika Gjuteri. Arbetet i detta projekt har finansierats via medlemsmedel.

SAMMANFATTNING

Detta projekt har undersökt hur Lantanhalt och gjuttemperatur påverkar stelningskrympning i segjärn. Före försöken utfördes gjutsimulering för att förutsäga uppkomst av krympporositeter och optimera försöks-planering. Försök med kommersiella ympmedel och MgFeSi- legeringar, med och utan La, har utförts. För varje smälta har prov gjutits i en standardmätkopp av typ Quik-Cup och Temperatur-Tid kurvan registrerats. Termisk analys värden som TElow, TS, REC, S1, GRF1, GRF2 och dT/dt_TS har beräknats från Temperatur - Tid kurvan. Dessa beräkningar jämfördes med de porositeter som hittades vid undersökning av provens mikrostruktur. Resultatet visar att kontroll av ympning och gjuttemperatur är viktigt för att minska risken för sugningar i segjärn. Avslutningsvis kan konstateras att det är mojligt att gjuta segjärn utan matare.

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ORIGIN This publication constitutes the final report of the project G790J. This project deals with the shrinkage solidification in ductile iron castings. The report was compiled by Vasilios Fourlakidis, Metallurgy and Materials Engineering, Swerea SWECAST AB, Jönköping. Casting experiments have been performed by Vasilios Fourlakidis and Sune Jansson in Nya Arvika Gjuteri. This project has been funded through member resources.

ABSTRACT

This paper investigates the influence of different lanthanum contents and pouring temperatures on the formation of shrinkage porosities in ductile irons. Two commercial products containing La, one FeSiMg alloy and one inoculant, were tested in a ductile iron melt as well as an Mg-treatment alloy and an inoculant without any lanthanum content in their composition. From each melt the cooling curves were measured by the use of grey iron solidification quick-cups and different solidification aspects, such as TElow, GRF1, GRF2 and TS were calculated and compared with the porosities which found from the microstructure examination. Also prior to the experiment a casting simulation was performed for the identification of castings porosities defects and for developing the design of the experimental samples. The results of this experiment confirmed that by using certain metallurgical conditions and treatments elements it is possible to produce ductile iron castings without using feeders.

1 INTRODUCTION

Shrinkage porosities are one of the most common defects in iron castings and it is caused by the contraction of the liquid metal during the solidification procedure. Proper use of feeders can eliminate the occurrence of such defects by supplying feed metal on the last solidify areas. Nevertheless the use of feeders extends the need for machining of the part after the casting; increase the risk for occurrence of surface defects near to the area where the sleeves are positioned and it is resulting to the increasing production cost in a foundry. During cast irons solidification the precipitation of graphite it is accompanied with an increase in volume which in some cases it can offset the contraction of the liquid and the iron matrix. Also in cast irons the equiaxial against the columnar growth of the primary austenite leaving larger free flowing passages for the molten iron and having as results the improvement of feeding capability. In a number of experimental works [7, 8, 9] it is suggested that an equiaxial austenite growth together with a controllable graphite precipitation it can provide cast irons without any shrinkage porosities defects. This research work investigates the effect of carbon content, lanthanum content and pouring temperature on the cooling behavior of ductile iron melts during solidification and the influence of those factors to the formation on the porosities defects.

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2 LITERATURE SURVEY

The microstructure in as-cast ductile iron is consisting primarily by tow material phases: a steel matrix (ferritic, pearlitic or a combination of those two) which surrounds the graphite nodules. During the solidification of an iron melt the metallic matrix will typically shrink 1,2% when cooling from 1400 ºC to room temperature. On the other hand the transformation of the dissolved carbon into nodules of graphite cause an expansion which can offsetting the metallic matrix shrink [1]. In Figure 1 is shown the local densities in cast iron during solidification. Line 1 depicts the shrink of the metallic matrix in the region where the cooling is very high and the microstructure is mainly metastable phase. In the line 2, where the cooling is slower, a local material expansion is taken into account due to the graphite nodules formation nucleated in that volume area [2].

Figure 1. Example of the local densities in cast iron during solidification [2].

2.1 Solidification of ductile iron

In Figure 2 is illustrated the solidification phase diagram in a cast iron melt.

Figure 2. Solidification phase diagram of iron melt.

1

2

3

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As can be seen from Figure 2 the ductile iron solidification can be divided into three different stages. In every cooling region contraction or expansion it is occurring depending on the phase precipitation. First the liquid is contract during the cooling from the superheat to the liquidus. This contraction is very predictable since it is dependent on the coefficient of expansion of the alloy. After the liquidus temperature is reached the melt shrink further and the liquid transforms to solid austenite for the hypoeutectic compositions. In case of hypereutectic irons graphite precipitates at the liquidus instead of austenite, resulting in expansion rather then contraction. At the eutectic an expansion occur and the remaining liquid transforms into austenite and graphite [3].

The volumetric changes in three different cast irons during solidification are presented in Figure 3.

Figure 3. The volume in irons during solidification [3].

It can be noted from the Figure 3 that all the irons undergo a net expansion during solidification. The volume of the solidified iron at the end of solidification (before solid contraction) is greater than the volume of the liquid poured into the mold. Also feed metal must be supplied by risers for all cast irons in zone A. Additional feed metal must be provided in zone b for hypoeutectic irons [3].

2.2 Shrinkage porosities

The shrinkages porosities are divided in four different categories [4]:

Pull downs or suckins porosities which are produced very early in solidification and can be seen on the outside of the casting usually as a smooth depression in the casting surface (Figure 4a).

Macro shrinkage porosities which are usually found inside the casting and close to the heat centers. They appear as holes larger than 5 mm with rough dendritic surfaces if the iron is hypoeutectic or with round smooth walls if the iron is hypereutectic (Figure 4b).

Micro shrinkage appear as small cavities (often less than 3 mm) with irregular surfaces often with signs of dendrites and they are usually located close to the heat centers of the casting (Figure 4c). Micro shrinkages appears very late in solidification. At this stage feeds paths are closed. This type of shrink commonly appears on isolated bosses

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and in areas far from feeders. The only possibility to solve the micro shrinkage problems is to rely on late eutectic graphite precipitation, with its inherent expansion, to compensate the contraction of the austenite. For this reason always the solidification patterns where most of the graphite precipitate early are undesirable and a uniform precipitation pattern is preferred instead.

Porosities which are small (less than 1 mm) dispersed cavities with irregular or rather smooth surfaces. They are more dispersed than micro shrinkage and they are usually located close to heat centers and in grain boundaries (Figure 4d).

Figure 4. The four different types of shrinkages found in cast irons [4].

Every type of defects is occurring at very different and distinct times during solidification. In Figure 5 is shown a typical cooling curve for a hypoeutectic iron and the time position for different defects [4].

Figure 5. A typical cast iron cooling curve and the position of different shrinkage porosities [4].

(a) (b) (c) (d)

2.3 Variables influencing the formation of shrinkages

There are many factors which cause the creation of shrinkage porosities. The formation of shrinkage porosities it can be related to design production variables such as feeding-gating system and the stability of the cast mold or it can be attributed to metallurgical factors such as carbon equivalent, pouring temperature, amount and method of inoculation and magnesium residual [5]. Some of the metallurgical factors which influence the formation of shrinkage porosities are presented in the follow paragraphs.

2.3.1 Pouring Temperature

It has been found that the shrinkage cavity volume becomes greater as the pouring temperature become higher. Figure 6 shows the shape of the shrinkage porosities and the relationship between the pouring temperature and the volume percentage of the shrinkage cavities [6].

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Figure 6. (a) Shrinkage cavities at various pouring temperatures. (b) Relation between pouring temperatures and volume shrinkage cavities [6].

2.3.2 Magnesium Content Magnesium content has a marked effect on the shrinkage tendency. In Figure 7 is presented the influence of the magnesium range on the shrinkage porosities. The increasing shrinkage with increasing Mg content in ductile irons is due to the formation of carbides with low melting point which segregates in the last solidified areas and their contraction causes the creation of porosities [5].

Figure 7. Effect of magnesium content on shrinkage [5].

2.3.3 Inoculation

Both under-inoculation and over-inoculation can cause shrinkage. In the case of under-inoculation, not enough dissolved carbon is precipitated as graphite. With over-inoculation, too many nucleation points are active early in the solidification resulting in an early expansion and sometimes large wall movement. Later in the solidification, when feeders become inactive and contraction take place, there is no graphite coming out from solution to counteract the contraction and the result is micro shrinkage between the eutectic cells. [7]

It has been reported [8] that the use of FeSi inoculants containing lanthanum favor the growth of equiaxial crystals in iron melt. As a result the thickness of the austenite columnar growth zone is reduced leaving larger free flowing passages for the remaining molten iron to flow and

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feeding the last solidifying areas in the casting and thus minimizing the formation of shrinkage defect.

Skaland et al [9] investigated the effect of lanthanum and cerium bearing MgFeSi alloys on the solidification shrinkage in ductile iron. The results show that shrinkage porosity is totally eliminated when using pure lanthanum bearing MgFeSi alloy but there are indications of scattered micro-porosity when pure cerium bearing alloys were used. The use of pure La also substantially increase the number of graphite nodules compared to Ce containing MgFeSi alloy.

2.3.4 Carbon Content

In order to minimize shrinkage, it is necessary to maximize the formation of late graphite that grows during the eutectic solidification. This late graphite can exert internal pressure to offset the contraction of the material [10].

It has been found that the volume of shrinkage cavity becomes smaller as carbon content increase and takes minimum values at eutectic composition. In hypereutectic range, the volume of shrinkage cavity becomes bigger reversely as C content is increased. In Figure 8 is shown the effect of C content on shrinkage cavity and the relationship between C content and the shrinkage volume. From Figure 8 it can be seen that near eutectic composition (CE value between 4.2 to 4.6) no shrinkage cavity is observed at all.

Figure 8. (a) Effect of carbon content on shrinkage cavities. (b) Relationship between carbon content and the shrinkage volume fraction [6].

From the level rule on the phase diagram (Figure 9) it can be seen that the main cause for the increase of the shrinkage volume in hypereutectic composition it is that the graphite amount during eutectic solidification decrease in hypereutectic range. From the formulas in Figure 8 it can be calculated that the graphite amount during eutectic solidification decreases 0,023% per 1% of C content in hypereutectic composition. This fact indicates that increasing of volume of shrinkage cavity in hypereutectic range is related to decreasing of graphite amount during eutectic solidification [6].

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Figure 9. Formulas for calculating the amount of eutectic graphite [6].

2.4 Thermal analysis

Thermal analysis can be used for the prediction of the potential risk for shrinkages porosities. In Figure 10 it is illustrated a typical cooling curve of a hypoeutectic iron and its first derivative together with the position of some parameters which influence the development of shrinkages according to Adaptive Thermal Analysis System (ATAS®) developed by NovaCast AB.

Figure 10. Cooling curve of a hypoeutectic iron [7].

The cooling curve in Figure 9 shows the different arrests during the solidification of a ductile iron melt. When the liquidus temperature is reached the cooling curve shows a horizontal plateau (TL). The sample cooling continues, austenite continues nucleating until the liquid iron reaches the eutectic composition (TElow). The size of the graphic area between TL and TElow indicates the primary austenite growth which is

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in direct correlation to macro shrinkage tendency. For this reason a low area value is desirable. At TElow the eutectic precipitation of austenite and graphite start. The temperature increases due to realize of latent heat until the high eutectic temperature (TEhigh) is reached. The difference between them is called recalescence. The eutectic solidification then continues until the solidus temperature (TS) is reached. It has been suggested that the low TS is an indication of high micro-shrinkage formation tendency. Further analysis of the curves in Figure 9 provides Graphite Factor 1 (GRF1) and Graphite Factor 2 (GRF2). GRF1 is the amount of time it takes to drop 15 degrees Celsius from the high eutectic temperature and it is an indication of overall eutectic graphite precipitation during eutectic freezing after TEhigh. A high GRF1 indicates more eutectic graphite value, means the iron is more shrinkage resistance. GRF2 represent the angle of the first derivative of the cooling curve at the end of freezing. A lower value of GRF2 indicated more precipitation of graphite at the very end of freezing, leading to an iron that is more resistance to micro shrinkage [7].

In Figure 10 it is presented the first derivative of the cooling curves of two ductile iron melts. The curve in Figure 11 (a) correspond to a base iron melt and Figure 10(b) shows the derivative of a melt treated with 0,2% stream inoculation. The curve in Figure 10(a) shows very erratic cooling. GRF1 is low indicating a low amount of eutectic graphite and GRF2 is high indicating less graphite precipitation at the end of solidification. In figure 11(b) the first derivative curve it is much smoother, indicating even cooling through out solidification. The evaluated values GRF1 and GRF2 shows that the inoculation improve these parameters [10].

(a) (b)

Normal Iron Inoculated GRF1: 72 GRF1: 90 GRF2: 69 GRF2: 29

Figure 11. (a). The first derivative of the cooling curve for a normal iron sample (b) The curve for the sample where 0,2% in stream inoculation was added [10].

2.5 Casting simulation

Casting simulation is a tool that provides detailed information of various casting parameters such as mold filling and material solidification process. Furthermore the casting simulation provides the opportunity to predict the various casting defects the microstructure development and the material properties at the very early stages of the casting component production. In Figure 12 it is presented an application example of the

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casting simulation. In this example it is depicted the actual shrinkage porosity found in the casting and the calculated shrinkage defect.

Figure 12. Comparison between actual (a) and simulated (b) shrinkage defects in an iron casting component.

(b)(a)

3 DESIGN OF THE EXPERIMENTAL HEATS AND SIMULATION

Purpose of this experiment was to investigate the effect of various carbon equivalent and lanthanum contents on the shrinkage formation.

The experimental casting pattern it is consisting of 2 cross shape parts and one feeder placed at the side in one of the crosses. The geometry of the experimental heats is depicted in Figure 13a.

For developing the test sample a number of simulations were carried out prior to the casting experiment. The aim of the simulations was to indicate the porosities location, to evaluate the feeder performance and ensuring that the chosen casting design can provide the necessary shrinkage defects for the comparison and the evaluation of the influence between the different metallurgical treatments. Figure 13b shows the simulated position of the porosities in the experimental design.

(b) (a)

Figure 13. (a) The 3d design model of the experimental casting. (b) The positions of the calculated porosities.


4 EXPERIMENT

The casting experiment has been conducted in Nya Arvika Gjuteri AB. Table 1 presented the different treatments additives and the targeting composition of the experimental castings. Compositions of eutectic, hypereutectic as well as varying amount of lanthanum content was planned to be tested. As can be seen from Table 1, Lanthanum planned to be added only in the hypereutectic alloys in two different composition levels, 0.005% and 0,10%. Also it is investigated the effect of lanthanum when it is added together with the Mg-treatment alloy and during the inoculation. The second goal was to examine the influence of the pouring temperature on the shrinkage formation. The term ¨Normal¨ in the follow Tables it is referring to the Mg-alloy and to the inoculant which does not contain any La. The ‘Normal’ alloy it’s the standard used alloy in this foundry.

Table 1. The design sequence of the experimental castings.

No Alloy FeSiMg alloy Inoculation CE %La content

1 Eutectic Normal Normal 4,2 Traces

2 Hypereutectic Normal Normal 4,7 Traces

3 Hypereutectic Normal Without 4,7 Traces

4 Hypereutectic Normal La-type 4,7 0,005

5 Hypereutectic FeSiMg+La Normal 4,7 0,005

6 Hypereutectic Normal La-type 4,7 0,010

The base melt has an eutectic composition and it was prepared in an induction furnace with a capacity of 2,8 tones. The MgFeSi and MgFeSi+La alloy treatment was made in a sandwich ladle with capacity 2000 kg and then the melt was transferred to a pouring ladle with a capacity of 300 kg. The extra graphite and the La-inoculant were added in the melt during the transfer to the pouring ladle. Also during the pouring in the mold, 0,1% in stream inoculation was used. In Table 2 it is presented the amount of the different treatment elements added in each of the experimental melts. The chemical composition of the inoculant which added in-stream during the casting, as well as the composition of the La-inoculants and Mg-alloy is shown in Tables 3-4 respectively.

In order to investigate the influence of the pouring temperature each experimental melt was casting twice. After the casting of the first mold, the molten iron remained inside the ladle approximately for 2 minute before start the pouring in the second mold. The pouring temperatures ranged between 1330-1350°C for the high casting temperature samples and 1297-1320°C for the low casting temperature heats. Unfortunately some of the pouring temperatures were not recorded and for this reason during the evaluation of the results the samples are divided into two


categories, the high pouring temperature and the low pouring temperature samples.

Table 2. The amount of different treatment alloys-elements added in the melt during the transfer or pouring procedure.

No Graphite (kg)

La-Inoculant (kg)

Inoculant without La (%)

FeSiMg alloy with La

Target %La content

1 - - 0,1 - Traces

2 1,5 - 0,1 - Traces

3 1,5 - - - Traces

4 1,5 0,6 0,1 - 0,005

5 1,5 - 0,1 36 Kg in 2,8t ladle 0,005

6 1,5 1,2 0,1 - 0,010

Table 3. Chemical composition of Mg-treatment alloys.

%Si %Mg %RE %Ca %Al %La

Normal MgFeSi-alloy 45 6,0 0,08 0,60 0,43 -

MgFeSi+La-alloy 44-48 5,2-5,8 - 0,4-0,6 1,2 max 0,25-0,40

Table 4. Composition of the inoculants.

%Si %Ca %Al %Ba %La

In-stream inoculant 73-78 0,75-1,25 0,75-1,25 0,75-1,25 -

La-inoculant 45 2,5 1,5 - 1,8

Chilled coin shape specimens were taken from all pouring ladles for contacted the chemical analyses. The composition of the different heats is summarized in Table 5.

Table 5. Experimental heats chemical composition. CE=%C+%Si/3.

No %C %Si %Mn %P Cu %Cr %Ni %Mo %Ti %Mg %La CE

1 3,59 2,32 0,40 0,039 0,314 0,054 0,027 0,010 0,010 0,044 0,003 4,36

2 3,62 2,30 0,38 0,036 0,313 0,050 0,025 0,008 0,010 0,041 0,002 4,39

3 3,65 2,30 0,38 0,038 0,308 0,047 0,025 0,009 0,010 0,041 0,003 4,39

4 3,55 2,43 0,40 0,036 0,304 0,050 0,026 0,009 0,010 0,041 0,005 4,36

5 3,62 2,27 0,39 0,034 0,308 0,049 0,028 0,011 0,009 0,047 0,005 4,38

6 3,60 2,44 0,40 0,38 0,040 0,051 0,026 0,008 0,010 0,040 0,005 4,41


As can be seen from the Table 5, despite the addition of the graphite and the increasing amount of La-inoculant the carbon equivalent it is eutectic in all of the castings and the lanthanum content remains in the same level for the heats No 4 and 6. The failure to obtain the targeting composition it is probably due to the fact that the graphite and the inoculant remained partly un-dissolved due to the short period between the graphite-inoculant addition and the casting procedure.

The evaluation of the shrinkage porosities formation was made nearby to the thermal center area in the feeder-less cross-shape cast. The metallographic specimens were grounded and polish and an optical microscope with an image analysis program were used for measuring the size of the shrinkage area in every sample. The diagonal red lines in Figure 14 indicate the selected area in which the metallographic examination performed.

Figure 14. Illustration of the cross section cut in the casting and the investigated area for the examination of shrinkage porosities.

For the collection of thermal analyses data, samples extracted from the ladle in every melt and poured into standard Electronite Quick cups. A Novacast ATAS software used for recording the cooling curves.

5 RESULTS AND DISCUSSION

5.1 Porosities

In Figures 15-20 are presented the cross-section of the metallographic investigated samples from the different heats. As can be seen in the follow Figures the occurred defects in the samples are mainly shrinkage porosities and pull downs defects (outer sunks or sinks).


High casting temperature Low casting temperature

Fig 15. Heat No1 Foundry’s standard alloy and treatment.

Fig 16. Heat No2 added 1,5 Kg graphite.

Fig 17. Heat No3 added 1,5 Kg graphite. No inoculation.

Fig 18. Heat No4 added 0,6 kg La-inoculant and 1,5 Kg graphite.

Fig 19. Heat No5 added FeSiMg+La and 1,5 Kg Graphite.

Fig 20. Heat No6 added 1,2 kg La-inoculant and 1,5 Kg Graphite.

The total area of the shrinkage porosities as well as the length of the pull downs defects have been measured in each sample. The results are presented in Table 6 and in the diagrams in Figures 22 and 23. The pull downs length (Lp) it is calculated by the equation: Lp = L1-L2. The features L1 and L2 are depicted in Figure 21.


Figure 21. Features for the calculation of the pull downs length.

Table 6. Measurements of total shrinkage porosities area and maximum pull downs length, in high and low casting temperatures.

Shrinkage porosities area (mm2) Length of pull downs defects (mm) Heat No

High casting Temp. Low casting Temp. High casting Temp. Low casting Temp.1 5,8 13,1 2,7 1,6

2 0,6 8,5 0,2 1,7

3 15 15,8 2 4,3

4 7,6 6,8 1,3 0,5

5 5,1 10,1 3,2 2,6

6 2,9 5,4 2,1 1,7

Area of shrinkage porosities in different experimental samples

0

2

4

6

8

10

12

14

16

18

1

Are

a o

f s

hri

nk

ag

es

po

ros

itie

s m

m^

2

High TEMP

Low TEMP

Figure 22. The area of shrinkage porosities in the samples with different treatments additives and casting temperatures.

Normal alloy Added 1,5 Kg Graphite

Without inoculation

La-inoculant 0,6 Kg

FeSiMg+La La-inoculant 1,2 Kg

L1

L2


Length of pull downs in different experimental samples

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

1

Le

ng

th o

f th

e p

ull

do

wn

s [

mm

]

High TEMP

Low TEMP

Figure 23. The length of pulls downs in different treatments additives and casting temperatures.

In Figures 22 and 23 it is shown that the amount of shrinkage porosities is increasing with decreasing casting temperature. On the other hand the size of the pull downs is decreased with the lowering of pouring temperatures. The latter observation it is in a good agreement with results found in literature [3-4]. On the other hand the investigation results from another research work [6, Figure 6], shows that the volume of the shrinkage cavities become greater as the pouring temperature become higher. A possible reason for this contradiction in the results could be that in the previous experimental work the tested pouring temperatures ranged between 1350 to 1500°C. In this work the high pouring temperatures was varying between 1350 to 1330°C. Maybe there is a temperature interval near to 1350°C where the shrinkage defects are minimized.

As can be seen in Figures 22 and 23 the shrinkage porosity and the pull downs are minimized when added 1,5 kg graphite into the normal alloy and when the pouring temperature is high. Although the chemical composition in the second melt it is not influence by the addition of graphite, compared to the first one (Table 5), the shrinkage formation tendency is effected a lot. The elimination of the casting defects in the second heat can be attributed to the graphite addition. It is not clear how the un-dissolved graphite particles increase the melt resistance against the formation of shrinkage porosities and the pull downs defects. As it has been mentioned in the literature review [8] ¨elements which promote the growth of equiaxial crystals in iron melt result to the reduction of the thickness of the austenite columnar growth zone, leaving larger free flowing passages for the remaining molten iron to flow and feeding the last solidifying areas in the casting and thus minimizing the formation of shrinkage defect¨. In another paper Loper et al [11] suggested that the graphite is an effective nucleation side for the austenite. Base on the above assumptions a probable explanation for the elimination of the

Normal alloy Added 1,5 Kg Graphite

Without inoculation

La-inoculant 0,6 Kg

FeSiMg+La La-inoculant 1,2 Kg


casting defects in the second heat, could be that due to the added graphite, many un-dissolved graphite particles remains suspended in the molten iron and they act as nucleation sides for the precipitation of austenite, resulting to the promotion of the equiaxial crystals growth and improving the feeding ability of the melt by providing more flow-feeding paths for the molten metal.

Nevertheless the graphite addition it is not the only influencing factor for the reduction of the shrinkage porosities in the samples. As can be seen in Figure 22 and 23 the shrinkage formation resistance and the pull downs formation tendency it is worsens by the use of the La containing nodularizer alloy in heat No5 compared to the heat No2 which have been treated with the foundry standard nodularizer.

The addition of different amounts of the lanthanum containing inoculants has also an impact on the formation of the shrinkages porosities although that the final composition of the %La it is not influenced as can be seen from the Table 5. Figure 24 and 25 shows the effect of different La additions amounts, on the formation of shrinkage porosities and pull downs defects.

Shrinkage porosity formation vs Amount of added Lanthanum

0

1

2

3

4

5

6

7

8

5 7 9 11 13 15 17 19 21 23

Amount of added La [grams]

Sh

rin

kag

e p

oro

siti

es a

rea

[mm

^2]

High TEMP

Low TEMP

Figure 24. Relationship between the amount of the used La and the shrinkages formation area.

The weight of the used La in Figure 24 it is calculated from the given percentage content of La on the inoculant. In Figure 24 it is shown that the increasing amount of La is beneficial for decreasing the total shrinkage area.

As can be seen in Figure 25 the addition of La-inoculant has a worsening effect on the formation of pull downs. In this case the increasing amount of the La promotes the formation of this kind of defects. Also the diagram in Figure 25 indicates that the higher casting temperature results to increasing size of the pull downs defects.


Pull downs defects vs Amount of added Lanthanum

0

0,5

1

1,5

2

2,5

3

9 11 13 15 17 19 21 23

Amount of added La [grams]

Len

gth

of

the

pu

ll d

ow

ns

de

fec

ts [m

m]

High TEMP

Low TEMP

Figure 25. Relationship between the amount of La and the size of the

pull downs.

The lanthanum containing inoculant has been used together with the in-stream inoculation. This combined treatment probably has over-inoculated the melt and worsened the performance of the La-inoculants. For this reason it is believed that is not possible to compare the effect between the La-inoculant and the inoculant without any La content.

5.2 Thermal analysis

Thermal analysis has been performed and the characteristic temperatures and values TElow, TS, Recalescence, S1, GRF1, GRF2 and dT/dt_TS have been collected. Example of the location for the different thermal characteristic parameters in a eutectic cast iron melt it is illustrated in the cooling diagrams in Figures 26.

The cooling curves and the different thermal analysis data are presented in Figure 27 and in table 7 respectively. The cooling curves have been obtained by extracted specimens from the ladle a few seconds before the casting of the high pouring temperature samples. For this reason in Table 7 are included also the shrinkage porosities measurements and the length of the pull downs, from the high pouring temperatures heats. In Figure 27 the cooling curves have been displaced on the time axis in order to facilitate the comparison between them.

According to literature [7, 9] the thermal parameters TS, Recalescence, GRF1, GRF2 and dT/dt_TS indicates the melts shrinkage formation tendency and the parameters TElow and S1 depicts the potential risk for pull downs formation. Figures 28-34 illustrates the correlations between the casting defects and those thermal parameters.


Figure 26. The temperature profile during solidification of an eutectic iron and the cooling rate as a function of time. Different thermal parameters are illustrated.

TEhigh

Cooling Curves

1100

1110

1120

1130

1140

1150

1160

1170

1180

1190

1200

0 500 1000 1500

Time [sec]

Te

mp

atu

re [

de

gre

es C

els

ius

]

FeSiMg+La

Normal alloy

La-inoculant 1,2 Kg

La-inoculant 0,6 Kg

Normal+1,5kg Graphite

Without inoculation

Figure 27. The cooling curves of different experimental heats.

Time

Time

Temprature

dT/dt

TElow

GRF1

TS

S1

REC

dT/dt_TS

GRF2


Table 7. Thermal data extracted from the cooling curves or from the first derivative of the cooling curves together with the measurements of the defects in the experimental samples.

Heat No

TElow (°C)

TS (°C)

REC (°C)

GR1 GR2 dT/dt_TS S1 Shrinkage porosities

area (mm2)

Pull downs length (mm)

1 1146 1104 6 90 53 -3,125 106 5,8 2,7

2 1148 1113 4 97 45 -3,5 129 0,6 0,2

3 1144 1102 7 86 74 -2,75 130 15 2

4 1142 1100 6 87 76 -2,625 121 7,6 1,3

5 1142 1095 2 90 62 -3 83 5,1 3,2

6 1141 1104 5 99 52 -3,083 110 2,9 2,1

Recalescence vs micro-shrinkage porosities

0

2

4

6

8

10

12

14

16

2 3 4 5 6 7

REC [degree Celsius]

Are

a o

f sh

rin

kag

e p

oro

sit

y [m

m^

2]

8

Figure 28. Shrinkage porosity as a function of re-calescence.

Solidus Temperature vs micro-shrinkage porosities

0

2

4

6

8

10

12

14

1090 1095 1100 1105 1110 1115

TS [degree Celsius]

Are

a o

f s

hri

nk

ag

e p

oro

sit

y [

mm

^2

]

Figure 29. Shrinkage porosity as a function of solidus temp.


GRF1 vs micro-shrinkage porosities

0

2

4

6

8

10

12

14

16

80 82 84 86 88 90 92 94 96 98 100

GRF1

Are

a o

f s

hri

nk

ag

e p

oro

sit

y [

mm

^2

]

Figure 30. Shrinkage porosity as a function of the GRF1.

GRF2 vs micro-shrinkage porosities

0

2

4

6

8

10

12

14

16

40 45 50 55 60 65 70 75 80

GRF2

Are

a o

f sh

rin

kag

e p

oro

sit

y [m

m^

2]

Figure 31. Shrinkage porosity as a function of the GRF2.

dT/dt_TS vs micro-shrinkage porosities

0

2

4

6

8

10

12

14

16

-3,7 -3,5 -3,3 -3,1 -2,9 -2,7 -2,5

dT/dt_TS [°C sec-1]

Are

a o

f s

hri

nk

ag

e p

oro

sit

y [

mm

^2

]

Figure 32. Shrinkage porosity as a function of the cooling rate at the end of solidification.


TElow vs Pull Downs

0

0,5

1

1,5

2

2,5

3

3,5

1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150

TElow [degree Celsius]

Le

ng

th o

f Pu

ll D

ow

ns

[m

m]

Figure 33. The length of pull downs defects as a function of low eutectic temperature.

S1 vs Pull Downs Length

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

70 80 90 100 110 120 130 140

S1 [°C sec]

Le

ng

th o

f P

ull

Do

wn

s [

mm

]

Figure 34. The length of pull downs defects as a function of S1 value.

As can be noticed from the above diagrams there is a quite good relationship between the amount of the shrinkage formation and the thermal values REC, TS, GRF1, GRF2 and dT/dt_TS as well as between the length of the pull downs and the S1 values. According to Figures 28, 31 and 32 low values of recalecence, GRF1 and dT/dt_TS indicates castings with high resistance towards shrinkage formation. In Figures 29 and 30 it is shown that high values of TS and GRF1 are desired for minimum shrinkage formation tendency. As can be seen from the Figures 33 there is pour correlation between the length of the pull downs and the low eutectic temperature.

6 CONCLUSIONS

An experiment of six different melt treatments and two different casting temperatures have been performed. The main formed defects in the samples were shrinkage cavities and pull dawns. The results shows that besides the carbon equivalent and chemical composition there are also other factors such as inoculation treatment and pouring


temperature which have a great impact on the formation of shrinkage porosities and pull downs defects in ductile iron.

In high casting temperatures the amount of shrinkage porosities is reduced. In contrast, the lower size of pull downs defects has been found in the low pouring temperatures samples.

The increasing amount of La has a beneficial effect on the reduction of shrinkage porosities formation but it is also increasing the size of the pull downs defects.

By using the MgFeSi+La treatment alloy together with the graphite addition, the resistance in shrinkage and the pull downs formation tendency it is worsened compared to the normal nodularizer alloy.

There is a high degree of correlation between the amount of shrinkage porosities and the thermal analysis values REC, TS, GRF1, GRF2 and dT/dt_TS. On the other hand the thermal parameters TElow cannot provide clear indication for the pull downs formation tendency.

In this experiment the use of graphite prior to the inoculation and the high casting temperatures (but lower than 1350°C) eliminates both shrinkage porosities and pull downs defects. The results of this experimental work indicates that the balance use of certain types of inoculation elements and the average casting temperatures are essential factors for decreasing or eliminating the casting shrinkage defects in ductile irons.

7 REFERENCES

1 David Sparkman, Offsetting Macro-Shrinkage in Ductile Iron, Ductile Iron News, Issue 3, 2001.

2 Modeling the Solidification of Cast Irons, http://www.esi-group.com / products / casting / Etips / e-tips/eTip30_.pdf.

3 David Eli, Shrinkage in Nodular Iron, Ductile Iron News, Issue 3, 2001.

4 Rudolf Val Sillen, Production of ductile iron castings whithout feeders, FTJ, May 2007.

5 C. M. Ecob, A review of common metallurgical defects in ductile cast iron, http://www.elkem.no /dav/9c2ceebc69.pdf.

6 T. Kanno, I. Kang, Y. Fukuda, T. Mizuki, Effect of pouring temperature and composition on shrinkage cavity in spheroidal graphite cast iron, AFS Transactions 2006 paper 06-084.

http://www.esi-group.com/

http://www.esi-group.com/


7 V. Popovski, C. Misterek, L. Kaiser, Metallurgical comparison between operating conditions, inoculant types and fade effects in gray iron, AFS Cast Iron Inoculation Conference, Illinois, 2005.

8 Siclari R., Margaria T., Berthelet E., Fourmann J., Koch M., Micro-shrinkage in ductile iron / mechanism and solution, 66th world foundry conference Instanbul 2004, pp 863-873.

9 Skaland T., A new method for chill and shrinkage control in ladle treates DI, 66th world foundry conference Instanbul 2004, pp 975- 986.

10 ATAS-Case story ‘Inoculation of Ductile Iron’, http://www.novacastfoundry.se /swe/sewatasinoculation.html

11 Loper C.L., Inoculation, what is it and how does it work, International inoculation conference proceedings, April 1998, USA.

http://www.novacastfoundry.se/

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091215 A study on Ductile Iron Production without the use