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T. Alp, A. A. Wazzan and F. Yilmaz

MICROSTRUCTURE­PROPERTY RELATIONSHIPS IN CAST IRONS

T. Alp and A. A. Wazzan

Chemical & Materials Engineering Department, King Abdulaziz University, Jeddah, Saudi Arabia

and F. Yilmaz Professor, Metallurgical Engineering Department, Sakarya University, Adapazari, Turkey

:

. . ) ( () . () . Fe­C . ( ) . .

* Address for correspondence: Professor T. Alp, Chemical & Materials Engineering Department, King Abdulaziz University, P.O. Box 80204, Jeddah 21589, Saudi Arabia e-mail: [email protected]

* [email protected]

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ABSTRACT Several cast irons, prepared with different chemical compositions and microstructures have been examined by extensive mechanical testing and optical and scanning electron microscopy (SEM). Properties arising from various microstructures are tabulated. Mechanical properties are shown to be a function of both the martix and graphite (or carbide) forms. Changing the matrix from ferritic-pearlitic to bainitic-martensitic type results in effects similar to those experienced in steels containing these phases respectively. The influence of graphite (or carbides) on the final properties, however, is dictated by the respective shapes and distributions of these microstractural constituents. The coupled zone-eutectic region in gray cast iron is asymmertical and inclined to the right-hand side in Fe­C equilibrium phase diagram. Consequently, hypereutectic compositions reveal denderites of primary austenite. In white cast iron, the coupled zone symmetry is thought to arise from the high volume fraction of cementite which compensates for its growth rate anisotropy. Key Words: Cast irons, microstructure, mechanical properties, graphite, carbide, coupled zone.

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MICROSTRUCTURE­PROPERTY RELATIONSHIPS IN CAST IRONS

1. INTRODUCTION

It is well established that the properties of any engineering material are determined by its detailed microstructure. The factors that influence the properties of cast iron include the chemical composition of the matrix, and the size, distribution, volume fraction, and morphology of the individual microstructural constituents. There is a wealth of published literature on the effect of microstructure on the mechanical properties of graphitic and alloy cast irons. Among the wider family of cast irons, spheroidal graphide (SG) cast iron is the most extensively studied material. Hence the published data on this material may be reviewed to highlight the close correlation between the microstructure and engineering properties. Particularly, austempered ductile iron (ADI) possesses superior combinations of strength, toughness, ductility, fatigue and wear resistance, damping capacity, design flexibility, and cost effectiveness compared to any other material. Austempering consists of full austenitization, quenching to an austempering temperature to allow isothermal transformation, followed by air cooling. Austempering occurs in two Stages. During Stage I the matrix austenite transforms to ausferrite, which consists of a mixture of acicular ferrite and stabilized austenite enriched with carbon. In Stage II, the stabilized austenite decomposes to ferrite and carbide, thus diminishing the stabilized austenite phase fraction. The effect of retained austenite on the mechanical properties of ADI has been a subject of extensive research. Relatively recent studies [1] indicate that fine dispersion and increased volume fraction of retained austenite enhance toughness and ductility [1,2] as well as the fracture toughness, KIC [3] and, together with bainitic ferrite, contribute to the flow stress of the material [1]. The volume fraction of retained austenite in ADI can be computed by using the lever rule, if the carbon content of the relevant phases are known. Very recently, Chang [4] compared the estimated amount of retained austenite with the corresponding measured value. Although the correlation between the two values for austenite is found to be fairly acceptable, computational prediction is thought to be more reliable. Depending on its matrix structure being constituted of upper or lower ausferrite, the fracture toughness (KIC) of conventional ADI may be increased by 50 to 100 % over that of the as-cast ferrite-pearlite variant, with its hardness nearly doubled [5]. The high KIC and increased hardness levels accomplished with two step austempering of ductile iron are thought to arise from the greater amount of retained austenite and the more intimately interwoven and less sharp ferrite needles of the ausferrite matrix. In alloy white cast iron too, the fracture toughness increases considerably with retained austenite content [6]. The high­stress abrasion­pin test results evidently manifest an unequivocal dependence on retained austenite in these alloys. The matrix structure has been shown to exercize a significant influence on the resonant vibration fracture (RVF) characteristics in SG cast irons [7]; the upper bainitic matrix is observed to exhibit a larger deflection amplitude and higher resistance to major crack formation than ferritic cast irons. The findings of Lin et al. [7] also indicate that the RVF resistance improves with increased nodularity in an identical matrix. In a more recent paper, Lin and co-workers [8] observed that increased amount of pearlite in the matrix results in a decreased logarithmic decrement and lower deflection amplitude. The ferrite rim around nodular graphite particles accounts for the lower initial deflection amplitude which, in turn, leads to the highest RVF resistance. The surrounding pearlite offers a better crack propagation resistance probably by acting as an effective crack front deflector. Luo et al. [9] report that the fatigue properties are affected by the matrix microstructure of the ductile irons. The fatigue limit increases with the increases in tensile strength in the order of ferritic iron, pearlitic iron, and ADI.

A

B

C

D

E

Figure 1. Types of flake graphite according to ASTM A247.

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Until recently, cast irons have been categorized according to the distribution of their respective graphite phase as typified by the specification ASTM A247. In view of the researches [10­12] that have shown cast iron structures to be much more complex than thought originally, there is a growing concern to amend the specifications used hitherto in order to take into consideration the newly observed morphologies. In grey cast iron, for instance, a variety of graphite shapes and distributions add to the complexity. The ASTM classification generally relies on the various patterns of graphite distribution as illustrated in Figure 1. Type-D flake morphology in this classification, for example, when examined by optical microscopy, closely resembles coral graphite. However, this doubt is fully resolved by using scanning electron microscopy (SEM). Hence, one of the objectives of the present work has been to redefine graphitic cast irons according to the morphology of their graphite phase, identfy the condition and mechanisms leading to their growth, and elucidate some of the important crystallographic and microstructural features. The second major objective has been to indicate the strong influence exercised by the microstructural state on the mechanical and physical properties. Since these properties are dependent upon graphite shape and distribution, in this work special emphasis is laid upon the shape factor, and all graphitic cast irons, including the malleable cast iron, are examined from this perspective. amechanical properties model has been developed to quantitatively describe the relationship between selected mechanical properties, such as hardness, tensile strength, and elongation, and microstructural features of cast iron. In white cast iron, carbide, as a competing phase with graphite, is also investigated. Consequently, in the present paper, graphitic cast irons are classified as flake­lamel, coral­fibrous, vermicular­cylindrite, and spheroidal­nodular respectively. 2. EXPERIMENTAL DETAILS Cast irons prepared individually by high frequently (HF) electromagnetic field induction melting were first subjected to tensile tests at a crosshead speed of 2 mm per minute, using an Instron universal testing machine model 6025. Additionally, Brinell hardness tests were carried out by a universal hardness tester. Metallographic studies of unetched or lightly/heavily etched samples were conducted by using optical microscopy. After cutting the specimen, grinding was performed by using emery papers of grade 80 to 800. Diamond polishing from 5 to 1 micrometer particle size followed. Specimen surfaces were deep etched for SEM investigations. Deep etching for 10 hours in 15% HCl acid solution was followed by cleaning for 10 minutes with 5% hydrofluoric acid. The period of deep etching depends on both the concentration of the etchant and its renewal. The cleaning in hydrofluoric acid was continued until the bubbling action subsided. Although etching and cleaning times are a function of fineness of structure, the matrix was also found to be an important factor. The rate of attack was large for the and matrix. In contrast, the pearlite seemed to exhibit little depth in long etching. Samples with their surface graphite and/or carbide extracted were rinsed in methanol to remove any acid residue from the surfaces and then thoroughly dried. It should be pointed out that the graphite and carbide phases, which are normally less affected by acid attack, show marked deterioration when exposed to acid media over prolonged periods of etching or cleaning. The chemical compositions, solidification conditions and the mechanical test results of various cast irons investigated in this study are presented in Table 1. 3. 3.1. MICROSTRUCTURAL OBSERVATIONS Cast Irons with Graphite

3.1.1 Flake­Lamel Graphite Cast Irons These irons may be subdivided into various groups according to the liquid composition and solidification conditions (Figure 1). In type-C coarse primary flake graphite crystals are observed at hyper-eutectic compositions. These crystals are a few mm in length and a few hundred µm in width. They can be differentiated from the eutectic flakes by their coarse forms and smooth surfaces. Cooling rate was found to exercise a dramatic effect on the size and distribution of the graphite crystals. Increasing the cooling rate leads to fine graphitization and favours type-A against type-B formation. Type D and E graphites form for all compositions at high cooling rates, and in the case of hypo-eutectic compositions they form irrespective of the cooling rate. Figure 2b indicates flake graphite in three dimensions associated with twist and rotational faults. Graphite has a hexagonal structure the unit cell of which is defined by two (0001) basal planes and six (100) rectangular planes perpendicular to the basal planes (Figure 3a). Carbon atoms in the basal planes have strong covalent bonding, while adjacent basal planes are held by weak van der Waal forces. The basal planes have the appearance of a stack of layers of several thicknesses [13]. Individual layers are 10-5 cm in thickness and show rotations about the c axis of ~ 13°, ~22°, and 28° (Figure 3c). Flake graphite grows from nuclei formed on twist boundaries. Crystal growth occurs along the basal plane by using these steps. This is referred to as A-growth. In the case of flake crystals there is little growth along the

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direction perpendicular to the basal plane. This is termed C-growth. Coarsening of flake crystals, changing of orientation, branching, and bending provide convincing evidence that C-growth in these crystals is negligibly small. Table 1. The Structure and Properties of Various Types of Cast Irons. No. Type of Cast Iron C 1. 2. 3. 4. 5. Flake graphite Flake graphite Fibrous graphite Cylindrical graphite Spheroidal graphite (Ferritic) Spheroidal graphite (Pearlitic) Austenitic flake graphite White iron Malleable iron (Ferritic) Malleable iron (Martensitic) 3.31 2.95 3.81 3.5 3.60 Chemical Composition (Weight %) Solidification Condition Mechanical Properties 0.2, MPa max, MPa 298 365 334 Ti(0.08) Mg(0.02) 44mm,dia. Mg(0.05) 150mm,dia. Mg(0.05) 44mm,dia. Ni(19.6) Cr(2.16) Cr(0.93) 340 0.040 Quenched 690 14 5.0 462 400 400 3.5 3.0 6.0

Si 2.48 2.45 2.60 2.30 2.20

Mn 0.54 0.45 0.01 0.40 0.40

S 0.037 0.033 0.002 0.010 0.010

P 0.019 0.140 0.012 0.020 0.010

Others Ni(1.25) Mo(1.07)

%

BHN 220 238

6. 7. 8. 9. 10.

3.60 2.45 2.23 2.40 2.4

2.20 1.83 0.49 1.40 1.40

0.40 1.02 0.43 0.50 0.50

0.010 0.025 0.031 0.040 0.180

0.010 0.018 0.022

680 210

6.0 131 120

0.2 = 0.2% offset yield strength max = Tensile strength

% = Percent elongation BHN = Brinell Hardness Number

a

b

Figure 2. Flake-lamel graphite cast iron (a) Optical micrograph, × 110, (b) SEM micrograph, × 400.

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Due to the presence of multiple steps on (100) planes, growth on these surfaces is relatively easier than that on faceted (0001) surfaces. The presence of thermal and compositional gradients in front of the growing crystal prevents the stable growth of graphite. Change of orientation, bending, and branching, which are seen in lamellar growth, in normal eutectics, are also observed in flake graphite growth. There are two mechanisms that could operate for the branching and bending of flakes. The first of these involves straight orientation change of growing (0001) plane by twinning [13,14] and the second is associated with flake branching in growing (0001) plane by the occlusion of foreign particles [15].

Basal plane surface

Prism plane surface

a

13o 22o

c

28o

b

Figure 3. Flake graphite (a) Basal and prism planes, (b) Growth in (a)-direction by step of twist boundary, (c) Rotations of growth layers.

3.1.2.

Fibrous­Coral Graphite Cast Irons

Increasing the cooling rate leads to a shift from A-type coarse flake to undercooled D-type flake. The undercooled flake, on the other hand, transforms to fibrous graphite with further increase of cooling rate. Flake graphite exhibits a lamellar feature in contrast to fibrous graphite, which manifests a circular section and rod­like character. In these crystals there is no abrupt corner so that growth continues uninterruptedly. Figure 4 illustrates observation of fibrous graphite. The SEM evidence shows a rounded growth feature as well as weak lamellar structure in fibrous crystals.

a

b

Figure 4. Fibrous coral graphite cast iron, (a) optical micrograph ×150, (b) SEM micrograph ×1200.

Lux, et al., [15] have convincingly shown that the crystal either grows along the c­ direction perpendicular to the (0001) planes as sketched in Figure 5(b), or along a-direction by scrolled or conical development of (0001) planes (Figure 5 (a , c). The growth of graphite along c-direction requires two dimensional nucleation or screw dislocation sites; but even then this type of growth is unfavorable. Relatively easy growth occurs when basal planes lie parallel (scroll) or semi-parallel (conical) to the axes of fibres.

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[0001] (0001)

a

b

c

Figure 5. Fibrous graphite growth models based on Lux, et al. data [15]; (a) Conical forms; (b) Growth by screw dislocation (c) Scrolled form

3.1.3 Cylindrical ­Vermicular (CV) Graphite Cast Irons This type of graphite growth is observed in alloys containing insufficient spheroidiser (0.02% Mg). Inter-connected cylindrical growth is evident in Figure 6. Mg additions can only neutralize the effect of S in the gray iron, leading to a half­rounded graphite shape. This crystal form lies in between degenerate flake­fibrous and spheroidal growth forms. While spheroidal graphite shows separate nucleation and growth, cylindrical graphite exhibits conjoint nucleation and continuous growth. Excessive Mg addition (0.05%Mg) leads to transformation from cylindrite to the nodular form.

a

3.1.4. Nodular­Spheroidal Graphite Cast Irons

b

Figure 6: Cylinderite vermicular graphite cast iron, (a) Optical micrograph ×110. (b) SEM micrograph ×1600.

Using differential thermal analysis (DTA) it has been shown [16] that a cooling rate in the range 1­20 K/min. affects the onset of the ferrite growth in spheroidal graphite (SG) iron. The temperature for the start of this transformation in a base Fe­C­Si alloy is decreased by small addition of Mn or Cu. SG growth occurs from individual and separate nuclei present in the liquid. It is suggested that spheroidal graphite develops as a result of two dimensional growth on (0001) planes. This type of growth in the c-direction can be aided by screw dislocations. An alternative suggestion involves growth in the [100] direction of wrapped (0001) planes. These planes develop in a continuous manner by giving rise to tilt­twist boundaries. Deep etching of polished surfaces has consistently revealed tilt­twist boundaries. However, extension is always along the a-direction. As the (0001) planes continue to warp, the overall growth of the nodule occurs in the [0001] direction. This "cabbage-like" growth, which is frequently observed on the surface of graphite crystals, is illustrated in Figure 7.

a

b

169

Figure 7. Nodular­spheroidal graphite cast iron; (a) Optical micrograph ×100, (b) Extracted graphite (SEM ×1600)

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SG sections are observed to embody conical segments when examined by optical microscopy (Figure 8 (a). This depicts a good example of symmetrical growth, which involves a tilt­twist boundary. As seen in Figure 8 (b), the traces in the segment are reminiscent of helical growth. Cylindrical graphites may grow either in a helical or wrapped manner (Figure 8 (b) and 8 (c). Optical and SEM evidence presented in Figure 6 supports this suggestion. In the optical micrograph three different features are noteworthy. These are the cylindrical structure, star-like structure, and spheroidal structure. The star-like structure, which is covered by a halo, is not related to the growth model of Figure 8 (d). This structure develops under active nucleation conditions. Following nucleation, this crystal showed rapid initial growth until actual obstruction by Mg occurred. Initial growth was edgewise in the a-direction which led to thickening. Two-stage growth as presented here for the Fe­C system has also been reported for the Al­Ge [17] and Al­Si [18]. Shape aspects and geometric considerations dominate the concept of cabbage-like growth. It has been suggested that a nodule may originate from a very fine flake that rolls into a ball. This mode of formation is a matter of controversy and there is no clear evidence of cabbage-like growth. Carbon, silicon, and germanium crystals grow in a faceted manner in liquid alloys. Since the number of growth directions is higher in Si and Ge than in graphite crystals, star-like growth and concomitant thickening mechanism can develop more easily in them.

c-direction

c c c

c

a-direction

a b

c-direction

c

d

Figure 8. SG growth models; (a) Optical micrograph of nodule × 800; (b) The nucleus of a spherulite; (c) Cabbage-like growth; (d) Star-like branching.

3.1.5. Temper Graphite Cast Irons White irons are heat­treated to decompose the metastable Fe3C into graphite. The final structure consists of graphite temper rosettes in a ferritic or pearlitic matrix. The development of temper graphite depends on solid-state diffusion and is influenced by the carbide structure, composition, tempering temperature, and time, as well as the furnace atmosphere. The microstructure of malleable cast iron is depicted in Figure 9.

a

3.2. Cast Irons with Carbide

b

Figure 9. Temper graphite cast iron: (a) Optical micrograph ×110, (b) SEM micrograph × 900.

Carbides in cast irons may have the general formulae M3C, or M7C3. These carbides may occur as lamel, rod, or continuous matrix. In unalloyed cast iron, Fe3C either develops in a lamellar manner or forms a continuous phase. Any graphite present in the structure leads to mottled appearance. The carbide phases impart hardness wear and abrasion resistance. The microstructure of the unalloyed iron generally consists of M3C carbides and pearlite as shown in Figure 10.

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a

b

Figure 10. Cast-iron with carbides: (a) Optical microstructure × 110, b) SEM micrograph × 510. However Ni, Cr, and Co additions both increase the hardness and toughness. Especially M7C3 carbides in these alloys show broken lamellar structure and display greater toughness. The M7C3 carbides are also reported to grow as rods and blades with their long axes parallel to the heat flow direction in the mould [19]. 3.3. Eutectic Growth Conventional eutectic melts of the Fe­C­Si system exhibit a tendency to precipitate dendrites upon solidification, so that dendritic structures are detected even in hypereutectic alloys. Recently, Lesoult et al. [20] presented a physical model which permits to describe the formation of such microstructures during the freezing of iron melts. In cast irons primary grows with non-faceted dendritic morphology. Whilst, primary graphite or carbide grows with faceted lamellar morphology. Primary phases in this alloy are surrounded by a eutectic phase. The eutectic growth region is a zone of composition and temperature. The more rapidly developing phase ( or graphite) dominates the eutectic structure. The purely eutectic structure extends on either or both sides of the eutectic point. The eutectic region develops without primaries. Especially high liquid temperature gradients expand this zone, causing coupled eutectic growth to develop easily. The formation of completely eutectic structures has a special significance in relation to certain mechanical properties. When a eutectic structure is produced with controlled orientation, composite-like properties are obtained to advantage. In gray cast iron, the eutectic region extends to the right hand side, while in (WCI) the eutectic region is highly symmetrical [21,22]. Asymmetrical eutectic zone in graphite containing irons is due to the anisotropic growth behavior of graphite. The near-symmetrical coupled zone in (WCI), on the other hand, is due to a high volume fraction of carbide (Figure 11). In the case of the spheroidal graphite eutectic, each crystal of graphite is taken to be a eutectic particle. This eutectic forms by separate nucleation and growth of graphite. Star-like crystals nucleate freely and grow by branching and behave as origins of eutectic cells. Each spheroidal form is to be regarded as an unbranched star-like crystal. In vermicular­ graphite eutectic, the amount of cylindrite graphite cells cannot be calculated, because the cell boundary is not clearly defined. Eutectic graphites are strongly interconnected. Therefore, the number of eutectic cells appears to be less than that of spheroidal eutectic. In fibrous eutectic, graphite and austenite crystals grow in a coupled manner and the eutectic cell number density changes according to the solidification variables. In the case of flake graphite, the eutectic is like a fibrous one and shows cooperative growth in the eutectic cell. All the graphite in the unit cell is interconnected. Directional solidification conditions lead to only one eutectic cell with elongated fibrous or flake-like growth. Eutectic growth in (WCI) presents similar complications. However, in the ledeburitic transformation it is very reminiscent of a pearlitic transformation in steel [23]. Extensive directional solidification experiments carried out showed that in -graphite eutectic, flake graphites do not develop at right angle to the solid-liquid interface. At a given growth rate, due to equal inter-lamellar distance, there is multiple branching in graphite. Branching compensates for compositional and thermal fluctuations. While carbon enrichment in front of the eutectic austenite occurs, the boundary of and graphite is relatively devoid of carbon. The high level carbon concentration in the center of austenite leads either to nucleation of graphite or its branching from the boundary. The graphite developed at austenite pockets provides a typical example for undercooling graphite. For high G/V rates (G being the liquid temperature gradient in front of the solid, and V the growth velocity of the solid interface) the interface is stable and the -liquid and the graphite­liquid interfaces share the same line. In this case, branching is

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less than lamellar development. At intermediate G/V values the graphite­liquid interface leads the austenite-liquid interface and frequent branching at non-stable solid-liquid interface is readily observed. At low G/V values, the -liquid interface leads eventually as obstruction of graphite growth becomes preponderant. Then, severe branching and transformation of graphite into fibrous forms is possible.

Growth velocity, mms-1

1

Fe3C eutectic only G eutectic and dendrites G eutectic

a

10-2

10-4 1180

b

-G - Fe3C eutectic

1 2

Temperature, oC

1160

Fe - G coupled zone

1140

Fe - Fe3C coupled zone

1120

3.50

4.00

4.27 4.50

5.00

Carbon content, wt%

Figure 11. The coupled zones in directionally solidified Fe­C alloys with G:70 oC cm-1.

(a) Velocity-composition plot showing calculated boundaries and experimental points, (b) Temperature-composition plot with superimposed phase diagram. Region 1. G plates and G eutectic. Region 2. Fe3C plates and eutectic [19]. This development can be explained by a competitive growth mechanism (Figure 12). In this mechanism alternative phases compete and grow at different rates under different solidification conditions. Thus, at a given undercooling, T1, flake graphite grows faster than austenite. In this way, while flake graphite leads, austenite shows pocket formation in the center. At T2 (high undercooling) the alternative phases develop with the same growth velocity. At T3, austenite grows faster than flakes whose growth is frequently interrupted. Consequently, under conditions of progressively higher undercooling (i.e. lower G/V ratios and chill casting) flake graphite transforms to fibrous form. Experimental studies invariably showed graphite and austenite competition. At high temperatures, graphite and austenite grow side by side and austenite forms a halo around graphite flakes or nodules. When Mg depresses the growth temperature of graphite, unaffected austenite growth leads the graphite, and thick austenite halos with dendritic extension are observed.

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I

V

G

G

T3

a

T1

T

T2 T3

T1

b

T2

Figure 12. -G, growth in positive temperature gradient. (a) Growth rate (V) dependence on undercooling (T) for and graphite. I represents flake- fibre transition; (b) Solid­ liquid surfaces.

4.

STRUCTURE­PROPERTY RELATIONSHIP

The solidification of cast iron with a hypoeutectic composition begins with the nucleation and growth of austenite on the mould wall. As the temperature falls, eutectic solidification commences in the interdendritic liquid. This eutectic is either Fe­Fe3C or Fe­graphite. The proeutectic dendrites strengthen the iron and can be compared to the fibers of a composite. Their strengthening effect depends on their composition, structure, continuity, and fineness. The latter features are promoted by solidification conditions unfavorable to nucleation, namely, high superheat, high pouring temperature, directional growth with a high temperature gradient, low growth velocity, and low solute content. The properties of the dendrites are influenced considerably by the solid-state transformation, which in turn, depends on austenite composition and cooling rate. Pearlite formation which increases the strength of the alloy (Figure 13) is favoured by fast cooling rate, low CEV and alloying elements, such as Mn, Ni, and Cr and trace elements like Cu, Sn, Sb, and As. Heat treatments, such as, austempering in nodular irons, or isothermal heat treatment to produce a bainitic matrix can lead to exceptionally high strengths coupled with good elongation [24].

900 UTS 800 700 N/mm2 600 500 400 300 0.1% proof elongation, %

30

10

0

20

40 60 Pearlite, %

80

10

0

Figure 13. Effect of pearlite fraction on mechanical properties in spheroidal graphite irons [13].

Elongation, %

20

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Table 1 illustrates the fact that in various cast irons mechanical properties are more dependent on microstructure than chemical composition. For example, coarse flake graphite irons (C-type) show lower ultimate tensile strength (UTS) than fine flake graphite irons (A-type). In SG iron, changing the matrix from ferritic structure to pearlitic increases the strength, but decreases ductility. The beneficial effect of stepped austempering on the fracture toughness of SG iron has been interpreted in terms of upperausferrite morphology. More intensely interwoven and less sharp needles of ferrite are believed to reduce the stressconcentration effect [25]. It is well established that cylindrical­vermicular graphite cast iron shows a thermal conductivity higher than that of spheroidal graphite but less than that of fibrous graphite, in which a highly intimate interconnection between graphite crystals exists. This contributes greatly to conductivity. Spheroidal graphites show no such interconnection, while in vermicular graphites a reasonable degree of connectivity between graphite crystals is observed. An analogous argument will hold when comparing mechanical properties. In fibrous graphite cast iron, due to the good connection between graphite crystals and fine homogeneous distribution, the ductility is 10 times higher than that of flake graphite cast iron. It is interesting to note that with recent progress in simulation of solidification processes, predictions of solidification microstructural evolution and resulting material properties are presently becoming possible[26]. 5. 1. CONCLUSIONS Cast irons constitute a family of casting alloys comprising flake-lamella, fibrous-coral, cylindrical-vermicular, nodular-spheroidal, temper graphite, and white types, all display excellent founding characteristics and satisfy a wider range of engineering requirements than any other metal. In cast irons the nucleation and growth of graphite with novel morphologies is influenced by the degree of undercooling, crystallographic characteristics and alloying additions. Increasing the cooling rate results in progressively finer graphitization. Graphite morphology exercises a strong influence on the mechanical and physical properties of cast irons. Impurity elements, addition (like Mg) and cooling rate play a major role in the development of Fe­ Fe3C or Fe­G morphologies. Structural shapes and growth modes such as a-direction growth or c-direction growth are the main indicators. In the case of flake crystals, a-growth is favored. c-growth is the main route in spheroidal graphite formation. In between the two extreme conditions, other graphite forms develop. Austenite growth parallel to graphite growth has a significant bearing on several engineering properties of cast iron. Off-eutectic growth, coupled growth, and competitive growth mechanisms (halo formation) add to the complexity of Fe­C structures. There is strong corelation between structure and mechanical properties of cast iron. While the malleable martensitic iron offers a high UTS (690 MPa) with modest elongation (5%), the ferritic variant exhibits a low strength (340 MPa) and impressive ductility (14%) SG iron with a pearlitic matrix exhibits a high UTS (680 MPa) coupled with a moderate elongation of 6%. In view of the novel microstractural data pertaining to various morphologies of the graphite phase, new specifications need to be expounded to modify the existing ASTM specifications.

2.

3.

4. 5.

ACKNOWLEDGMENTS The authors are indebted to Professor E. Smith for the provision of research facilities at the University of Manchester, Institute of Science and Technology (UMIST), Materials Science Centre, Manchester, England. REFERENCES

[1] J. Aranzabal, I. Gutierrez, J. M. Rodriguez-Ibabe, and J. J. Urcola, 1997, "Influence of the Amount and Morphology of Retained Austenite on the Mechanical Properties of an Austempered Ductile Iron", Metallurgical & Materials Transaction 28 A (1997), pp. 1143­1156. H. Bayati, and R. Elliott, "Relationship between Structure and Mechanical Properties in High Manganese Alloyed Ductile Iron", Mater. Sci. Technol., 11, (1995) pp. 284­293. C. Hsu, S. Lee, H. Feng, and Y. Shy, "The Effect of Testing Temperature on Fracture Toughness of Austempered Ductile Iron", Metallurgical & Materials Transaction, 32 A. (2001), pp. 295­303. L. C. Chang, "An Analysis of Retained Austenite in Austempered Ductile Iron", Metallurgical & Materials Transaction, 34 A (2003), pp. 211­217. C. H. Hsu, and T.L. Chuang, "Influence of Stepped Austempering Process on the Fracture Toughness of Austempered Ductile Iron", Metallurgical & Materials Transaction, 32 A (2001), pp. 2509­2514.

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T. Alp, A. A. Wazzan and F. Yilmaz

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[10] C. R. Loper Jr., R. C. Voigt, J. R. Yang, and G. X. Sun, "Electron Microscope in Studying Growth Mechanisms in Cast Irons," AFS Transactions, 172 (1981), pp. 529­542. [11] P. C. Liu, C. R. Loper Jr., T. Kimura, and H. K. Park, "Observations on the Graphite Morphology in Cast Iron," AFS Transactions, 88 (1980), pp. 97­188. [12] D.D. Double, and A. Hellawell, "Growth Structures of Various Forms of Graphite", in The Metallurgy of Cast Iron. Geneva: Georgi publishers Co., (1974), pp. 509­528. [13] D.D. Double, and A. Hellawell, "The Structure of Flake Graphite in Ni­C Eutectic Alloy," Acta Metallurgica, 17 (1969), pp. 1083­1083. [14] H. Nieswaag, and A. J. Zuithoff, "The Effect of S, P, Si, and Al on the Morphology and Growth Structure of Directionally Solidified Cast Iron," in The Metallurgy of Cast Iron. Geneva: Georgi publishers Co., 1974, pp. 327­351. [15] B. Lux, I. Minkoff, F. Mollord, and E. Thury, "Branching of Graphite Crystals Growing from a Metallic Solution," in The Metallurgy of Cast Iron. Geneva: publisher, 1974, pp. 494­508. [16] J. Lacaze, A. Boudot, V. Gerval, D. Oquab, and H. Santos, "The Role of Mn and Cu in the Eutectoid Transformation of Spheroidal Graphite Cast Iron," Metallurgical & Materials Transaction, 28A (1997), pp. 2015­2025. [17] C. Lemaignan, D. Camel, and J. Pelissier, "In-Situ Electron Microscopy of some Solidification Processes in Metallic Alloys," Journal of Crystal Growth, 52 (1981), pp. 67­75. [18] F. Yilmaz, Ph.D. Thesis, University of Manchester, Manchester, England, 1979. [19] O. N. Dogan, J. A. Hawk, and H. G. Laird, "Solidification Structure and Abrasion Resistance of High Chromium White Irons," Metallurgical & Materials Transaction, 28A, (1997), pp. 1315­1328. [20] G. Lesoult, M. Castro, and J. Lacaze, "Solidification of Spheroidal Graphite Cast Iron­I. Physical Modelling", Acta Mater, 46 (3) (1998), pp.983­995. [21] R. Elliott, Cast Iron Technology. London; England: Butterworths, 1989. [22] H. Frediksson "The Coupled Zone in Gray Cast Iron," Metallurgical Transaction, 6 A, (1975), pp. 1958­1960. [23] M. Hillert, and H. Steinhauser, "The Structure of White Cast Iron," Jemont Ann, 144 (1960), pp. 520­559. [24] G. J. Cox, "The Heat Treatment of SG Iron," The Metallurgist and Material Technologist, 12 (1980), pp. 629­632. [25] C. H. Hsu, and T. L. Chuang, "Influence of Stepped Austempering Process on the Fracture Toughness of Austempered Ductile Iron", Metallurgical and Materials Transactions, 32A (2001), pp.2509­2514. [26] R. D. Pehlke, "Computer Simulation to Solidification Processes­The Evolution of a Technology, Metallurgical and Materials Transactions", 33A (2002), pp. 2251­2273.

Paper Received 6 September 2003; Revised 14 August 2004; Accepted 13 October 2004.

October 2005

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