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MATADOR SPECIAL ISSUE

Effect of Orientation, Thickness and Composition on Properties of

Ductile Iron Castings

Vasudev D. Shinde1, +,

B. Ravi2 and K. Narasimhan

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1) PhD Research Scholar, Department of Metallurgical Engineering & Materials Science, Indian

Institute of Technology Bombay, Mumbai-400076 (India) shinde@iitb.ac.in ,

vasu.metal@gmail.com

+ Corresponding author. Tel.:+912225764399; Fax: +912225726875;

2) Professor, Department of Mechanical engineering, Indian Institute of Technology Bombay,

Mumbai-400076 (India) b.ravi@iitb.ac.in

3) Professor, Department of Metallurgical Engineering & Materials Science, Indian Institute of

Technology Bombay, Mumbai-400076 (India) nara@iitb.ac.in

Abstract

In this work, the effects of casting orientation (horizontal, side and vertical), section thickness

(4-16 mm) and composition (Cu, Mn) were investigated on the cooling rate, microstructure and

mechanical properties (tensile strength, yield strength, elongation, hardness) of hypereutectic

ductile iron castings. Overall, horizontal castings were found to cool faster than side and

vertical oriented castings. Thermal analysis (using cooling curves) showed a wide difference

among the four sections. Thinner sections exhibited significant undercooling and thereby

carbide formation, leading to poor ductility. The combined effect of Cu and Mn showed an

increase in amount of pearlite to 82% and nodularity to 94% along with a reduction in nodule

count to 323 and amount of ferrite. Also, increased tensile strength (659 MPa) and hardness

(264 BHN) were observed along with a drop in ductility to 2.5% in 4 mm thin section, which

helps offset carbide formation. Thermal analysis was found to be a useful tool in understanding

the combined effect of orientation, thickness variations and processing parameters.

Keywords: Thin wall, ductile iron, solidification, inoculation, microstructure

1. Introduction:

Ductile iron provides a range of mechanical properties, often comparable to steels,

while having good castability [1]. It is therefore gradually replacing forged and welded

components in automobiles [2]. Since castings now constitute a significant proportion

of vehicle weight, manufacturers are increasingly redesigning the parts with thinner

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walls to reduce the total weight [3]. The challenge is to obtain the desired combination

and values of mechanical properties [4].

The properties of as-cast ductile iron are largely controlled by chemical composition,

melt treatment and cooling rate [5]. Melt treatment includes the addition of magnesium

alloy followed by inoculation to increase the nodule count and to suppress carbide

formation [6]. The magnesium treatment eliminates oxide bifilms and produces multiple

nuclei in the melt. Post inoculation treatment too is beneficial, especially in thin wall

ductile iron castings, since it further increases the active number of nucleation sites.

Graphite nucleates on these particles, and their further growth is controlled by austenite

dendrites [7]. Austenite formed during solidification undergoes solid state

transformation at eutectoid temperature, which modifies the solidified structure and

leads to other complexities in solidification morphology [8]. Since early nucleation of

graphite nodules helps prevent carbides in thin sections, ductile irons with hypereutectic

composition are preferred for such castings [9]. However, due to higher cooling rates in

thin wall ductile iron castings, sufficient pre-eutectic graphite nucleation is required

[10].

Thin wall ductile iron castings require an optimal combination of composition and melt

processing to be free of carbides that occur due to chilling. Hypereutectic ductile iron,

with CE ranging from 4.45 to 4.9 %, is usually recommended for plates of thickness

below 5 mm [11]. To avoid primary carbides in ductile iron, the eutectic temperature

should be greater than 1140oC [12]. Further, more homogenous microstructures are

obtained by using multiple gates instead of risers [13].

The alloying elements such as Cu, Mn, Sn, Sb and Cr are known to increase tensile

strength and hardness with subsequent decrease in ductility and impact energy. This is

due to an increase in the amount of pearlite with subsequent decrease in ferrite. Even

small changes in the amount of the above elements show significant increase or

decrease in mechanical properties of ductile iron. Silicon is a strong solid solution

strengthener; it reduces undercooling and avoids carbide formation by nucleating

graphite. It increases volume fraction of ferrite and nodule count. Copper is a strong

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pearlite promoter; its addition up to 1% converts ferritic structure into pearlitic [14].

Manganese increases hardness and strength by stabilizing pearlite but promotes carbides

in heavy sections. It segregates at grain boundaries and thus increases hardenability

[15]. Arsenic, tin and antimony promote pearlite and carbides, and are hence kept to

lower limits; their effect can be counteracted by cerium additions [16].

The ratio of ferrite to pearlite in the matrix and the morphology of graphite decide the

mechanical properties of ductile iron castings [17]. This depends upon the cooling rate

during eutectoid transformation, nodule count and alloying elements [18]. The ferrite

being softer gives higher ductility but lower tensile strength than pearlite. Also, the

graphite morphology plays an important role; deviation from spheroidal shape reduces

the ductility and impact properties [19]. The time span between spheroidal treatment

and pouring has a significant effect on elongation, but less effect on the tensile strength

and hardness of castings [20].

The cooling curve generated by inserting suitable thermocouples in the casting cavity

reflects the effect of solidification variables such as chemical composition, inoculation

and its effectiveness [21]. There is a high temperature drop in liquid metal due to heat

transfer between flowing metal stream and mould walls [22]. The casting orientation

affects the solidification behaviour, and thereby leads to variations in graphite nodules

as well as deviation from nodularity. The top portions of the castings were observed to

have a higher nodule count but lower values of nodularity compared to bottom portions

[23]. Further, as solidification proceeds more rapidly in thin wall ductile iron castings,

the feeding pattern in these section influences the final microstructure and thereby

mechanical properties [24].

Previous work shows that for improving the strength of a casting, copper is an important

constituent. Manganese is also used in the present work for strengthening the casting by

promoting pearlitic matrix, but in limited amounts, since it can alter the structure by

promoting carbides in different section thicknesses. An attempt is made to balance the

strength and ductility of ductile iron by varying the amount of manganese and copper.

Further, the effects of different section thickness and casting orientation on the

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microstructure and properties of ductile iron castings, which have not been reported

earlier, have been included in the present investigation.

2. Experimental work

Experiments were designed and conducted to study the solidification behaviour in

varying thickness ductile iron castings, as shown in Table 1. A step casting was

designed with four sections having thickness 4, 8, 12 and 16 mm, respectively. Each

step is 50 mm long, making the total length of casting 200 mm. The width of the casting

is 100 mm, so as to avoid end freezing effects in all sections. Four gates were provided

for rapid and uniform filling. Total four melt compositions with code A-D were used to

pour a total of 12 castings. Initially, four castings with melt composition A were

produced in vertical and side orientation (two AV and two AS). Another eight were

produced in horizontal orientation, with melt composition A to D (two castings of each

composition).

The moulds were prepared in green sand using a wooden pattern of the step casting. The

casting is moulded in drag box whereas runner and sprue were in cope. The gating

systems for the vertical and horizontal orientation of the casting are shown Fig.1. The

total mould height of vertical casting was 250 mm whereas in horizontal castings it was

200 mm. K-type thermocouples were inserted in the middle of each step to record the

thermal history of casting solidification. A DAQ-3005 (MCC-USA) data logger for data

acquisition synchronised with Desylab 12.0 software was used.

The melt charge consisted of 50 kg pig iron, 150 kg cold rolled steel scrap and balance

foundry returns with suitable chemical composition. The charge mix was melted in 300

kg capacity coreless medium frequency induction furnace in a production foundry. The

molten metal was tapped into a preheated ladle containing Ferro-silicon- magnesium

(FeSiMg) alloy granules of size 10-15 mm at the bottom covered with steel scrap

(sandwich process). The tapping temperature was 1450 oC.

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Table 1: Experimental castings and study parameters

Casting code Casting orientation Composition Study parameters

AV1, AV2 Vertical Trace Cu, 0.2% Mn Under-cooling and

delay in solidification AS1, AS2 Side Trace Cu, 0.2% Mn

AH1, AH2 Horizontal Trace Cu, 0.2% Mn

BH1, BH2 Horizontal 0.2% Cu, 0.3% Mn Phase and variation

in properties CH1, CH2 Horizontal 0.4% Cu, 0.4% Mn

DH1, DH2 Horizontal 0.5% Cu, 0.5% Mn

Fig. 1: Castings with gating system in (a) horizontal, (b) side and (c) vertical orientation

The inoculant was added in the melt stream while transferring metal into pouring ladle

of 50 kg capacity for proper mixing. Inoculant particles were of 6 to 10 mm in size so as

to dissolve easily and dust free to avoid oxidation losses. The spectroscopic melt

samples was taken just before pouring into the mould, and analysis was carried out

using a spectrometer (BRUKER, model Q-4 Tasman). The treated iron was poured into

mould cavity at a temperature of 1380 oC for all castings. Six experiments (two vertical,

two side, and two horizontal castings) were conducted for studying the effect of casting

orientation with the same chemical composition (A). The thermocouples were inserted

in one casting of each orientation (AH2, AS2 and AV2).

a

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Another six castings were poured to study the effect of chemical composition for a

given orientation (horizontal). In all the melts (labelled A, B, C, and D), 3.6% carbon

and 2.5-2.78% silicon gave a carbon equivalent of 4.44-4.57, which is in hyper-eutectic

range. Cu varied from 0.035 to 0.512% and Mn varied from 0.216 to 0.518% as shown

in Table 2. Other elements present in the melt were Pb<0.01, Al=0.006, Cr=0.015,

Mo<0.002, Ni<0.002 and Ti=0.02.

Table 2: Chemical analysis of the melts A, B, C and D

Melt code C Si Cu Mn P S Mg

A 3.62 2.51 0.035 0.216 0.005 0.011 0.035

B 3.63 2.69 0.214 0.310 0.004 0.009 0.034

C 3.68 2.68 0.401 0.392 0.004 0.010 0.032

D 3.61 2.78 0.512 0.518 0.006 0.010 0.039

3. Results

The solidification temperature history recorded using thermocouples and stored in the

data logger was used to plot the cooling curves. The cooling curves in the four sections

(different thicknesses) of castings AV and AH are shown in Fig. 2.

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Fig. 2: Cooling curve at the middle section (different thicknesses) of horizontal, side

and vertical castings.

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Fig. 2: Cooling curve at the middle section (different thicknesses) of horizontal, side

and vertical castings. (new)

Fig. 3: Cooling curve and its first derivative (cooling rate) in 8 mm side oriented casting

The first derivative of the cooling rate indicates the evolution of cooling rate during

solidification. Its interpretation can reveal microstructural information that can not be

easily obtained from standard metallographic techniques. The typical cooling curve and

cooling rate in 8 mm side oriented ductile iron casting are shown in Fig.3. The graphite

nucleation starts at liquidus temperature (TL), followed by horizontal eutectic portion.

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The cooling rate curve passes through zero indicating end of eutectic solidification

(TEend), but due to presence of trace elements end of freezing (EOF) extends further.

Key parameters of the cooling curves are shown in Table 3. The solidification

temperature range is approximately 1170 oC to 1120

oC. It has been observed that the

cooling rates during solidification in different sections range from 6.4 to 0.4 oC/s in

horizontally oriented castings, 3.6 to 0.7 oC/s in side orientation and 2.6 to 1.1

oC/s in

vertically oriented castings. In other words, horizontal castings have nearly twice the

cooling rates of side oriented castings, owing to increased rate of heat transfer from the

larger surfaces in horizontal orientation. A wide range of under-cooling was observed in

horizontal (1-4 oC) and side (2-8

oC) castings due the presence of both thick and thin

sections. The comparatively slower cooling rates in side oriented castings gave wider

total solidification times. The difference in solidification times of horizontal, side and

vertical orientated castings is found to increase with increasing casting wall thickness.

Table 3: Thermal analysis of cooling curves in different sections of casting (Melt A)

Thickness

(mm)

Cooling rate (oC/s) Undercooling (

oC) Solidification time (sec)

Hori Side Vert Hori Side Vert Hori Side Vert

4 1.8 1.0 5.00 18 16 0 40 57 20

8 0.8 0.4 1.42 11 7 10 70 95 50

12 0.7 0.3 0.83 6 9 0 120 150 60

16 0.5 0.2 1.43 5 7 0 145 180 70

Thickness

(mm)

Cooling rate (oC/s) Undercooling (

oC) Solidification time (sec)

Hori Side Vert Hori Side Vert Hori Side Vert

4 6.4 3.63 2.63 2 2 1 25 33 38

8 1.11 0.97 1.03 1 2 4 63 72 77

12 0.64 0.61 0.51 4 4 6 125 110 118

16 0.41 0.68 1.1 4 8 6 170 100 160

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Fig. 4: Microphotographs of 12 mm section castings with composition A-D

The samples for microstructure studies were taken from the middle portion of the

casting and polished. These were etched with 2% Nital (2% concentric Nitric acid and

98 ml Methanol solution). Optical micrographs were taken using a camera attached to a

Leintz microscope (Fig. 4).

The polished samples were studied using an Image Analyzer (Pro-metal-11) for

microstructural studies to compare fraction of pearlite content and nodule count in each

casting sections; these images are shown in Fig.5. Tensile test specimens were prepared

from each casting as per ASTM standard E8M-04. The Brinell hardness is measured on

the samples taken from the middle portion of each casting. The average values (derived

from two samples of each composition) of tensile and hardness are shown in Fig. 6.

More undercooling that is observed in thin sections, indicates the possibility of carbides

in these sections. This also indicates the failure of inoculation in generating a sufficient

number of nucleating sites for graphite, resulting in 5-6% carbides observed in 4 mm

thin sections.

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Fig. 5: Values of (a) percentage pearlite and (b) nodule count in the castings

Fig. 6: Values of (a) tensile strength and (b) hardness of the castings

4. Discussion

The factors influencing the solidification process, such as metal composition (including

trace elements), melt modification, nodularization treatment and inoculation influence

the shape of the cooling curve too. The part of the cooling curve from the liquidus

temperature to the end of eutectic solidification represents the solidification range. Two

separate cooling curves can be compared in terms of their shape and temperature values.

The thermal analysis of the four sections within the horizontal castings indicates faster

cooling rate compared to corresponding sections in side oriented castings. Solidification

time in horizontal castings ranged between 25-170 seconds for various thicknesses,

compared to 33-110 seconds for side orientation. The longer solidification time of side

oriented castings can be attribed to hot metal continuously feeding from the the top. The

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amount of undercooling observed in 4 and 8 mm sections of horizontal castings was

more than in side orientated castings.

The nucleation potential depends on the number of potential heterogeneous nuclei,

which can be indirectly assessed by carbide content (chill depth). A high nucleation

potential will result in low formation of carbides. The nodule count is a direct measure

of the nucleation potential of ductile iron but it can be measured only after completion

of melt processing. The amount of Mg residual needs to be maintained above a critical

value of 0.03% to achieve the desired nodule count [1]. Nodule count can be maximized

by oxide-free base iron melting along with good inoculation practice. Nodule count and

nodularity, both are affected by cooling rate. Thin sections (due to rapid cooling) result

in better nodule shape than slower cooled sections for the same magnesium residuals.

High nodule count associated with the increased cooling rates of thin wall castings is a

major factor in matrix evolution. It was found that because of high nodule count

produced by rapid cooling rate, the pearlite content was higher in the thinner plates.

In 4 mm section approximately 4-5% primary carbides are found in casting where

eutectic undercooling temperature is below 1140 oC. The cooling curve shown in Fig. 3

indicates undercooling (minimal) temperature of 1143 oC, which is above the critical

limit (1140 oC) [12] and therefore gives carbide free casting. The castings BH-DH were

found to be free of carbides even after increased manganese up to 0.5 %. The overall

nodule count observed in side oriented castings was found to be higher compared to

horizontal and vertical castings produced in the same composition indicating continual

graphite nucleation throughout solidification.

The simultaneous increase of both Cu and Mn enhances both tensile and yield strengths

without a significant decrease in ductility (as compared to that observed by an increase

in Cu alone). The microstructure study indicates no traces of carbides in 16, 12 and 8

mm sections and only 4 to 5% carbides in 4 mm thick sections. The nodule size

distribution affects shrinkage tendency, since it reflects graphite formation and

expansion throughout the entire solidification sequence. Small nodules and uniform

distribution indicate early graphite nucleation and wider range of graphite nodule sizes,

indicating continuous nucleation of graphite during solidification. In side and vertical

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oriented castings, the metal is more turbulent, which delays solidification and results in

less carbide formation, which is reflected in the cooling curves of the respective

castings. The tensile strength and hardness variations due to thickness variations are less

in vertical orientation compared to horizontal and side orientation, with the same

chemical composition.

5. Conclusions

The properties of ductile iron as indicated by their grades are largely determined by

their microstructure, which in turn is affected by section thickness of the casting and

chemical composition of the melt. It is found that the casting orientation also alters the

nodule count and final microstructure in the casting. The difference in solidification

times of horizontal and vertical orientated castings is found to increase with increasing

casting wall thickness. Thin wall (4 mm) sections are more prone to deep undercooling

and carbide formation, especially in horizontal orientation. The microstructure and

thereby mechanical properties (especially tensile strength and hardness) can be

improved in thin wall ductile iron castings by simultaneously increasing the amount of

copper and manganese. The combined addition of Cu and Mn varying from 0.1 to 0.5%

increased the amount of pearlite from 10 to 82% in the ductile iron castings, which in

turn increased the strength from 467 to 659 MPa. The corresponding fall in ductility (%

elongation) was 15 to 3 in 4 mm thick castings. Thus, ductile iron castings with Cu and

Mn upto 0.5 % with produce carbide free structures in side oriented 4 mm wall castings.

Acknowledgement

This work is partially supported by the E-Foundry project, funded by the National

Knowledge Network Mission of the Ministry of Communications and Information

Technology, New Delhi. The authors gratefully acknowledge the assistance of Ganesh

Foundry and S.S. Industries, Ichalkaranji for arranging melting trials. The first author

acknowledges the support of his parent organization, Textile and Engineering Institute,

Ichalkaranji for carrying out the present research work.

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