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  • A R C H I V E S O F M E T A L L U R G Y A N D M A T E R I A L S

    Volume 57 2012 Issue 3

    DOI: 10.2478/v10172-012-0084-6

    E. FRA, M. GRNY



    The paper presents a solidification sequence of graphite eutectic cells of A and D types, as well as globular and cementiteeutectics. The morphology of eutectic cells in cast iron, the equations for their growth and the distances between the graphiteprecipitations in A and D eutectic types were analysed. We observed a critical eutectic growth rate at which one type ofeutectic transformed into another. A mathematical formula was derived that combined the maximum degree of undercooling,the cooling rate of cast iron, eutectic cell count and the eutectic growth rate. One type of eutectic structure turned smoothlyinto the other at a particular transition rate, transformation temperature and transformational eutectic cell count. Inoculation ofcast iron increased the number of eutectic cells with flake graphite and the graphite nodule count in ductile iron, while reducingthe undercooling. An increase in intensity of inoculation caused a smooth transition from a cementite eutectic structure to amixture of cementite and D type eutectic structure, then to a mixture of D and A types of eutectics up to the presence of onlythe A type of eutectic structure. Moreover, the mechanism of modification of cast iron was studied.

    Keywords: Cast iron, solidification, inoculation, structure, eutectic cells

    W pracy podano sekwencj krystalizacji ziaren eutektyki grafitowej typu A i D oraz kulkowej a take eutektyki cemen-tytowej. Przeanalizowano morfologi tych ziaren w eliwie oraz rwnania na prdko wzrostu ziaren oraz odlego midzywydzieleniami grafitu typu A i D. Wykazano, e istniej krytyczne prdkoci wzrostu eutektyki, przy ktrych jeden rodzajeutektyki przeksztaca si w drugi. Wyprowadzono oglne rwnanie wice stopie przechodzenia eutektyk z szybkoci sty-gnicia eliwa, liczb ziaren eutektycznych i prdkoci ich wzrostu. Wykazano, e istnieje prdko transformacji, temperaturatransformacji i transformacyjna liczba ziaren, przy ktrych jeden rodzaj eutektyki przechodzi pynnie w drugi. Modyfikacjaeliwa zwiksza liczb ziaren eutektycznych w eliwie z grafitem patkowym i liczb kulek grafitu w eliwie sferoidalnymoraz zmniejsza przechodzenie. Powikszenie intensywnoci modyfikacji powoduje pynne przejcie od eutektyki cementytowejpoprzez mieszaniny eutektyk typu D i cementytowej, eutektyk typu D i A a do wycznie eutektyki typu A. Podano mechanizmmodyfikacji eliwa.

    1. Introduction

    Cast iron is the most important and most widelyused casting alloy and its inoculation phenomenon wasdiscovered in 1920 [1] and patented by Meeh in 1924[2]. There are many studies on this phenomenon, whichare summarised and analysed in [3]. Elements such asBa, Ca and Sr, which are usually introduced to a bathin ferrosilicon, are the most important inoculants of castiron. Ferrosilicon that contains these elements is treatedas a complex inoculant.

    The purpose of this study were to analyse the inoc-ulation effects and explain the inoculation mechanism ofcast iron.

    2. Solidification of graphite eutectic

    After undercooling below the graphite eutectic equi-librium temperature, Tr (Fig. 1a), in the liquid alloy,graphite nuclei are created that take the form of rosettesduring growth (Fig. 1b). On the concave surface ofgraphite rosettes, an austenite nucleates and surroundsthe central part of the rosette, rising along its branchesand leading to the creation of eutectic cells. From eachnucleus, a single eutectic cell is formed. Therefore, thenumber of nuclei also represents the number of eutecticcells. In ductile iron, each graphite nucleus gives rise toa single graphite nodule (Fig. 2a). As the solidificationprocess continues, the austenite shell nucleates directlyon the graphite nodule and the eutectic transformation


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    Fig. 1. (a) Sequence of solidification of graphite eutectic cell, (b) scanning electron photography of eutectic cell, (c) graphite eutectic cellboundaries on metallographic specimen (d) cooling curve of cast iron

    Fig. 2. Sequence of solidification of ductile iron, (b) graphite nodule at the fracture of ductile iron, (c) microstructure of ductile iron

    begins. Eutectic cells may contain a lot of nodules(Fig. 2a). Thus, in ductile iron, the number of nucleican be identified only by the number of graphite nodulesrather than the number of eutectic cells (Fig. 2).

    3. Solidification of cementite eutectic

    After undercooling of cast iron below the cementiteeutectic equilibrium temperature, Tc (Fig. 1d), in the

    liquid alloy, cementite nuclei are created that take theform of plates during growth. On this plate, the austen-ite nucleates and grows in a dendritic form to coverthe cementite (Fig. 3a). A common solidification frontof the eutectic structure is created. During its growth,plate-to-fibre transition of cementite takes place (Fig.3b,c) and a cell of cementite eutectic is formed (Fig.3d).In the central part of the cell, cementite takes the formof plates, while in the periphery, it assumes the form

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    of fibres. Cross-section of eutectic cells in white castiron (visible on metallographic specimens) is shown inFig. 3e.

    Fig. 3. Sequence of the development of cementite eutectic cell (a,b,c),a scheme of cell (d) and microstructure of a cross section of cementiteeutectic cell (e) [4]

    4. Morphology of eutectic cells

    In a typical grey cast iron with flake graphite, thereare two types of eutectic structures.

    The A type of eutectic cells are formed at a lowdegree of undercooling, while the D type is formed at ahigh degree of undercooling. The appearance of skele-tons in the graphite eutectic cells of A and D typesare shown in Fig. 4a,b while microphotographs of theircross-sections are given in Fig. 4c, d. The skeleton of an

    A type of eutectic structure exhibits a small number ofcrystallographic errors of graphite crystal [5] and conse-quently, is poorly branched (Fig. 4a). On the other hand,the skeleton of a D type has a large number of crystal-lographic errors and is therefore much more branched.

    5. Cell growth rate

    The theory of eutectic growth [4] gives the generalequation that combines the eutectic growth rate, u, withthe degree of undercooling, T.

    u = T 2 (1)

    where - is the growth coefficient of eutectic cell.The value of a growth coefficient for the eutectic

    cells with flake graphite depends on the chemical com-position of the iron and is given in [6]. Therefore, forthe cast iron containing 2% of Si, the equation (1) takesthe following form:

    u = 1, 7 106T 2; cm/s (2)

    You may notice that the higher the degree of undercool-ing, the higher the cell growth rate.

    6. Interfacial distance

    Interfacial distance is the distance between thebranches of a continuous skeleton of graphite in the eu-tectic cell, which is observed on metallographic speci-mens (Fig. 5a and 5c show A and D types, respectively).The distance was much lower in the D type than in theA type.

    The theory of eutectic growth [4] indicates that theinterfacial distance in eutectic depends on their growthrate. According to the study [7], the interfacial distancecan be determined from the following equations:

    Fig. 4. A scheme of a graphite skeletons in A (a) and D (b) types of eutectic cells, microphotograps of cross sections of cells:with A type of eutectic formed at smal undercooling (with low growing rate) (c) and with D type formed at high undercooling(with high growing rate) (d)

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    A type eutectic

    = 136.8 u0,50; m (3)

    D type eutectic

    = 16, 1 u0,25; m (4)

    The graphs of these equations indicate a discontinuitywithin the interface distance between the A and D typesof graphite eutectic structure (Fig. 6).

    Fig. 5. Schemes of solidification of hypereutectic cast iron and in-terfacial distances between graphite precipitations (a, d), schemes ofA and D type of eutectics (b, e) appearance of graphite of A and Dtype in scanning electron microscopy (c,f)

    Fig. 6. Influence of growth rate on interfacial distance

    From Fig. 6, there is a critical velocity growth range(around 7 to 10 m/s) at which the interval change occurswithin the interface distance.

    7. Eutectic transformation in cast iron

    In the fundamental theoretical work of [8] studyingthe relationship between free energy and eutectic growthrate, a critical growth rate of eutectic was demonstrat-ed, called the transition rate, at which one type of eu-tectic structure transformed into another. This generalprinciple of energy, confirmed by the experimental stud-ies of cast iron [2, 9], indicates that the transformationrate of the A type into the D type of a graphite eutec-tic structure amounts to ukr = 1 30 m/s. Below thisrate range, the A type is formed since it has the lowestfree energy, while above this range, the D type is pro-duced because its free energy is lower than that of theA type (Fig. 7a). Similarly, there is an experimentallydetermined rate range for the graphite-cementite eutec-tic transition (uD,c = 85 250 m/s) [9]. Above this raterange, cementite eutectic is formed because its free ener-gy is lower than that for the D type of eutectic (Fig. 7b).Near the transformation range, the smooth transitionof one type of eutectic into another through the eutecticmixture of different proportions (Fig. 8) is observed.

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    Fig. 7. Influence of growth rate on free energy of eutectics (a,b); uA,D trasformation rate of A to D type of eutectic, uD,C - trasformationrate of D type to cementite eutectic, microstructures


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