revealing and characterising solidification structure of ductile cast iron

Revealing and characterising solidi cationstructure of ductile cast ironG. Rivera, R. Boeri, and J. Sikora

This study deals with the solidi cation and microsegregation processes of near eutectic ductile cast iron. A detailedanalysis of old and new solidi cation models was made. Metallographic techniques, developed elsewhere by theauthors, were used to reveal the solidi cation microstructure and macrostructure. The results allow a new soli-di cation model to be proposed, where each solidi cation unit is a grain of eutectic austenite that has a dendriticsubstructure and contains a very large number of graphite nodules. A pattern of microsegregation exists inside eachsolidi cation grain, while no intergranular microsegregation has been detected. A procedure to characterise thesolidi cation re nement was developed considering the location of the last to freeze areas. MST/5169

The authors are in the Metallurgy Division INTEMA, National University of Mar del Plata (CONICET), B7608FDQMar delPlata, Argentina (jsikora@ .mdp.edu.ar). Manuscript received 25 July 2001; accepted 4 October 2001.# 2002 IoM Communications Ltd.

List of symbols and abbreviations

Ac e ll average area per cellAg ra in average area of eutectic grains

An od u le average nodule areaDAAS direct austempering after solidi cation

DI ductile cast ironLTF last to freeze

MA, MI1, MI2 meltsMN multinodularNc average number of cells per unit area

Nc ,V number of cells per unit volumeNg ra in average number of grains per unit areaNg ,V number of grains per unit volumeNn average number of nodules per unit area

Nn ,c average number of nodules per cellNn ,g ra in average number of nodules per grainNn ,g ,V number of nodules per grain in volumeNn ,V number of nodules per unit volumeUN uninodular

Introduction

Ductile cast irons are materials of great technologicalimportance that are used in the construction of a largenumber of components. Despite their extensive application,there are still a number of aspects of their metallurgy thatare not thoroughly understood. In particular, signi cantresearch efforts have been carried out to understand thesolidi cation of these alloys and the resulting microsegrega-tion, aiming to reach a better control of their microstructureand mechanical properties.

The literature shows two major physical models to explaineutectic DI solidi cation. Most solidi cation experts agreetoday that the solidi cation of eutectic DI involves theformation of austenite dendrites and graphite nodules orspheroids. The interaction of the growing austenite withthe nodules originate solidi cation units, described asengrossed austenite dendrites, which contain several gra-phite nodules,1 ,2 as shown schematically in Fig. 1. This willbe referred to as the MN model. On the other hand, otherresearchers, mostly involved with the modelling of micro-structure and microsegregation, use an earlier solidi cationmodel in which solidi cation units are formed by graphitenodules enveloped by austenite shells, as shown in Fig. 2.3 7

This will be referred to as the UN model. The discrepanciesregarding the solidi cation of DI are dif cult to resolve

because, after solidi cation is complete, solid state trans-formations take place during cooling to room temperature,making it impossible to reveal the solidi cation structure bystandard metallographic techniques. As a consequence, thestandard characterisation of DI structures includes only thedescription of the matrix microstructure at room tempera-ture and the graphite count and morphology. The coarsen-ing of the structure as the solidi cation rate decreases ischaracterised indirectly by the coarsening of the graphitenodules. There is no method based on the observation of thetrue solidi cation structure of the matrix.In recent publications, the authors reported the develop-

ment of novel methods to reveal micro- and macroscopicallythe solidi cation structure of near eutectic DI cast in sandmoulds.8 10 The evidence obtained through the use of thesemethods contradicted the previous solidi cation models,described above, and led the authors to propose a newexplanation of the solidi cation mechanism of eutectic DI.According to this explanation,10 the solidi cation begins withthe independent nucleation of graphite and austenite in themelt. The austenite grows dendritically, trapping and enve-loping graphite nodules as they collide with the dendrites.Further graphite growth requires the diffusion of C to thenodules through the austenite envelope. The macrographictechnique developed by Boeri and Sikora10 showed that eachsolidi cation unit is formed by a large austenite dendrite thatcontains many graphite nodules. It has been shown that smalldepartures from the eutectic composition do not markedlyaffect the solidi cation process.11 Earlier experimental resultsand existing models must be critically reviewed and comparedwith this new solidi cation model.The objectives of this study are to make a detailed analysis

of old and new solidi cation models, and to develop a pro-cedure to characterise the solidi cation structure of neareutectic DI cast in sand moulds.

Experimental procedure

MATERIALSDuctile iron melts used were of near eutectic composition,and were produced in commercial foundries and pilotplants using medium frequency induction furnaces. Chargematerials were DI quality pig iron (low P, S, and Mn), lowC steel scrap, foundry returns, ferroalloys, electrolyticNi and Cu, and graphite. The melt was nodularised byusing 2 wt-% of Fe Si Mg Ca Ce (9 wt-%Mg), andpost-inoculated with 0.5 wt-% of Fe Si (75 wt-%Si).

DOI 10.1179/026708302225003668 Materials Science and Technology June 2002 Vol. 18 691

Table 1 lists the chemical composition and the graphitecharacteristics (ASTM A247)12 of the melts used.

The Y blocks of 13, 25, and 75 mm (ASTM A395),13 andround bars of 20 mm diameter and 100 mm long, were castin resin bonded sand moulds. Metallographic samples wereobtained from the Y blocks and round bars at the locationsshown schematically in Fig. 3.

METALLOGRAPHIC STUDYThe metallographic techniques used in this study weredeveloped elsewhere by the authors.

Micrographic techniqueThe technique reveals the solidi cation microstructurethrough the use of a reagent that brings up the micro-segregation patterns generated during solidi cation, asreported by Rivera et al.8 For better results, it is recom-mended to carry out a ferritisation anneal on the samplesbefore etching. In this study, the etching procedure wasapplied on samples taken from Y blocks. Typical results areshown in black and white in Fig. 4a*. The light patches

1 Schematic of sequence of solidi cation of eutecticductile cast iron according to multinodular model:black lines represent boundary between solidi cationunits; dark grey zones at which three solidi cationunits meet correspond to last to freeze areas in melt

2 Schematic of sequence of solidi cation of eutecticductile cast iron according to uninodular model: blacklines represent boundary between solidi cation units;dark grey zones at which three solidi cation unitsmeet correspond to last to freeze melt

*Colour versions of all micrographs, which provide enhancedinformation on the microstructures, are available from the authors.

692 Rivera et al. Solidi cation structure of ductile cast iron

Materials Science and Technology June 2002 Vol. 18

(arrowed) surround and make it possible to identify thelocation of the microsegregated regions corresponding to theLTF melt. To characterise the distribution of such regions,a procedure was developed as described below. On a 6100micrograph of the colour etched samples, lines are manuallydrawn between the location of neighbouring LTF regions(Fig. 4b). This de nes a pattern of closed regions, that havebeen referred to as `cells in the past and were considered tobe the solidi cation units.14 Such nomenclature is still usedin the present article, although it must be pointed out that,under the current understanding of the authors, the divisioninto cells does not have physical meaning, but is only ameans to characterise the dispersion of LTF.

On each sample analysed, at least 50 cells were identi edand the average number of nodules per cell Nn ,c was mea-sured. The number of nodules per unit area Nn and theaverage nodule area Ano du le were also measured on eachsample, and the number of cells per unit area Nc wascalculated as the ratio between Nn and Nn ,c . The averagearea per cell Ac e ll was calculated as N

1c .

Macrographic techniqueThe observation of the macrostructure requires DAAS10 tobe carried out. In this process, the cast parts are shaken outfrom the mould when their temperature is ~ 950C, and

transferred to a furnace held at 900C, where they remainfor 30 min to allow temperature stabilisation. The partsare then austempered in a molten salt bath held at 360C,for 90 min. After DAAS treatment, the matrix microstruc-ture of the samples shows a ne mixture of ferrite needlesand retained austenite, typical of austempered ductile iron.The retained austenite retains the crystalline orientationde ned during solidi cation. Therefore, later etching withPicral (5 wt-%) reveals the grain structure of the eutectic

(a)

(b)

a Y blocks; b round bars

3 Experimental castings: arrows show location of sam-ples used for metallographic characterisation: a~13,25, and 75 mm

4 a black and white print of melt MI1 after colour etch-ing and b same area as a with LTF zones and cell con-tour marked manually

Table 1 Chemical composition and characterisation of graphite phase*

Chemical composition, wt-% Graphite characteristics

Melt Fe C Si Mn Cu Ni Cr Mo MgCast size,mm NL, % NS

Nn , nodulesmm 2

MA Bal. 3.00 3.10 0.27 0.87 0.74 0.060 Bar u ~20 100 6/7 291MI1 Bal. 3.50 2.09 0.80 0.83 0.26 0.027 Y block~13 100 5/6 253

Y block~25 100 5/6 200Y block~75 90 5 78

MI2 Bal. 3.20 2.95 0.22 1.00 0.71 0.20 0.056 Y block~13 100 6/7 348Y block~25 100 6 225Y block~75 100 5 120

*NL nodularity; NS nodule size; Nn nodule count.

Rivera et al. Solidi cation structure of ductile cast iron 693


austenite. This procedure was carried out on samples takenfrom round bars. Figure 5a shows the result of the etchingof a sample of melt MA, obtained using the DAAS tech-nique. Figure 5b shows the results of the etching of anothersample of the same melt cast without the use of DAAS, andaustempered using the same temperature and time as thesample of Fig. 5a. It is very clear that DAAS allows themacrostructure to be revealed, while no evidence of such astructure is present in samples cooled down conventionallyafter solidi cation.

The result shown in Fig. 5a is typical for all DIs inves-tigated. It shows a grain structure similar to that found incast parts of most metallic alloys. The average area of theeutectic grains Ag ra in was measured. The number of grainsper unit area Ng ra in was calculated as A

1grain

, and the numberof nodules per grain Nn ,g ra in was calculated as the product ofNn and Ag ra in .

Results and discussion

ANALYSIS OF VALIDITY OF EARLIERSOLIDIFICATION MODELSUninodular modelThe progress of the solidi cation according to the UN model,shown schematically in Fig. 2, would lead to a distribution ofthe microsegregation as that shown schematically in Fig. 2c,where if it is assumed, for example, that the average nodulediameter is 40 mm and the average distance between nodulesis 120 mm (approximate values for a 25 mm thick plate castin a sand mould), typical distances between microsegregated(LTF) regions would be similar to the average distancebetween nodules. The actual distribution of microsegregation,shown in Fig. 4a, is signi cantly different from what is pre-dicted by the UN model. Note that the average nodule sizeof the sample of Fig. 4a is ~ 40 mm, similar to what has beenassumed in the schematic of Fig. 2c. The actual distancebetween LTF is about three times larger than that predictedby the UN model. If this ratio is taken to a three-dimensionalspace, it would mean that the UN model predicts a dispersionof the microsegregation regions about 27 times larger thanthat actually observed. This indicates that the UN model failsto predict the microsegregation pattern. Moreover, an evenmore negative fact is found when the predictions of the UNmodel are compared with the macrostructure shown in Fig. 5,where the grain volume is about three orders of magnitudegreater than that of the UN model. It must be pointed out thata large number of mathematical models of microsegregationand solidi cation of DI are based on the UN model.3 7

Multinodular modelThe progress of the solidi cation according to the MNmodel would lead to the distribution of the microsegrega-tion shown schematically in Fig. 1c. This is in good coinci-dence with the actual pattern, shown in Fig. 4a. This is notsurprising, since the MN model has been developed throughthe observation of the microsegregation patterns.8 The MNmodel proposes that areas between microsegregated regionsare the actual eutectic solidi cation units. Nevertheless,the size reported for such units is much smaller than thatrevealed by macrography (Fig. 5). Moreover, according tothe MN model the microsegregation inside each solidi ca-tion unit is very small. Therefore, it is advisable to examinesimultaneously the micro- and macrostructure. In orderto examine the microstructure while keeping track of themacrostructure, the location of a grain boundary revealedby macroetching was marked by Vickers hardness indenta-tions. The same sample was repolished and colour etched, asshown in Figs. 6 and 7 (in higher magni cation). Notice-ably, no predominant microsegregation is seen at theboundary between solidi cation units, but extensive micro-segregation is observed inside each solidi cation unit. Thispoints out the inadequacy of the MN model, and the needto formulate a new approach.

PROPOSED OF IMPROVEMENT TO NEWSOLIDIFICATION MODELThe model proposed by Boeri and Sikora10 agrees with mostof the experimental evidences, but can still be improvedto account for the distribution of the microsegregation

a direct austempering after solidi cation; b conventional aus-tempering

5 Macrographs of melt MA etched with 5 wt-% Picral

6 Micrograph of melt MA after direct austempering aftersolidi cation procedure and colour etching: hardnessindentations show position of grain boundary ofeutectic austenite

7 Micrograph of same sample as Fig. 6 at higher magni- cation



in better detail. A new version of this model is proposedhere, as shown schematically in Fig. 8. Figure 8a c repre-sent different stages of the formation of the macrostructure.The growth of austenite dendrites that originate the grainsis shown. In this sequence, the presence of graphite nodulesis ignored, since the size of the graphite particles is too smallat this scale. Figure 8e g represent a higher magni cationview of the stages corresponding to Fig. 8a c respectively,where the nature of the interaction of austenite dendritesand graphite nodules is portrayed. According to the model,the solidi cation of eutectic DI begins with the independentnucleation and growth of austenite and graphite from themelt, with the austenite growing dendritically, as shown inFig. 8a and e. As the solid fraction increases, the austenitedendrites grow (Fig. 8b) and they contact and envelopmost of the graphite nodules, as shown in Fig. 8f. Furthergrowth of enveloped nodules is controlled by the diffusionof C from the liquid to the nodules, through the austenite.The advance of the austenite dendrites leaves signi cantamounts of melt between the secondary arms. In thismanner, at the point at which growing dendrites interact,de ning the grain size, there is still a large fraction of meltinside each grain. The structure after solidi cation is com-plete is represented in Fig. 8c and g. Figure 8c showsthe dendritic substructure of the eutectic austenite grains.The dark grey regions of Fig. 8g represent the location ofthe LTF areas that can be revealed by colour etching. Thedotted lines, manually drawn between the location ofneighbouring LTF regions, de ne the closed regions called`cells.

According to this explanation of the solidi cation processof eutectic DI, the solidi cation unit is the grain of eutecticaustenite that has a dendritic structure and contains a largenumber of graphite nodules and several `cells. A pattern ofmicrosegregation exists inside each austenite grain.

CHARACTERISATION OF SOLIDIFICATIONSTRUCTUREHaving developed micro- and macrographic techniques toreveal the solidi cation structure of DI, it is now possibleto characterise it in detail. For example, the characterisationof the macrostructure of melt MA is: Ag ra in~1.8 mm

2 ;

Ngra in~0.6 grains mm 2; and Nn ,grain~524 nodules grain

1.

Note that the average grain size is relatively large. TheDAAS technique is certainly useful for research, since itallows the observation of features that were earlier ignored.Nevertheless, it is also certain that this technique cannotbe applied on samples obtained from regular foundryproducts, therefore it is not useful for characterising thesolidi cation structure of such products. The micrographictechnique, on the other hand, can be applied on everyDI sample. Colour etching reveals the location of LTF.According to the solidi cation model introduced recently,the size of the regions lying between LTF is related to thedendrite secondary arm spacing. This spacing is a parameterusually measured to evaluate the in uence of the coolingrate on the re nement of the dendritic structure.15 There-fore, the cell count Nc, determined by the procedure des-cribed in the experimental methods section, is closely relatedto the degree of re nement of the austenite in DI, and willbe used to characterise it. The re nement of the graphite isproperly characterised by the parameter Nn .Figure 9 shows black and white prints of micrographs of

samples of melt MI2 obtained from Y blocks of 13 and75 mm thickness, after ferritisation and colour etching. Thecoarsening of the austenite dendritic structure caused by theincrease in solidi cation time is seen clearly in Fig. 9b,where the LTF regions are larger and lie farther apart thanin Fig. 9a. The coarsening of the graphite nodules is alsoevident, as they become larger and the nodule count dimi-nishes as the section size increases.Table 2 lists values of Nn , Nn ,c, and Nc for melts MI1 and

MI2 cast in different sections. The results show that as thesection size increases, the number of nodules per cell Nn ,cincreases, and the cell count Nc diminishes more markedlythan the nodular count Nn . This is seen in detail in Table 3,which lists the percent of reduction of Nn and Nc as thesection size increases.

STEREOLOGIC CHARACTERISATIONIn order to advance in the characterisation of the soli-di cation units of eutectic DI, it seems relevant to performsome calculations to describe the size of the features involumetric terms. The number of nodules per unit volume

a c solidi cation macrostructure (equiaxed grains); d actual macrostructure; e g solidi cation microstructure; h actual microstructure

8 Schematic illustration of sequence of solidi cation of eutectic ductile cast iron



NnV can be estimated by using Hilliards equation16

AA (z)~A(z)=L2 ~V=L3 : : : : : : : : : (1)where L is the length of the test plane, A(z) is the area ofintersection of a particular test plane with the graphitephase, AA (z) is the fraction of the area of the test plane thatis occupied by the graphite phase, AA (z) is the expectedvalue of AA (z), averaged over the domain of axis z (that is,all positions of test planes in the structure), and V/L3 is thevolume fraction occupied by the graphite phase.Assuming

AA(z)~Nn :Anodule : : : : : : : : : : : : (2)

and

V=L3~Nn,V :Vnodule : : : : : : : : : : : (3)

where Anodule is the average nodule area and Vnodule is theaverage nodule volume. Equating equations (2) and (3)

Nn,V~Nn :(Anodule=Vnodule) : : : : : : : : : (4)

Assuming that cells and grains are spherical and have aradius r, the number of cells per unit volume Nc,V, and thenumber of grains per unit volumeNg,V, can be calculated asfollows

rcell~(Acell= p )1=2)Vcell~(4=3) p r3cell)Nc,V~V {1cell (5)

rgrain~(Agrain=p )1=2)Vgrain~(4=3) p r3grain

)Ng,V~V {1grain : : : : : : : : : : : (6)The number of nodules per cell in volume Nn,c,V, and thenumber of nodules per grain in volume Nn,g,V, can becalculated as

Nn,c,V~Nn,VVcell : : : : : : : : : : : : (7)

Nn,g,V~Nn,VVgrain : : : : : : : : : : : (8)

The results of the stereologic characterisation of melt MAare listed in Table 4. It is important to note that, for thesamples investigated, each eutectic grain contains approxi-mately 18000 nodules and 400 cells. This is very far fromthe single nodule inside each grain assumed by the UNmodel, and also far from the few nodules assumed by theMN model.

Future work

Future efforts will be aimed at identifying the in uence ofthe cooling rate and the equivalent carbon content on thecoarseness of the solidi cation structure of DI. The rela-tionship between the re nement of the solidi cation grainsize and the re nement of the microsegregation pattern willbe also investigated.

Conclusions

1. The knowledge about the solidi cation structure ofductile cast iron gained through the use of recently deve-loped micro- and macrographic techniques, has led to theproposal of a new solidi cation model, in which eutectic

Table 3 Reduction of nodule count Nn and cell count Ncas the section size increases, %

Increase from13 to 25 mm

Increase from25 to 75 mm

Melt Nn Nc Nn Nc

MI1 21 40 61 72MI2 35 50 46 71

a 13 mm; b 75 mm

9 Black and white micrographs of different Y blocks ofmelt MI2 after colour etching

Table 4 Stereological characterisation of melt MA*

Microstructural parameters Macrostructural parameters

Nn,V , nodulesmm 3

Nc,V , cellsmm 3

Nodules percell volume Vcell, mm

3Ng,V, grainsmm 3

Nodules pergrain volume

Cells pergrain volume Vgrain, mm

3

23557 47 503 1.99610 3 1.3 18375 392 7.8610 1

*Nn ,V nodules per unit volume; Nc,V cells per unit volume; Vcell average cell volume; Ng ,V grains per unit volume; Vgrain average grains volume.

Table 2 Metallographic characterisation of different sec-tions*

Metallographic parameters

MeltSectionsize, mm

Nn, nodulesmm 2

Nn,c, nodulescell 1

Nc, cellmm 2

MI1 13 253 6.0 4225 200 7.8 2575 78 11.2 7

MI2 13 348 8.3 4225 225 10.5 2175 120 20.4 6

*Nodule count Nn; nodules per cell Nnc; cell count Nc.



solidi cation units or grains are large austenite dendritesthat contain a very large number of graphite nodules.

2. The inadequacy of the earlier solidi cation models hasbeen demonstrated.

3. The microsegregation of the alloying elements, result-ing from solidi cation, takes place intradendritically whileinterdendritic or intergranular microsegregation is notevident.

4. The degree of re nement of the solidi cation austenitestructure can be assessed by the cell count Nc value.

5. Under the present experimental conditions, the aver-age size of equiaxed solidi cation units measured on neareutectic 20 mm round bars cast in sand moulds, is appro-ximately 1.2 mm in diameter and 0.8 mm3 in volume. Unitsof this size contain ~ 18 000 graphite nodules.

References

1. r. boeri and f. weinberg: AFS Trans., 1989, 97, 179 184.2. d. banerjee and d. stefanescu: AFS Trans., 1991, 99, 747 759.3. e. fraS , w. kapturkiewicz, and a. burbielko: in `Physical

metallurgy of cast iron V: advanced materials research , (ed.G. Lesoult and J. Lacaze), 499 504; 1997, Swizerland, Scitec.

4. ch. charbon and m. rappaz: in `Physical metallurgy of castiron V: advanced materials research , (ed. G. Lesoult andJ. Lacaze), 453 460; 1997, Swizerland, Scitec.

5. l. liu and r. elliott: Int. J. Cast Met. Res., 1998, 10, 301 305.6. l. liu and r. elliott: Int. J. Cast Met. Res., 1999, 12, 75

82.7. b. liu: Int. J. Cast Met. Res., 1999, 11, 259 266.8. g. rivera, r. boeri, and j. sikora: Cast Met., 1995, 8, 1 5.9. j. massone, r. boeri, and j. sikora: Int. J. Cast Met. Res., 1999,

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11. g. rivera: `Solidi cation structure of SG cast iron , Doctoralthesis, National University of Mar del Plata, Argentina, 2000.

12. Standard A247, ASTM, Philadelphia, PA, 1978.13. Standard A395, ASTM, Philadelphia, PA, 1956.

14. g. rivera, r. boeri, and j. sikora: in `Physical metallurgy ofcast iron V: advanced materials research , (ed. G. Lesoult and

J. Lacaze), 169 174; 1997, Switzerland, Scitec.15. m. flemings: `Solidi cation processing ; 1974, New York, NY,

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16. j. hilliard: `Quantitative microscopy , chapter 3; 1968, NewYork, NY, McGraw Hill.



revealing and characterising solidification structure of ductile cast iron

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