The 5th PSU-UNS International Conference on Engineering and Technology (ICET-2011), Phuket, May 2-3, 2011

Prince of Songkla University, Faculty of Engineering Hat Yai, Songkhla, Thailand 90112

Abstract: The production of compacted cast iron with

desirable microstructure and mechanical properties is

known to be difficult due to its narrow operating

window. This research investigated effects of % carbon

equivalents (CE) at 4.1, 4.3 and 4.5 on graphte/matric

structure and hardness property of the compacted cast

iron. The results revealed an alteration of matrix

structure due to the varied % CE. Higher %CE led to

increasing amounts of ferrite in thin sections; thereby,

providing lower hardness of the compacted cast iron.

Key Words: Compacted cast iron /Carbon Equivalent

/Microstructure/Hardness

1. I�TRODUCTIO�

Thailand automobile industry has been under focus

for decades due significantly to its growing demands in

the production of automobile parts and assembly.

Compacted cast iron (CGI) is one of the most versatile

materials liable for use in the high-efficiency engine

parts due to its good mechanical properties, damping

capacity and high thermal conductivity. However, the

production of CGI is rather difficult due to its limited

casting parameters, e.g., chemical composition,

spheroidizing agent additions (%SA), magnesium

treatment temperature and section thickness. These

parameters control the residual magnesium in the melt

and the cooling rate of the casting which eventually

affect both matrix structure and morphology of graphite,

to be spheroidal, compacted (vermicular) or flake [1-2].

Previous study [3] has emphasized on the effects of

%SA (0.3-0.7 wt.%), magnesium treatment temperatures

(1450-1550oC) and specimen thickness (3.2-50.8 mm) on

graphite morphology. However the effect of %CE has

not been reported. Since mechanical properties of cast

iron (CI) is related to matrix structure and morphology of

the graphite, this research therefore investigated the

effects of % carbon equivalent (%CE) on microstructure

and hardness property of the cast iron.

2. EXPERIME�TAL

Cast iron was melt in a 35 kilogram crucible using an

induction furnace. A sandwich process was selected for

magnesium treatment at 1450, 1500 and 1550oC by

adding spheroidizing agent of 0.3, 0.5 and 0.7 wt.%.

Chemical composition was analysed prior to casting into

step bars of six different thickness varied from 3.2 to

50.8 mm.

Metallographic examination was carried out. Image

analyzer was employed for phase analysis to determine

amounts of ferrite, pearlite and graphite using at least ten

fields of measurements for each condition.

Rockwell hardness (scale E) was performed such that

relationships among %CE, microstructure and hardness

property can be established.

3. RESULTS A�D DISCUSSIO�

3.1. Chemical composition and microstructure

Examples of carbon and silicon contents are listed in

table 1, giving the predetermined %CE values of 4.1, 4.3

and 4.5. Percents of residual magnesium were varied

according to %SA additions. Fig. 1 illustrates three

distinct levels of %CE obtained from each melt to

produce the castings.

Table 1. Chemical composition of cast irons magnesium

treated at 1500oC

%SA C Si Mg %CE

0.3 3.45 2.08 0.013 4.12

0.5 3.35 2.08 0.016 4.04

0.7 3.38 2.15 0.023 4.10

0.3 3.45 2.62 0.015 4.32

0.5 3.40 2.75 0.021 4.32

0.7 3.38 2.77 0.028 4.30

0.3 3.38 3.28 0.015 4.47

0.5 3.30 3.49 0.016 4.47

0.7 3.32 3.36 0.024 4.44

EFFECTS OF %CE O�

MICROSTRUCTURE A�D HARD�ESS OF

CAST IRO�S

Tapany Udomphol1*, Rattana Borisutthekul

1, Usanee Kitkamthorn

1,

Panya Buahombura1, Thumrongsak Witchanantakul

1, �arong Akkarapattanagoon

1

1Suranaree University of Technology, School of Metallurgical Engineering

*Corresponding author email: tapany@sut.ac.th

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Cast iron melt

0 1 2 3 4 5 6 7 8 9 10

% Carbon Equivalent

3.8

4.0

4.2

4.4

4.6

4.8

CE 4.1

CE 4.3

CE 4.5

Fig. 1 %CE of the cast iron melts.

Fig. 2 Microstructures of different %CE cast irons,

magnesium treated at 1500oC.

3.1.1 Graphite morphology

Extensive microstructure investigation revealed the

graphite shape has changed from spheroidal to virmicular

with increasing section thickness especially in thin

sections from 3.2 mm to less than 12.7 mm, as shown in

fig. 2 in the case of 0.3%SA addition. This is due to

slower cooling rates in thicker sections (12.7 to 50.8

mm), which are less preferable for nodular graphite

formation. Moreover, the size of the graphite became

larger in the thicker sections, owing to the slower cooling

rates as addressed previously. The alteration in the

graphite shape with increasing section thickness is also

consistent with a reduction in %nodularity, characterized

by eq. 1) as follows [2]

1005.0

% ×+

=∑

∑ ∑total

teIntermedia,odular

A

AA,odularity 1)

Where ∑A,odular and ∑AIntermediate are summation of

area of nodular graphite and summation of area of

nodular graphite and compacted graphite respectively.

∑Atotal is the summation of total graphite area. It is noted

that cast iron with %nodularity of higher than 80 (N>80)

provides nodular or spheroidal graphite (SG) cast iron.

%Nodularity of 50<N<80 yields SG+CG while

20<N<50 gives CG+SG. CG is obtained when 0<N<20

and N<0 gives flake graphite (FG).

Increasing %SA additions from 0.3 wt.% to 0.7 wt.%

has considerable changed the graphite morphology, as

illustrated in figs. 2 and 3. The % nodularity curves have

been lifted up significantly when %SA increases from

0.3 wt.% to 0.5 wt.% and 0.7 wt.% additions. This is

primily due to higher percent of residual manesium

obtained in higher % SA additions, see table 1.

Unlike the effect of %SA additions, increasing

magnesium treatment temperatures from 1450 to 1550oC

has been reported [4] not to evidently change the

graphite morphology, observed from the cast

microstructures thoughtout thickness of 3.2 to 50.8 mm.

Although higher losses of magnesium at higher treatment

temperatures were expected, it was however found that

%SA additions at 0.3 wt.% and 0.5 wt.% provided

comparable percents of residual magnesium obtained

from the cast iron melts after magnesium treatment;

thereby, resulting similar graphite morphology. The

addition of 0.7 wt.% SA although was observed to offer

only slightly higher loss of magnesium, no considerable

change in graphite morphology was obtained.

The effect of section thickness on %nodularity of the

4.1% CE cast irons (manesium treated at 1450oC with

0.3-0.7 wt.% SA additions) is exhibited in fig. 4.

Increasing section thickness promotes compacted

graphite formation due solely to reducing cooling rates.

It should be further noted that higher %SA addtions

provides enhanced degree of nodularity. Similar trends

obtained from fig. 3 were also found in cast irons with

higher %CE. The graphite shape was observed not to

significantly change with increasing %CE when other

parameters such as magnesium treatment temperatures

and %SA additions were fixed.

Section thickness, mm

0 10 20 30 40 50 60

%Nodularity

0

10

20

30

40

50

60

70

80

90

100

SG

SG+CG

CG+SG

CG

0.7%SA

0.5%SA

0.3%SA

Fig.3 Alteration of %nodularity obtained in 4.1% CE

cast irons with increasing section thickness (magnesium

treated at 1450oC).

From %CE and section thickness investigated,

%nodularity maps can be constructed as demonstrated

for example in fig. 4. %Nodularity map of 0.5 wt.% SA

addition cast irons, magnesium treated at 1500oC, as

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shown in fig. 4a), suggests that smaller sections provides

greater degrees of nodularity whereas larger thickness

offers compacted graphite. It should be noted that %CE

closed to the eutectic composition provided a significant

trend in producing nodular graphite cast irons especially

in thin sections in the case of 0.5 wt.% SA addition.

50

50

50

50

50

40

40

40

40

50

50

60

60

6060

60

6060

60

30

70

70

70

80

80

Section thickness, mm

10 20 30 40 50

%Carbon Equivalent

4.25

4.30

4.35

4.40

4.45

a) 0.5 wt.% SA addition

30

30

3030

30

30

30 3030

20

40

40

40

40

50

50

50

50

Section thickness, mm

10 20 30 40 50

%Carbon Equivalent

4.20

4.25

4.30

4.35

4.40

4.45

b) 0.3 wt.% SA addition

Fig. 4 %Nodularity map of cast irons and magnesium

treated at 1500oC a) 0.5 wt.% SA addition b) 0.3 wt.%

SA addition.

Moreover, extensive analysis on %nodularity map of

the investigated cast irons (%CE = 4.1-4.5, magnesium

treatment temperature = 1450-1550oC, %SA addition =

0.3-0.7 wt.% and section thickness = 3.2-50.8 mm) has

also suggested that 0.3wt% SA cast iron over the

investigated magnesium treatment temperatures could

offer a practical window for the production of compacted

graphite cast irons with section thickness varying from

6.3-50.8 mm, see for instance in fig. 4 b). These results

have been summarized elsewhere [5]. Nonetheless the

production of the compacted cast iron in the thin section

(3.2 mm) seemed to be problematic in this study. Futher

work [6] has however recommended the use of shell

mold added with exothermic powder is feasible to

achieve compacted graphite in thin section for eutectic

cast iron (C = 3.3-3.5 wt.%, Si = 2.5-2.6 wt.%).

3.1.2 Matrix structure

The matrix structure has been observed to change

significantly with %CE as illustrated in fig. 2. Matrix

phase analysis has shown trends of decreasing pearlite-

to-ferrite ratios with increasing section thickness over the

casting parameter ranges for both high and low %CE cast

irons, see fig. 5. Considering at similar %SA additions,

high %CE cast irons provided the matrix structure with

higher amount of ferrite in comparison to those observed

from lower %CE cast irons especially in the thinner

sections (3.2 and 6.3 mm). As the section thickness

increased, higher amount of ferrite as a result of slower

cooling rates were obtained; similar to those observed

from low %CE structures. Since silicon is more soluble

in ferrite, higher silicon content in the high %CE cast

irons might explain the promotion of ferrite phase in thin

sections. However, the diffusion process has dominated

in the thick sections. It seems that the effect of slower

cooling rates in the thicker sections has prevailed and

then gradually suppressed the effect of silicon in

promoting ferrite formation in thick sections, see fig. 5.

Section thickness, mm

0 10 20 30 40 50 60

Pearlite / ferrite ratio

0.0

.2

.4

.6

.8

1.0

1.2

1.4

1.6

Low %CE (4.1)

High %CE (4.3)

Fig. 5 Comparison of pearlite to ferrite ratio of low and

high %CE cast irons with 0.3 wt.% SA addition,

magnesium treated at 1500oC.

3.2. Hardness

According to figs. 6 a) and b), hardness values

decrease non-linearly as the section thickness increases.

Hardness of low %CE (4.1) cast iron is observed to be

dependent on %SA additions. This can be seen by

comparing the hardness of low %CE cast irons having

0.3 wt.% and 0.7 wt.% SA additions, see fig. 6 a). It is

refered that higher %SA additions aided the formation of

spheroidal graphite while lower %SA addition was only

sufficient for compacted graphite formation, leading to

lower hardness in the latter case.

Hardness of high %CE (4.5) cast irons noticeably

lower than that of low %CE cast iron observed in thinner

sections (3.2 and 6.3 mm), see fig. 6. This is associated

with the higher amount of ferrite in the matrix structure

in the high %CE cast irons as previously discussed in

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section 3.1.2, and see fig. 6. This in turn has resulted in

reduced amount of pearlite, leading to lower hardness

values in this case. Hardness values of the high %CE cast

iron then decreases as the section thickness increases

until reaching hardness levels comparable to those of the

low %CE cast irons, as section thickness approaches

50.8 mm. The differnce in hardness property of high and

low %CE cast iron specifically in the thin sections, might

be plausibly due to the effect of silicon content in

promoting ferrite formation.

Fig. 6 Hardness of low and high %CE cast irons,

magnesium treated at 1500oC.

% Pearlite

0 10 20 30 40 50 60 70 80

Hardness, HRE

90

95

100

105

110

115

120

0.3 wt% SA, %CE 4.5

0.5 wt% SA, %CE 4.5

0.7 wt% SA, %CE 4.5

0.3 wt% SA, %CE 4.1

0.5 wt% SA, %CE 4.1

0.7 wt% SA, %CE 4.1

High CE Low CE

Fig. 7 Hardness and %pearlite relationship of low and

high %CE cast irons, magnesium treated at 1500oC.

Relationship between hardness and %pearlite of high

and low %CE cast irons is illustrated in fig. 7. It can be

seen that there are two groups of hardness-%pearlite

information. Data of higher %CE (4.5) are confined in

the lower hardness and % pearlite values, suggesting

inferior hardness property owing to the prevailed effect

of higher ferrite amount, regardless of the garphite shape

(%wt. SA additions). More scattered data in lower %CE

cast irons (4.1) signifies the intevention of graphite

morphology on hardness property, aside from pearlite

structure. The hardness property in this latter case can

therefore be tailored by the morphology of the graphite.

4. CO�CLUSIO�S

Experimental results showed that matrix structure and

the hardness property of the cast irons investigated were

affected by %CE. High %CE of 4.5 resulted in higher

amount of ferrite in thin sections (3.2 and 6.3 mm),

giving inferior hardness property. Effect of %CE on

hardness did not evidently reflect in thicker sections as

slower cooling rates have prevailed, resulting in large

amount of ferrite, regardless of %CE.

5. ACK�OWLEDGEME�TS

Authors would like to acknowledge technicians and

project students at Suranaree University of Technology

for kind supports as well as the National Research

Council of Thailand (NRCT) for research funding.

6. REFERE�CES

[1] X.J. Sun, et al., “Optimization of the process

parameters affecting the microstructures and

properties of compacted graphite iron”, J. of Mat.

Pro. Tech, Vol.476, May 2009, p.728-732.

[2] X.J. Sun, et al., “Identification and evaluation of

modification level for compacted graphite cast iron”,

J. of Alloy and Com., Vol.200, May 2008, p.471-480.

[3] Kumma, et al., “On the relationship between

specimen thickness and graphite morphology of

compacted graphite cast iron (CGI)”, 2nd

TMETC,

Century Park Hotel, Bangkok, October 2008. [4] Prakhongklang, B., et al, Effects of magnesium

treatment parameters on graphite morphology of cast

iron, The 5th Thailand Materials Science and

Technology Conference, Bangkok, September 16-19,

2008.

[5] Borisutthikul, R., et al, “Study and development of

methods for controlling the production of compacted

graphite cast iron for future automotive and

machinery parts”, NRCT research report, 2009.

[6] Romposomchoke, A., et al, “Feasibility of using shell

mold added with exothermic powder to achieve

compacted graphite shape in thin gauge”, 4th

TMETC,

Greenery Resort, Nakorn Ratchasima, November

2010.

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