The effect of twin/tilt initiated in the process of graphite growth making the graphite curling and change from flake to vermicular and spheroidal is discussed. With the developing of the solidification process,the modifying elements enrich in the front of solid-liquid interface, the amount of twin defects in the graphite increases,its tilt fashion changes and the graphite formed varies from flake to vermicular and spheroidal. The modifying elements promote the formation of twin/tilt. When the modifying elements are insufficient for spheroidizing,the tilt orientation of twins is changeable,and the graphite formed is twisted. When the modifying elements are sufficient enough, the tilt orientation of twins becomes singular, and the graphite formed tends to be round. According to the energy and kinetics consideration of the formation of twin/tilt boundary, it is predicted that the twin plane would firstly adopt (10Tm), especially the (10T2) plane. This result coincides well with the experimental observations. It is proposed that the formation of SG can be divided into two steps: growth of graphite nucleus into spherulite by twin/tilt mechanism and brancing on it in a spiral mode.

Graphite flakes and spheroids, extracted from rare-earth treated cast irons, have been examined using electron microscopy. Transmission microscopy has been used to investigate the defect structure of flakes and to demonstrate the concentric aromatic shell structure of spheroids. Each spheroid is shown to possess a rare-earth containing core or nucleus.

High resolution lattice images are presented as evidence for the presence of amorphous regions within flake graphite crystals. For spheroidal graphite, the lattice images indicate a greater degree of crystalline perfection. In addition, they suggest:

1) The presence of intercalation structures

2) The presence of an amorphous phase, on the graphite-iron interface, from which crystalline graphite forms at growth sites.

The implications for possible mechanisms of graphite crystallization are discussed.

In a previous publication, preliminary results obtained using secondary ion mass spectrometry to determine selective elemental distributions in cast iron were reported. These results were used to tentatively assign possible roles to the important solutes in controlling the graphite morphology. The current paper is a continuation of this work and presents the most recent results of the SIMS analysis.

The spherulitic growth of graphite from the melt is described in terms of instability mechanisms. Initially a pyramidal form grows which becomes unstable on the bounding face and a stellar polyhedral form results. The number of polyhedral facets increases in growth and the polyhedral apex angle decreases with the decrease of undercooling due to recalescence. Finally a rounded spherical form is produced.

The observations are taken from different experimental programmes, and from observations reported in the literature. Some comparison is also made with the behaviour of silicon under similar growth conditions.

Graphite morphology in cast iron is analyzed in terms of the growth kinetics of graphite crystals in liquid iron. At small driving forces, i.e., low supersaturation or small kinetic undercooling, graphite growth is characterized by faceted growth, resulting in flake, compacted and spherulitic graphite morphologies. However, at large driving forces, there is a transition from facted to non-faceted growth, resulting in a dendritic growth morphology.

Flake morphology is rationalized in terms of impurity dependent crystal growth mechanisms, whereas a spherulitic morphology is attributed to a defect controlled spiral growth mechanism. Compacted graphite morphology is considered as a transition between flake and spherulitic morphology.

A thermodynamic approach is used to inter-relate the residual concentrations of impurities of technological interest, i.e. S and 0, as a function of the residual concentration of the reactive elements, Mg, Ca, and Ce in a typical cast iron melt at 1500'C and atmospheric pressure. Such a diagram that quantitatively relates graphite morphology in thick cast iron sections to soluble concentrations of impurities is referred to as a graphite morphology control diagram.

In thin section castings that freeze at faster cooling rates and large kinetic undercoolings, the basal spiral growth mechanism dominates over the impurity controlled prism growth mechanism, leading to deviations from predictions based simply on the graphite morphology control diagram. In the ase of compacted graphite, where growth on both the prism and basal faces is involved, the degree of nodularity increases with the cooling rate, giving rise to section sensitivity.

At large undercoolings, the prevention of the nucleation and growth of cementite is an essential feature of graphite morphology control. It is estimated that the mobility of the cementite interface exceeds that of the prism interface in flake graphite growth by an order of magnitude and that of the basal interface in spherulitic graphite growth by three orders of magnitude. In practice, the driving force for graphite growth is raised selectively through the addition of graphite stabilizing elements, such as silicon, which raise the temperature of the graphite eutectic and depress the temperature of the carbide eutectic. Kinetic growth undercooling can be decreased by increasing the number of heterogeneous nuclei for graphite growth through inoculation. The application of the above concepts for the control of graphite morphology in shaped automotive castings is discussed.

Graphite structures formed during the cooling of molten cast irons and during their solidification were examined in alloys as a function of composition, treatment (magnesium and/or rare earths) and inoculation. The effect of cooling rate and solidification conditions on the morphology of the resultant graphite was investigated using a variety of techniques (optical and scanning electron microscopy, etching procedures, etc.) to reveal the growth characteristics of this phase and its relationship to other phases developing during the solidification process. These graphite forms have been classified according to their morphology and the factors affecting their development. Examples of these graphite forms are presented and their growth conditions and charateristics analyzed.

Kish graphite is shown to form, not as a result of internal development in a melt and subsequent flotation to the surface, but by a surface growth mechanism forming extensive graphite plates. Proeutectic graphite also forms in the melt just prior to the appearance of the eutectic, however, its growth in the absence of austenite is shown to be limited.

While a variety of graphite morphologies have been identified in eutectic solidification, it is shown that these graphite shapes may develop from the same nucleation sites, that growth conditions determine the graphite morphology, and that as the growth conditions are altered the morphology of the resultant graphite is correspondingly affected. The transition between graphite shapes such as flake, compacted, spheroidal, exploded, etc. is depicted in various microstructures.

In addition, the relationship between certain compounds (e.g., titanium carbo-nitride, etc.) and graphite growth is demonstrated and discussed.

An experimental device has been designed in order to simulate the solidification of large castings by means of a small-sized sample (1 kg). Once the spheroidizing and inoculation processes have been completed, this sample is slowly cooled at controlled rate and under protective atmosphere in an insulated crucible placed in a laboratory resistance furnace. The maximum freezing time is about 90 min. These tests relate to a base iron having a good degree of purity.

SEM, optical microscopy and automated electron microprobe have been used for studying structure and mechanism of chunky graphite formation.

Results of over forty tests confirm several data abstracted from literature and provide some more information. For instance, there are different ways of preventing chunky graphite formation : acceleration of solidification, use of Ni-Mg or pure Mg as spheroidizers and, in any case, an antimony addition (0,02%).

A rather low carbon equivalent is also a favourable factor. However, for a given carbon equivalent, a final relatively high Si content improves graphite spheroidizing, chunky graphite appearing only if this content results from Fe-Si-Mg additions and/or from overinoculation.

Besides, confirmation is given of the detrimental effect of useless rare earth additions (e.g. 0,02%).

The matrix surrounding chunky graphite is on average more rich in Si than the one surrounding spheroidal graphite. An oxidizing etching reveals that chunky eutectic has formed at first.

Aiming an interpretation of the results, structure examination keeps going on, in order to find out if there could be several kinds of chunky graphite.

Synthetic and commercial Fe-C-Si irons have been frozen under controlled conditions after vacuum purification in order to correlate graphite forms with freezing conditions and alloy composition. It was found that graphite morphology was influenced prirmarily by the sulphur and oxygen contents and the rate of cooling. The various modes of graphite growth are discussed, particularly the appearance of the coral and compacted graphite surfaces at high magnification after the addition of oxygen or nitrogen.

The ability of graphitic materials to serve as inoculants during the solidification of flake graphite cast irons has been examined and related to the physical structure of these materials. Carbon dissolution in cast iron melts is reviewed relative to dissolution rates and carbon recovery obtained from a variety of materials. Maximum dissolution rates and carbon recovery are shown to be attained with high purity carbon products, where the ash present does not form a network structure as carbon enters solution, and where the sulfur content of the carbon product is low.

Successful inoculation requires that the carbon product be graphitic, or possess a high degree of graphiticity, and that the solution rate be controlled so that the substrate graphite is present in the molten iron at the time of graphite formation during solidification. The characteristics of inoculants whereby this controlled solution rate can be achieved are discussed.

Inoculation with graphitic materials is shown to be effective in flake graphite cast irons, but is not effective in spheroidal or compacted/vermicular graphite cast irons. Factors contributing to this effect are discussed.

A theoretical analysis of inoculation shows that homogeneous nucleation of CaO, CaS, Al2O3 and ZrO2 is possible arid subsequent heterogeneous nucleation of silicates yields substrates for graphite growth. Excessive silicate growth leading to separation of nuclei could result in fade.

Results show that CaO/CaS embyros are most effective via silicate formation, Zr and Al control fade arido xidation enhances inoculation and fade. Added stable oxides give only limited inoculation.

The Spheroidal Graphites in cast iron and Ni-C alloy are extracted by using chloric acid or aqua regia and clean up using hydro-fluoric acid and hot water. The spheroidal graphite is crushed down till under five micron thickness wrapped by aluminium foil. The nucleus of spheroidal graphite can be observed by electron probe micro-analyser. Magnesium, Calcium, Cerium, Oxygen, Sulphur and Silicon etc. are recognized in the nucleus of spheroidal graphite.

The influence of additions of different SiC grades on the solidification of hypoeutectic and eutectic gray cast iron has been studied and compared with the additions of “FeSi 75” and pure silicon.

It is shown that additions of SiC change undercooling, microstructure as well as graphite shape and distribution in cast iron melts in a different manner than either FeSi or pure Si do. The difference is explained by the special dissolution kinetics of SiC which leads to formation of graphite and a lonqer-lastirg influence of carbon agglomerates in the melt. It is shown that SiC grades which are partially protected by SiO2 films from rapid dissolution in the iron melt show the most pronounced effect.

From the laboratory experiments it can be concluded that melt additions of special SiC grades can modify the microstructure of gray cast iron. A-graphite is more evenly distributed and formation of the Fe/Fe3C eutectic is more difficult. Similar influences are known from industrial production and can now be partially explained.

In the paper we have analysed some aspects of the structural,mechanical, technological and casting properties of compacted/vermicular graphite(C/VG)cast iron,with reference to our works published in the past 10 years.As regards the primary structure(eutectic solidification)we were interested in the composition and growth conditions of eutectic cells, of C/VG formation,on the basis of Mg and Si distribution,of intergranular phases,in different cooling conditions.We have also analysed the carbide formation and the inverse chill tendency. C/VG is characterised by form factors(compactness)on whose basis we propose some new typical graphite forms. The tendency to ferritic shells formation in the case of C/VG is compared with that recorded for NG,coral graphite,and FG type A,B,D. We note the possibility of favouring pearlite Dy thermal treatment,alloying and the use of a treatment-alloy with Al,Ti and other elements with a pearlitic action.We have also analysed the influence of wall thickness,cross-section geometry,holding time,anti- spheroidizing elements,nature of casting mould upon structure.Finally,we present a synthesis of the applicational properties,based on our data.

Using both liquid quenching and SEM technique is found that the vermicular graphite (CVG ) is formed Mainly in two ways. First, it grows out from flake graphite, and second, it degenerates, from spheroidal graphite (SC).According to this, two models of morphology and formation process.of CVS are sugguted. It. is discovered that the qrowth.direction of CVG of ten changes from a-to c-direction, or vice versa. Because of thu influence of modifying elements, the CVG is. twistad sq-verely, changing its growth direction continually and branching frequently. The growth of the branches may be in a-and/or c-direction; and their tips may be round and/or sharp. In all cases, there exists a large fraction of thick flake graphithe in the vermicular graphite. It is Suggested that the redistribution of modifying elements plays an importeant role in the formatinn of CVG.

When trying to understand the thermal behavior of solidifying cast irons, two experimental procedures seem to be especially helpful in generating the expected type of information, which then can be used for a quantitative understanding of the phenomena. They are thermal analysis and directional solidification.

The solidification morphology dictates the type of crystal growth for crystallites during solidification. As a result of this process, the casting may be unsound due to the presence of shrinkage cavities, internal shrinkage, sinks and hot tears. Therefore, it is important to have a knowledge of the solidification process to avoid these defects and to produce a sound casting.

The conditions were studied under which either eutectic graphite or carbide or both, Can form during freezing in low carbon silicon irons. Experimental variables included: iron composition, (C 1.32 to 2.17, Si 1.56 to 2.35%) small additions of alloying elements, (Ca, Mg, Al, S arid 0) and cooling rate (varying from quenching to slow cooling).

Thle results obtained show that without any alloying additions these irons solidify witb the formation OF eu~tectic carbide. A fully graphitic cast structure can he readily obtained, however, with inoculating additions of either Ca or Mg. The inoculating potential of these elements increases in the presence of small amounts of Al ard 0, hut decreases with the addi tion of S. Bal ancing S with Mn restores and improves graphite nucleation by other elements. Both Ca and Mg promote spherulitic, whilst Al and high C levels, promote flake graphite morphologies. Conditions and mechanisms of formation of euitectic graphite or carbides or their mixtures in low carbon and silicon cast. irons are discussed in thle light Of current theories of solidi ficati on of east irons.

Solidification of a nodular iron casting was simulated by an undercooling nucleation and diffusion controlled growth model. Simulated cooling curves were similar to measured ones but estimating nodular size distribution requires improving the model.

An attempt has been made to construct a model of the solidification of the globular eutectic grains which would account for an effect of interblocking the grain growth and would enable calculations starting with the pouring temperature and proceeding until the complete solidification of a casting is obtained. The family of equations proposed in this work enables a simulation of the kinetics of the transformation using a computer and basing only on the knowledge of the materials-designing properties of a foundry mould and the technological parameters of cast iron. The examples of computations given in the paper and the discussion of the results obtained makes it possible to explain the effect of various technological factors on the course of the solidification kinetics of a casting in the cause-effect relation.

The coefficients for partition of an alloying element (X) between austenite and liquid iron (), and between eutectic liquid and its solid (PS and PM in the stable and metastable eutectic solidifications, respectively) have been investigated thermodynamically. The coefficient depended significantly on the interaction between carbon and the element X. A simplified method for the evaluation of was introduced from the thermodynamic analysis of factors influencing on . The coefficients, PS and PM, calculated from and the coefficient for partition of the element X between cementite and ausfenite, agreed well with those obtained experimentally. It was indicated from the thermodynamic stability for the coexisting phases that the effect of an alloying element on graphitization occurring during eutectic solidification wvs closely related to the values of Δ(+PM-PS and and also to the effect of the element on the difference between the stable and metastable eutectic temperatures. No strict relation was found between the effects of an alloying element on the carbon activity in liquid iron and on the graphitization.