solidification, processing and properties of ductile cast iron

25 YEAR PERSPECTIVE

Solidification, processing and properties ofductile cast iron

N. S. Tiedje*

Ductile cast iron has been an important engineering material in the past 50 years. In that time, it

has evolved from a complicated material that required the foundry metallurgist’s highest skill and

strict process control to being a commonly used material that can easily be produced with

modern process technology. Yet, for the skilled metallurgist and foundry engineer, it is a material

that can be engineered to meet extreme demands with regard to mechanical properties and

geometrical complexity. It is therefore a material that has been in growing use since its discovery.

And the results of the latest years of research indicate that ductile cast iron in the future will

become a highly engineered material in which strict control of a range of alloy elements combined

with intelligent design and highly advanced processing allows us to target properties to specific

applications to a much higher degree than we have seen previously. It is the aim of the present

paper to present ductile iron as a modern engineering material and present the many different

possibilities that the material hides. Focus will be on the latest research in solidification and melt

treatment. But for completeness and to illustrate how ductile iron’s properties are optimised, the

essentials of heat treatment are described too. It is the hope that researchers will find a

comprehensive treatment of ductile cast iron metallurgy and that engineers and designers will be

presented with the latest information on, and references to, the properties and possibilities in

ductile cast iron.

Keywords: Ductile cast iron, Solidification, Mechanical properties, Processing

IntroductionCast iron has been a popular and important structuralmaterial since the technology for melting iron wasdeveloped in the 1700s. For more than 200 years, castiron has been used for many structural and artisticconstructions.1,2 In the 1800s, grey cast iron wasconsidered the most advanced material of the time andwas the basis for the technological advances during theindustrial revolution such as steam engines, trains andproduction machines. Also architecture benefited fromthe development of a strong material that is easilyshaped to any conceivable design. One of the mostsplendid examples is the Royal Pavilion in Brighton.3

Grey cast iron is a very brittle material, and forcenturies, cast irons could only be made ductile throughdays of heat treatment of the so called ‘malleable irons’.2

In the late 1940s, it was demonstrated that ductile castiron could be made by adding Mg to the liquid metal.2

In the late 1950s, there was a steadily increasing interestin the new ductile irons and laboratory results weregradually adopted by the industry. In the late 1950s and

early 1960s, ductile iron emerges as high quality castmaterial, but it was a material that required muchtighter control with raw materials and processes thanother cast irons.4–6 Many foundry engineers consideredit a complicated and unpredictable material to producebecause small variations in raw material quality wouldbe detrimental to casting quality.

The potential in ductile iron was clear from thebeginning. It is a material that is much easier to castthan steel but with similar properties. The goodproperties could be achieved as cast, avoiding the longheat treatment required to for the production ofmalleable iron. Therefore, there was a strong interestin overcoming the difficulties.

In the early years, researchers put much effort intounderstanding the mechanisms that control microstruc-ture and properties.1,2,4,6–9 Research was quickly com-mercialised by companies that built their reputation onsupplying raw materials of suitable quality and convey-ing the knowledge of ductile iron process technology tofoundries.10–13

It is the combined efforts of researchers and industrythrough the 1960s until today that has lead todevelopment of reliable raw materials, modern processtechnology and quality control that allows us to producehigh quality castings at a very low cost.

Department of Mechanical Engineering, Technical University of Denmark,DK-2800 Kgs. Lyngby, Denmark

*Corresponding author, email [email protected]

� 2010 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 6 January 2010; accepted 16 February 2010DOI 10.1179/026708310X12668415533649 Materials Science and Technology 2010 VOL 26 NO 5 505

It is interesting, however, that in spite of this large,combined effort, it is only very recently that we arebeginning to really understand the fundamental chemi-cal, physical and thermodynamic processes that controlthe microstructure in ductile cast irons.14–23

Ductile cast iron today is essentially very differentfrom the historical irons produced from the early 1700suntil the late 1950s. It is a material that can be designedto meet a wide range of engineering applications withregard to high and low temperature strength, ductility,fatigue and corrosion properties. It is easily cast in verycomplex shapes. It is also a very complex material wherealloy composition, casting design and productiontechnology all are important factors for the performanceof the final product. Figure 1 shows examples of typicalmodern ductile iron castings. They represent small andmedium sized heavily loaded parts with high demandsfor consistent quality for the automotive industry andvery large industrial components with extreme demandsfor mechanical properties, particularly fatigue strengthand fracture toughness.1,2,24

As a result, the consumption of ductile iron castingshas increased at the same rate as that of cast aluminiumin the last decade.25 And as our fundamental knowledgeof the material increase, new possibilities and perspec-tives unfold to the benefit of modern society.

What is ductile cast iron?Cast irons are in principle near eutectic Fe–C–Sialloys.1,2 Although for many applications, a number ofother alloy elements are added to control mechanical,physical and chemical properties.

At the equilibrium eutectic transformation, austeniteand graphite are formed. The shape of the graphite and

the growth morphology of the eutectic are highlydependent on the chemical composition of the melt.

Since cast irons are near eutectic, they flow well andare well suited for large, thin walled castings of highcomplexity.26

In grey cast iron. graphite grows as large flakesembedded in the austenite matrix. Fracture occurs alonggraphite flakes and renders the fracture surface grey, hencethe name. The large, faceted flakes reduce ductility toalmost zero but they give the material other qualities: as theaustenite shrinks during solidification, graphite expandsso that shrinkage porosities are minimised and sometimescompletely avoided. The large graphite flakes serves todampen noise and they increase thermal conductivity.27,28

In white cast iron, the equilibrium reaction issuppressed so that metastable carbides form instead ofgraphite. The metastable eutectic, called Ledeburite,29 isvery hard and brittle and has good strength. White ironshave excellent wear properties, but can only be machinedwith difficulty. Since graphite is the stable form of carbonin cast iron, the white irons can be heat treated overmany days during which the carbides dissolve and aretransformed to graphite with a nodule-like shape.

Figure 2 shows typical microstructures of grey, ductileand white cast irons. The difference in graphite shapeand distribution is clear and is a good illustration of whythere is a large difference in properties of the materials.

With the discovery that additions of Mg, Ce and/orLa change the shape of graphite from lamellar tonodular, a whole new world of cast materials wasrevealed. It was now possible to cast irons that areductile without the following heat treatment.2,4,9

As the graphite shape is changed from lamellar tospheroidal, nodules are embedded in a coherentaustenite matrix. Because the graphite has a nodular

1 Typical ductile iron castings. a) Control arm 2?9 kg. b) Brake calliper, 3?4 kg. c) Wind turbine hub 22 ton. a) and b)

are published with permission from Disa Industries A/S and c) with permission from Vestas Wind Systems A/S

2 Micrographs from optical microscope, all etched with nital. a) Ferritic-pearlitic ductile iron. Black spheres are graphite

nodules, grey areas are pearlite, and white is ferrite b) Ferritic-pearlitic grey iron. Largeblack lamels are graphite

flakes, light grey areas are pearlite, and white is ferrite. c) White cast iron. Primary austenite dendrites, grey, trans-

formed to pearlite. Between dendrites is Ledeburite, eutectic carbides (white) and pearlite (grey) from originally eutectic

austenite

Tiedje Solidification, processing and properties of ductile cast iron

506 Materials Science and Technology 2010 VOL 26 NO 5

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shape and the matrix is continuous, the material’sproperties are now primarily determined by the steelmatrix and as such, it becomes ductile and strong.

Typically, cast irons are characterised by their carbonequivalent (CEV). There are several suggestions tocalculate CEV in the literature, but the most commonlyused is1,2,30–33

CEV~%Cz%Siz%P

3(1)

Typical ductile cast irons contain 3?0 to 4?3 wt-%Cand 1?8–2?8 wt-%Si with CEV varying from 3?6 to4?9 wt-%.2,6,9,24,31,34,35 The eutectic point is at CEV5

4?3 wt-% so that ductile irons are near eutectic.

In equilibrium, carbon will precipitate as graphite andlike in grey iron, the graphite expands during solidifica-tion and compensates for the shrinkage.36,37

If the initial eutectic undercooling is too large,metastable carbides may form instead of graphite.Figure 3 shows a section of the Fe–C–Si phase diagramclose to the eutectic point for a Si content of 1?8 wt-%(black lines) and 2?5 wt-% (grey lines) respectively. Theequilibrium eutectic temperature (TEG) is increasedwith addition of Si while the metastable eutectictemperature (TEC) decreases. Addition of Si movesthe eutectic point towards lower carbon content asdescribed by equation (1).

Near the eutectic point, the lines in the phase diagramcan be approximated by simple linear equations, asdescribed by several researchers.8,30,33,38–42 The follow-ing are commonly used examples, where TAL is theaustenite liquidus temperature, TGL is the graphiteliquidus temperature, TEG is the equilibrium (graph-ite) eutectic temperature and TEC is the metastable(carbide) eutectic temperature

TAL~1576{97:3(%Cz0:23%Si) (2a)

TGL~{534:5z389(%Cz0:29%Si) (2b)

TEG~1153z4:865%Si (2c)

TEC~1150{12:5%Si (2d)

The exact values of the constants vary little fromone source to the other as long as the alloy is primarily

ternary. The equations show the strong effect that Cand Si have on the liquidus temperature, particularlyon the graphite side of the eutectic point. It is alsoclear that Si content is very important to control thegap between the equilibrium and metastable eutectictemperatures.

The size and distribution of nodules are important forthe development of the material’s microstructure andproperties because it has strong influence on thefollowing solid state transformation.

After solidification, the equilibrium concentration ofcarbon in austenite is reduced. According to Wessen andSvensson, the fraction of graphite will increase fromy0?07 wt-% after solidification to y0?1 wt-% aftercooling to room temperature.43

At y800uC, austenite transforms to ferrite andgraphite and/or pearlite in a eutectoid reaction. Theamount of pearlite formed during solidification deter-mines strength and ductility. Therefore, cast irons ofvarying grades are often referred to as ferritic, ferritic/pearlitic or pearlitic.

Owing to the eutectoid reaction, it is possible to heattreat ductile cast irons and by that modify mechanicalproperties considerably.1,2,7,24,44

Further improvements of ductile irons is possible byalloying with Ni (10–40 wt-%) or Si and Mo incombination (3–5 and 0?5–1?5 wt-% respectively)2 forcorrosion resistance, high strength and high temperatureapplications. Such alloyed irons will not be discussedfurther here.

Table 1 shows properties of a selection of ductile ironsaccording to the European standards. It illustrates wellthe wide range of properties that can be achieved withductile cast irons.

Solidification of ductile cast ironWhen cast iron is made from high purity Fe, C and Si inthe laboratory, graphite grows as nodules duringsolidification. Since the raw materials used in industrialprocesses always contain a range of trace elements,particularly S and O, it is necessary to remove thesefrom the melt. After melting, the base metal undergoes acleaning process which is often referred to as ‘magne-sium treatment’. The liquid metal is poured over a Ni–Mg or Fe–Si alloy which contains 3–10 wt-%Mg and, insome cases, rare earth elements.2 During this process,Mg (and the other active elements such as Ce and La)reacts with O and S, reducing the melt’s content of theseelements. After the treatment, residual Mg should be inthe range 0?03–0?05 wt-%. Mg additions may becalculated from2,5

Mg~0:75%Szresidual Mg (0:03{0:05%)

Expected Mg yield(3)

The expected yield depends on local process conditionsin the foundry and among other things, it is verysensitive to melt temperature and time passed after melttreatment.1,2 The effect of the Mg treatment fadesquickly so that the melt should be cast withinapproximately 20 min after treatment.

As already mentioned, cast irons may solidify follow-ing a stable reaction or a metastable reaction. Becausethe two systems are relatively close to each other, it is thefoundry engineer’s job to ensure that the right system is

3 Schematic section of the Fe-C phase diagram for a Si

content of 1?8 wt% (black lines) and 2?5 wt% (grey

lines) respectively


Materials Science and Technology 2010 VOL 26 NO 5 507


activated. For grey and ductile irons, this is in practicecarried out by adding inoculants to the melt that willpromote formation of graphite.45

Nucleation of phasesIn the course of time, a number of theories regardingsolidification of ductile iron have been suggested.2 Thesedifferent theories will not be dealt with here. Instead, asummary of what today is believed to be correct ispresented.

In general, it is agreed that austenite forms readily assoon as the austenite liquidus temperature isreached.17,18,33,46–48 There is no complete description ofhow austenite dendrites nucleate; however, it has beenobserved that dendrites grow from the mould surfaceand that they can grow from the surface of graphitenodules as well.48–52 It is also very likely that broken-offdendrite arms that float in the liquid may serve as nucleitoo, as it has been observed in other cast alloys.45,46,53

At the eutectic temperature, it s generally assumedthat an austenite shell forms around the nodules as soonas the nodule has grown to a stable size, typically with adiameter y5 mm.18,54

Also carbides will nucleate readily when the meta-stable eutectic temperature is reached.33,55

In contrast to that, graphite requires undercoolingand the presence of nuclei to form.19,45,56 Since thephysical and chemical conditions in a casting always aretransitional with regard to cooling rate, temperaturegradients and local melt chemistry, and that nucleationof graphite is very dependent on these factors, it hasbeen a complicated task to get to the bottom of themystery of graphite nucleation. Typically, graphiterequires some undercooling to form, but it has beenfound in the literature57 that some graphite nodules areable to form at temperatures higher than predicted bythe equilibrium phase diagram. The reason for this isunknown, but it is indicated that there are nucleationmechanisms in operation that are not well understood assuggested by Campbell.58

The early research focused on finding processparameters that control graphite nucleation. It wassoon discovered that even minor variations in thequality of raw materials could have great effect ongraphite nucleation because trace elements such asMn, S, P, Ni, Cr, Cu, Mo, Ti, Te, V, As and Sbthat exist in very dilute concentrations in the rawmaterials may vary sufficiently to influence the effectof Mg treatment and inoculation, and lead to defectivecastings.4–6,59 These efforts lead to a thorough de-scription of how melt chemistry, inoculation, coolingconditions and raw material quality influence nuclea-tion of graphite.34,38,60,61

Thermal analysis has been used extensively to describesolidification of cast irons.8,30,31,33,47,62–67 These investi-gations have shown how melt chemistry, cooling rate ofthe casting and different types of treatment alloys, andinoculants promote specific microstructures. It is nowpossible to design treatment alloys and inoculantsspecifically for a given combination of casting wallthickness, type of raw materials and melt treatmentmethod. And methods exist that allows foundries to usethermal analysis to test the quality of the melt and adjustprocess conditions accordingly immediately before cast-ings are made.T

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These efforts have led to increased stability inthe manufacture of ductile cast iron and have been thekey to developing procedures that give high qualityand reliability of ductile iron castings, but they havenot offered any explanation to the physical andchemical processes that take place in the melt duringsolidification.

Therefore, in the past two decades, more detailedstudies into nucleation mechanisms have been con-ducted. In the experiments, focus has been on the core ofgraphite spheroids.21–23,68 Skaland et al.22 have shownthat inoculants added to the melt form complex particlesthat serve as nuclei for the graphite nodules. Thecomposition of the particles can be linked to thechemical composition of the inoculants. From theseinvestigations, it is clear that the nucleus contains a coreof MgS and CaS around which a shell of complexsilicates forms. The composition of the silicates reflectsthe type of inoculants used, so that it may contain Al,Ca, Ba or Sr. This indicates that when inoculants areadded to the melt, they may dissolve completely orpartially and then through a reaction with trace elementsin the melt, form sulphides that again react with silicatesthat provide the right surface on which graphite maynucleate as described by Fredriksson et al.69

Figure 4 shows a section of a graphite nodule taken ina focused iron beam–scanning electron microscope. Thenodule has been cut to reveal the centre of the nodule.The chemical analysis of the inclusions shows that thecore contains Mg and S (Table 2).23

Experiments show nucleation to be a continuousprocess which primarily depends on the potency andamount of inoculants added to the melt and theundercooling at which nucleation occurs.21,66,70

Several suggestions for mathematical nucleationmodels have been proposed and used in the literatureas it has been described in a review by Stefanescu.71

Almost all models have in common that nucleation is afunction of undercooling DT and that they containparameters that describe the effect of inoculation18 andin that way they reflect the experimental observationswell. The following example is based on Oldfield’swork46

dN~An DTgL

� �n{1f L d DT

gL

� �

dtdt (4)

where An and n describes the amount and the efficiencyof the inoculants respectively; and f L is the fractionliquid. This term is active all through solidification sothat nuclei may form at any time depending on theundercooling of the melt.

The equation reflects experimental observations basedon acquisition of cooling curves and analysis ofmicrostructures of ductile cast irons, and links theefficiency of inoculation to solidification in a practicalway. The constants An and n can then be determinedfrom experiments. However, the model does not in anyway reflect the physical, chemical and thermodynamicevents that lead to the creation of sulphides followed bythe formation of a shell of complex silicates aroundthem that is described in the investigations of thecrystallography of the core of graphite nodules.

The models do give some insight into how processparameters influence the microstructure. For instance, ifthe inoculation is good, graphite will form at a lowundercooling, but because the undercooling is low, theamount of nodules formed at a given time is low too.Figure 5 shows a cooling curve from a ductile iron andits derivative. On the curve, some key points are marked.TEU is the temperature at which eutectic solidificationcommence, corresponding to the undercooling DTe.TEM is the maximum release of heat (equivalent tomaximum growth rate) during recalescence. TER is themaximum temperature during eutectic solidification. TFis the temperature at which solidification ends. DTe isequal to TER–TEU and describes the reheating duringrecalescence. It can be seen that at the beginning ofeutectic solidification, the undercooling is relativelylarge. While the first eutectic grows and the temperature

Table 2 Chemical analysis of core the nucleus ofgraphite nodule shown in Fig. 4, wt-%: Ga and Ptare artefacts of milling process23

Spectrum C O Mg Si S Ca Fe Ga Pt Total

IV 79.8 1.4 4.9 0.1 7.3 0.2 3.9 1.3 1.2 100.00V 75.4 1.3 3.4 0.1 4.9 0.1 9.3 1.7 3.9 100.00

4 Image of a milled cross section of a graphite nodule in

sample 2. (I) Platinum deposition (II) Graphite nodule

(III) Re-deposited iron after milling (IV) Core inclusion

(V) Lighter inclusions near the core (VI) Lighter inclu-

sions around the core. Chemical analysis of the core is

shown in table 2

5 Schematic drawing of a cooling curve for solidification

of ductile cast iron. The figure shows temperature vs.

Time and cooling rate (derivative) of cooling curve.

Some important points related to nucleation and

growth are marked on the curves





gradually increases, many nodules will form. The slopeof the cooling curve from TEU to TER indicates howquickly new nodules are formed at the beginning ofeutectic solidification. This can be described by themaximum on the derivative curve, TEM, which is aparameter describing the inoculants potency at thatstage.62,66,72,73 As the temperature increase to TER,nucleation is reduced to a minimum. Towards the end ofsolidification, the undercooling increase again becausethe heat generated from the diminishing amount ofliquid can no longer balance the heat extracted by themould. Again more nuclei are activated and a newpopulation of nodules may form if the inoculant is stillactive.16,73,74 A steep slope of the cooling curve at thispoint combined with a high TF indicates that sufficientnodules are formed at the end of solidification.

It should be clear from the discussion above that byadjusting the potency of the inoculants to a given castinggeometry (which determines local cooling rate), it ispossible to actively modify the final properties of thecasting. In particular, it is possible to suppress themicroporosity that form at the end of solidificationusing an inoculant that is active then as well as wheneutectic solidification starts.16

However, it is important to carefully control thegraphite nucleation because insufficient graphite nuclea-tion will cause shrinkage. However, excessive graphiteexpansion in the early stage of solidification will causethe material to expand and distort the mould. Later asthe last liquid solidifies, the material will shrink andpores are formed.36,53

To achieve a more consistent base melt, it is becomingmore and more common to precondition the meltalready in the melting furnace.75,76 This is carried outby adding an Fe–Si alloy containing traces of Ti, Zr andAl to the melt. These strong oxide formers react to formnuclei and are shown to increase TEU, TER and TF anddecrease DTe. This gives a more homogeneous micro-structure and less shrinkage porosity, and suppressesformation of carbides.

Solidification of hypoeutectic and eutectic meltsSolidification of ductile iron will be treated as twodifferent cases: hypoeutectic and eutectic melts as onecase and hypereutectic alloys as another.

In a hypoeutectic alloy, austenite dendrites form whenthe temperature reaches TAL according to the precedingdiscussion. Figure 6 shows a section of the Fe–C–Siphase diagram similar to that shown in Fig. 3, but onlyfor one Si content. Lines a, b and c indicate the liquidcomposition of three different cast irons.

In the hypoeutectic alloy, as temperature drops belowTAL, dendrites grow and the liquid is enriched incarbon. This is indicated by the arrow a on the figure.When the temperature reaches TEG, graphite shouldform. If the alloy is eutectic, solidification should beginhere (line b in Fig. 6).

If inoculation is good and cooling rate is moderate,the first nodules will form at a relatively low under-cooling, DTe.30,62,73 If the cooling rate is larger or theinoculant is less potent, the initial eutectic undercoolingwill increase before nucleation sets in. In such case, it ispossible that primary dendrites may form before theeutectic even in eutectic melts.

Eventually, nodules form and as they grow, thecarbon content of the liquid is reduced. Austenite willquickly nucleate on the surface of the nodules and growto completely encase them. From this point, nodules canonly grow at a rate determined by the rate of diffusion ofcarbon through the austenite shell.18,77,78 As a result, therecalescence following nucleation is slow compared towhat is found in other types of alloys.38,47 Because thegraphite nucleates in the melt, separated from theaustenite, this type of eutectic has been named divorcedeutectic.

Solidification of hypereutectic alloysIn hypereutectic alloys, the primary phase that formsduring solidification is graphite. Line c in Figure 6represents the solidification sequence for a hypereutecticalloy. As the temperature reaches the graphite liquidusline, it is thermodynamically possible for graphitenodules to nucleate. However, some undercooling isrequired and as a result, nodules form at a temperaturebelow the liquidus temperature. The primary nodulesgrow freely in the melt and may reach a considerable sizebefore the eutectic temperature is reached.74 As thenodules grow, the melt is depleted in carbon.

At the eutectic temperature, an austenite shell willform on the primary nodules. As the shell grows, thegraphite formation will slow down. Eventually, theundercooling increases until a new population ofeutectic nodules is formed.42,74 From here eutecticsolidification continues as described above.

Thin walled castings are often hypereutectic to ensurethat a sufficient population of nodules is formed beforethe eutectic reaction. But even in those alloys, it has beenobserved that off-eutectic dendrites form before theeutectic. This happens because the temperature fallsbelow the extended austenite liquidus line before theeutectic growth sets in. While the dendrites grow, thecarbon content of the melt increases again untilthe beginning of eutectic solidification, as shown in linec in Fig. 6.42,74,79 Figure 7 shows a colour etching of ahypereutectic ductile cast iron where off-eutectic den-drites, nodules and remains of austenite shells arevisible. The etching reveals that dendrites and austenite

6 Schematic of the Fe-C-Si phase diagram, showing the

eutectic temperature, and austenite and graphite liqui-

dus lines. The Austenite liquidus has been extended

by a broken line. The concen-tration of carbon in the

liquid for a hypo eutectic (a), eutectic (b) and a hyper

eutectic alloy (c)




shell form early in solidification. The effect is termedcoupled zone growth80 and describes the fact that insome materials, two phases of the eutectic may grow atthe same time with a higher growth rate than one of theeutectics alone. Because of the difference in nucleationkinetics between austenite and graphite, the regionwhere dendrites and eutectic may grow is skewed asindicated by the grey shading in Fig. 6. In cast irons,coupled zone growth is highly dependent on alloycomposition, cooling rate and inoculant potency.

Carbide formation during solidificationAs it is shown in equation (2), TEG and TEC depend onthe chemical composition of the melt. The equationsshown here are limited in the number of elementsincluded, but several researchers has pointed to elementsthat increases TEG.2,20,33,81 These graphite stabilisingelements are: Si, Ni, Cu, Co and Al.

It is referred to in the literature that too low Si contentresults in carbides in even relatively thick walledcastings,33,81 which is the reason that cast irons ingeneral contain more than 1?8 wt-%Si.

Other elements, such as Cr, Ti, V, Ce and Mg, areknown to be strong carbide formers and as suchdecrease the difference between TEG and TEG.2,81,82

The first three of these are normally only present insmall amounts as impurities from raw materials, butboth Ce and Mg are added in the melt treatmentprocess. Residual Mg contents from 0?05 wt-% andabove are known to promote carbide formation.80,81

If the undercooling becomes so high that thetemperature drops below TEC, carbides will form sincethey nucleate easily and grow fast.31 Such carbides arehard and brittle and therefore, detrimental to thecasting’s properties.

There are two sets of conditions under which eutecticcarbides may form: primary carbides that form at thebeginning of eutectic solidification and those that formin the final stage of solidification.

In the first case, where graphite nucleation isinsufficient, the temperature of the melt drops to TECbefore the eutectic solidification begins. In hypoeutecticand eutectic castings, primary dendrites will form first,and then between the dendrites, carbides form.

During solidification, as nodules and the surroundingaustenite shells grow, alloy elements will segregate. Sisegregates inversely so that the Si concentration is the

highest in the first austenite that forms. Other elements,among those the carbide formers, are rejected into theliquid2 and as a result, TEG is reduced while TECincreases in the residual liquid during solidification.

When the undercooling increases towards the end ofsolidification, carbides may form if the temperature TFin Fig. 5 is lower than TEC in the remaining melt. Thisphenomenon is called inverse chill and is typicallyencountered in low Si alloys. It is a result of theincreased undercooling of the last liquid to solidify(Fig. 5), and a result of contaminated raw materials oruse of an inoculant that is not sufficiently potent at theend of solidification.81,83,84

Nodule size distribution and nodularityDuctile iron’s mechanical properties can to a verylarge degree be linked to nodule shape and sizedistribution.1,24,44,85–87

Nodule shape is defined by a roundness shape factorthat indicates how close nodules are to being perfectlyspherical when a polished sample is viewed in amicroscope. To achieve best mechanical properties,nodules should be perfectly round.24,87 Incomplete Mgtreatment or the presence of small amounts of Ti, Al andCa combined with low Si content will lead to a change ingraphite morphology from nodules towards a morevermicular type of graphite.67 The change is gradual.Initially, nodules appear rough and irregular, and thenthey become more and more vermicular to finallycompletely flake-like as in grey iron. As nodulesdegenerate, so does elongation, fatigue strength andseveral other mechanical properties.24,87

Vermicular types of cast irons have in the past twodecades been developed as a high strength material tosubstitute grey iron where extra strength is required suchas in high power diesel engines.27,88 Although it is aninteresting new material, vermicular or compactedgraphite iron will not be dealt with here.

The nodule size distribution and nodule density arealso an important parameters in determining mechanicalproperties.1,2,24,44,87 There is, however, no simple rela-tion between nodule count and properties. A highnodule count is indicative of high cooling rates or largeundercooling during solidification. This leads to a finermicrostructure which again gives better properties aslong as carbides do not form. A high nodule count alsotends to reduce the amount of pearlite formed in theeutectoid reaction which makes the material softer andmore ductile.

Towards the centre of heavy section ductile ironcastings, graphite nodules may degenerate to ‘chunkygraphite’ (Fig. 8).89 The structure resembles that ofvermicular graphite, but the growth mode is different.The change in graphite morphology is believed to be aresult of segregation of S, O and rare earth elementscombined with a change in constitutional undercooling.It has recently been suggested that chunky graphiteforms as stage 3 in solidification (the first two stages areformation of primary nodules and growth of austenitedendrites). After that, the melt is under cooled to adegree where growth of chunky graphite allows acoupled eutectic to grow rapidly into the under cooledmelt.63

Building on this, Campbell has proposed that sincethere is a lack of nuclei in the centre of large castings,eutectic growth proceeds in a pulsating mode in which

7 Microstructure of hypereutectic ductile cast iron. The

sample is heat treated to become ferritic. The sample

is etched with Klemm 1 which reveals the Si content in

the ferrite. Dark blue and red is the first material to

solidify and the last solidified material is white




austenite partly overgrows the graphite. Then asaustenite growth slows down, graphite overtakes theaustenite.58

This type of defect is detrimental to mechanicalproperties. It can be suppressed by addition of Sb andstrict control of rare earth elements.64 However, themechanism behind the formation of chunky graphite isnot yet understood. The transition from good nodules tochunky graphite takes place over a very short distance(equivalent to a nodule diameter) in the casting.89 Theouter several centimetres of the casting may be perfectwith good strength and ductility, but the central volumeis weak and brittle. Because the defect is buried in thecentre of the casting, it can be very difficult to find itwith non-destructive testing methods.

Solid state phase transformations andheat treatmentFigure 9 shows a schematic and simplified drawing of abinary section of the Fe–C–Si equilibrium phase diagram(black lines and text) overlaid the metastable phasediagram (grey lines and text) for 2?5 wt-%Si. At the tem-perature a1, ferrite nucleates and grows into the austenitegrains, while excess carbon diffuses to the graphitenodules. The transformation takes place from a1 to A1

at which the remaining austenite transforms to ferrite andgraphite. This transformation depends on nucleation offerrite and diffusion of carbon to the nodules in solidstate. If the diffusion distance is too long or if cooling issufficiently rapid, then the austenite will become super-saturated in carbon and cementite will form in a eutectoidreaction where pearlite is formed. In this case, the castingwill become fully or partially pearlitic.1,2

In the vicinity of nodules, it will be more difficult toform pearlite because carbon can easily diffuse to thenodule. As a result, ferrite is often seen as a ring aroundthe nodules with areas of pearlite between (Fig. 2).

Strength increases and ductility decreases with pearlitecontent.7,24,44,90,91 In practice, it is not possible tocontrol cooling through the eutectoid transformation,while the casting is in the mould to any large degree. It istherefore common to heat treat castings to achieve thedesired mechanical properties.2

This is easily carried out by austenitising at y900uCfollowed by controlled cooling to room temperature.

The eutectoid transformation opens a range ofpossibilities to modify the mechanical properties of castirons as shown in Table 1.

Typically, high strength heat treated cast irons arealso to some degree alloyed with elements that eitherpromote formation of carbides and increase the pene-tration depth of the heat treatment such as Mo, Mn andCu in the case of pearlitic or bainitic ductile irons; oralloying is done to stabilise austenite and ferrite in the socalled austempered ductile cast irons (ADI), see com-positions in Table 1.7,44

In the past 25 years, there has been much focus onheat treated ductile irons. Traditional heat treatmentsknown in steels to produce bainitic irons are well known,and such materials are widely used for high strength andwear applications.24

Bainitic irons are rapidly cooled to temperatures inthe range 350–550uC where they are held while thebainite transformation takes place.

In ADI, the heat treatment resembles that of baini-tic ductile irons but the resulting microstructure isdifferent.95 After austenisation, the castings are rapidlycooled to a temperature between 200 and 350uC wherethey are held while the austenite gradually transforms.At the beginning of the process, very fine needles offerrite, typically referred to as ausferrite, grows intothe austenite. As the ferrite grows, excess carbonis rejected into the austenite that become heavilysupersaturated.2,94,94 The high Si content (typicallyaround 2?5 wt-%) in cast irons postpones the forma-tion of carbides and thus increases the processingwindow in which the desired microstructure can beachieved. Eventually, fine carbides form near thegrowing ferrite needles at which point the casting israpidly cooled to room temperature fixing a matrixconsisting of ferrite, supersaturated austenite and fewcarbides.90 An example of the microstructure is shownin Fig. 10. The result is a material that has bothextremely high strength and good ductility.

Future outlook for ductile ironDuctile cast iron is an important engineering materialthat has found widespread use in the industry forcomplex, highly loaded constructions. In recent years,

8 Microstructure in heavy sectioned nodular cast iron in

the area where the transition from nodules to chunky

graphite occurs. Eutectic cells of chunky graphite

forms in between the nodules. With permission from

Rikard Kllbom, Swecast

9 Schematic section of the Fe-C-.Si phase diagram at a

Si content of approximately 2?5 wt%. Black lines show

the equilibrium phase diagram and grey lines show the

metastable phase diagram





with the need to optimise constructions with regard toweight and yield, the interest in ductile cast iron hasbeen steadily increasing.25 Modern ductile irons offer thedesigner a material that, if one understands that themetallurgy and processing, can be engineered to meet awide range of applications. From Table 1, it is clear thatthe ratio of strength and stiffness to weight supersedesthat of aluminium alloys. Further, the resistance tofatigue failure is much better than that of aluminiumalloys and welded constructions.2,24 As such, ductileirons are potential candidates for optimised, complex,light weight, high performance constructions, which isthe reason for the increasing industrial application of thematerial.

Our increasing need for environmentally friendlyenergy has lead to an immense growth in the windturbine industry which is now one of the most importantusers of large (.1 ton) ductile iron castings.95–97 Theseare constructions where the demands for structuralsoundness, mechanical properties and light weight areextreme. As a result, the scientific interest in solving theoutstanding problems in ductile iron has increased inrecent years.

At present, great efforts are made to solve the problemsrelated to formation of chunky graphite in large castingsbecause when that problem is under control, it is possibleto reduce the weight of such castings considerably.63,64,89,97

In the years to come, it is very likely that we will seenew types of ductile cast irons that are engineeredspecifically for individual purposes. Standard irons arevery broad in their specifications, but taking advantageof the latest advances in melt treatment methods andmodern processing methods, it will be possible for theskilled foundry engineers to control metallurgy anddimensions of castings much more accurately thanspecified in the standards and thus optimise mechanicalproperties and tolerances.

The need to optimise constructions to reduce produc-tion costs while integrating more functionality andcomplexity into castings and at the same time minimis-ing machining costs forces designers and foundrymen tocooperate and constantly improve their skills. The routeto reach that goal will be through advanced designmethods where the casting design is based not only onanalysis of an ideal component with uniform properties,but through taking the full manufacturing process intoaccount. An example is the redesign of a valvehousing,98,99 where an analysis of the casting processshowed that a combined redesign of the casting and thegating and feeding system would give more uniformmechanical properties in the casting while at the same

time remove the need for heat treatment increasingcasting and at the same time increase casting yield by20%.

In the analysis, it is shown that not only can theproduct itself be optimised, but there is also considerablesavings to be made by improving gating and feeding.This area has been overlooked for some decades, but it isclear that that there are important issues here that needto be addressed in the future to really be able to get thefull benefit of modern cast materials.98–101

Such integrated design methods require that designersand process engineers have sufficient skills and compli-cated modelling tools at hand to predict how processing(casting, heat treatment, machining, etc.) influencesmicrostructure and properties (strength, ductility, fati-gue, residual stress, etc.) locally in the casting as it isafter processing.

These methods rely on that we have detailed knowl-edge of how structures and properties form in thematerials during processing. It is a complicated andcomprehensive task to achieve this goal, but much workhas been carried out, and much scientific effort isdirected towards achieving this goal, so that in the futureit will be possible to optimise castings and reduceresource spending considerably.

Acknowledgement

I would like to thank the staffs at the TechnicalUniversity of Denmark, who are not already creditedin this paper, but who over the years have made greatcontributions to the research carried out in the field ofductile cast iron: O. Munch, F. Lehm, L. Leth, J. S.Nielsen and H. Kanstrup.

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