Graphite Growth Morphologies in Cast Iron

Haji Muhammad Muhmonda and Hasse Fredrikssonb Materials Science and Engineering department, KTH, Sweden a,b

haji@kth.se a, hassef@kth.se b

Keywords: Cast iron, graphite morphology, transition, clusters, micro-segregation, ion etching, and thermodynamics.

Abstract. Graphite growth morphology was studied by using InLense detector on FEG-SEM after performing ion etching on the samples. Star like and circumferential growth mechanism of graphite was observed in the graphite nodules. Pure ternary alloy of hypo eutectic and hyper eutectic composition was treated with pure Mg, Ca and Sr, to study the effect of O and S concentration in the melt, on the transition of graphite morphology from nodular to vermicular/compacted and flake graphite. The change in the melt composition between the austenite dendrites due to micro-segregation of S, O and inoculants and their possible effects on the transition of graphite morphologies as well as the nucleation of new oxides/sulfides particles is discussed with the help of thermodynamics.

Introduction

The transition of graphite morphologies in cast iron from nodular to compacted/vermicular (CG/VG) and to flake graphite was not fully understood. Strong de-oxidizers and desulphurizer are often added to cast iron as inoculants which provide nucleation sites for the graphite. Certain elements such as Ca, Mg, La and Ce, are known as nodular graphite (SG) promoter. There have been reported many types of nuclei, such as Igarashi et. al [1] found MgO/MgS enveloped by (Mg,Si,Al)N, nucleating a nodule. Also CaS, MgO and Al2O3 were observed in the core of graphite nodule while (Al,Mg,Si)N in the shell. FeClx was found together with (Mg,Al,Si)xOx particle in the core of a graphite nodule in high purity cast iron [2]. Nakae et. al [3] analysed different types of compounds as nuclei for nodules such as (Mg,Si, Al)N-oxides, (La,Ce,Nd)OxSy, FeClx, (Ce,La,Nd)S and SiO2 as core compounds, and concluded that the formation mechanism of nodular graphite is not based on the nuclei . In flake graphite, MnS was heterogeneously nucleated by different types of oxides which became nucleation site for the flake graphite nucleation [4].

Contrary to the inoculation effect there have been evidences where nodular graphite was obtained without any inoculation such as Sadocha et. al [5] found only nodular graphite in a spectroscopic purity ternary alloy (Fe,C,Si). At slightly lower purity level many curved and bent graphite crystal appeared. According to Gruzleski [6] the morphological change is brought about by the ability of graphite to grow as curved crystals under certain condition while under other conditions this ability does not exist and only flakes are formed. According to Double et. al [7] and Johnsson et. al [8] Mg does not have direct action on graphite spherodization but it rather acts as a scavenger for those elements which stabilizes the flake form. According to Velichko et. al [9], vermicular graphite grows initially as spheroids, but later due to sufficient amount of O and S in the residual melt [10] develops branches with the crystal structure similar to flake graphite. Chang et. al [11], suggested the presence of Fe-C cluster in the melt which creates different polarity due to difference of the ionization energies in Fe and C, where C atoms exhibit weak negative polarity, and Fe weak positive polarity.

Franklin et. al [12] made secondary ion mass spectroscopy (SIMS) analysis on flake and nodular graphite. Higher oxygen content was found in the flake graphite but sulfur content was much lower than oxygen. In the nodular and compacted graphite the oxygen and sulfur content was lower compared to flake graphite. According to Hofmann et. al [13] about of the oxygen content was present as SiO2, 8 ppm as FeO + MnO, the remaining being bound as silicates, mixed oxides or

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more stable oxides than SiO2 such as Al2O3 or TiO2. Elbel et. al [14] also recorded the oxygen activity for FG, CG and SG at 1350 C which are 0.97 ppm, 0.30 ppm and 0.16 ppm respectively.

In this investigation, nodular graphite and flake graphite growth morphology was examined in Field Emission Gun (FEG) SEM after performing ion etching on the samples. The effect of O and S concentration in the melt on the graphite growth morphology was studied experimentally and theoretically with the help of thermodynamics.

Experimental procedure

The composition of alloys under investigation is provided in Table 1. Two alloys A1, A2 were prepared in the laboratory using pure Fe (99.99), C(99.9999) and Si (99.9), in the ceramic coated crucibles. The composition of these alloys was measured with Spark Emission Spectroscopy (SES). T640-6 sample was inoculated with Vaxoninoculants (Provided by Jaques Lacaze, CIRIMAT, ENSIACET, Universit de Toulouse, France). High Frequency (HF) induction furnace was used with a graphite suscepter, placed between the coil and the crucible, in a protective atmosphere of high purity Argon. A few de-oxidation experiments were performed by adding pure Mg, Ca and Sr to the melt and cooling down to room temperature, while using MgO crucibles. The final composition was not measured after de-oxidation experiments. Samples were examined along the perpendicular section in the SEM. Experiments A2-4 and A2-5 were performed on the sample which was treated with Ca (A2-2), in the DSC equipment using zirconia crucible, under argon atmosphere. Ion etching was performed perpendicular to the surface for 5 min (PECS Gatan: Beam 10 KeV, Etching gun ~380 A, left and right gun 10 and 5 A) on the sample T640-6 and A1-1. The morphology of graphite was studied with InLense detector attached to FEG-SEM. Cross-Section Ar ion polishing was used on Mg treated sample, parallel to surface while removing 75 micron deep surface.

Results

Sample T640-6 was examined with InLense detector in FEG-SEM, after performing ion etching. A star like growth of the graphite was observed in the centre of the graphite nodules (Fig.1(a,d)). The branches were extended outward to some distance from where onward the circumferential growth mechanism was dominant. Those nodules with star like growth, emerging from the middle of the graphite, do not seem to have any inclusion in the middle as the one shown in the Fig. 1 (except c). The nodules with an inclusion (nuclei) in the centre had circumferential growth by following the shape profile of the inclusion. In Fig. 1e, the central part did not have any inclusion, but the graphite formed in layers as indicated in the magnified image. There should be no effect of the argon ions on the surface of graphite due to the usage of low voltage and for short time.

Table 1. Experiments and experimental conditions.

Fig. 1: T640-6 sample, quenched at eutectic temperature. InLense detector images after Ar ion etching using PECS.

Table 2. Investigated alloys composition.

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De-oxidation of the melt using pure Mg. The microstructure contained combination of nodular, compacted/vermicular and undercooled fine graphite (Fig. 2a). In Fig. 2c, a bundle of graphite layer have changed its direction of growth from circumferencial to tangential direction which at certain distance again have changed the growth morphlogy towards circumferencial growth mechanism, resulting in two adjacent nodules. Two step growth could be observed in many nodules as shown in Fig. 2d, where the outer shell around the nodule was formed during the solidification process. The circumferencial profile of the growing graphite layers exactly followed the shape profile of the nucleating agent (MgO/S inclusio, Fig. 2d). In Fig. 2(e,f,g,h) after cirtain growth of the graphite, a transition towards flake like morphology was occurred.

As shown in Fig. 3, many MgS particles were observed both in the undercooled graphite region as well as in the nodular graphite region, with and without any connection to graphite. Most of them were below 1 m in size. A relatively large round MgO particle was found in the austenite region, sorrounded by large number of nano sized MgO particles (Fig. 3c).

De-oxidation of the melt using pure Ca. The microstructure showed large number of perfectly round nodules of various sizes and some other forms of compacted graphite which were mostly precipitated inside the shrinkage pores (Fig. 4d), with a complete absence of undercooled fine graphite (Fig. 4). The fraction of graphite in this sample was less due to the formation of CaC2 which can stabilizes or nucleate cementite. Due to multiple nuclei of CaO/S, a compacted like graphite was produced as indicated with arrows in Fig. 4c. C was often found in the middle of the noduels as shown in Fig. 4(b). Sample A2-2 when re-melted (A2-4), with low oxygen content resulted in fine undercooled graphite together with some course graphite flakes (Fig.4e), but with high oxygen content (A2-5), the microstructure was consist of extremely fine vermicular type of graphite, with no trace of course flakes (Fig. 4f).

De-oxidation of the melt using pure Sr. The sample treated with pure Sr contained mostly flake graphite; however a small area (about 5%) at the top edge of the sample surface, contained large size irregular graphite particles of various sizes as shown in Fig. 5. After ion ethcing there

Fig. 4: (a,b,c,d) Ca treated sample A2-2, Ar Ion etched, (e) Re-melted Ca treated samples A2-4, (f) Re-melted Ca treated samples and 1 wt. % O addition.

Fig. 2: A1-1 sample, FEG-SEM InLense detector images. Highlighted black

Fig. 3: A1-1, FEG-SEM images of Cross-Section Ion Polished surface, shows MgS particles.

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were some particles which contained amorphous graphite surrounded by layers of graphite (about 5-10 nm thick) as indicated in Fig. 5(d,e), also the inner part of Fig. 5(f,g) seems like amorphous. The growth mechanism in the other large size irregular graphite was flake like but with many different orientations.

Discussion

By adding different types of elements to liquid melt, one changes the concentration of dissolved trace elements (O, S, N, P etc.) in the melt by forming different types of inclusions depending on the affinity of the element with the trace elements. Ellingham diagram (updated by the creators) [15] is re-drawn (Fig. 6c) to calculate the energy of formation of some important types of reactive elements. Those elements which have the highest affinity towards oxygen such as Y and Ca might be consumed mostly prior to solidification starts; however if oxygen reduces to the equilibrium value with those active elements, the remaining elements can form sulfide particles during solidification. In the melt, the amount of sulfur is usually higher than oxygen which provides opportunity for new particles formation. From EDX analysis, only few of the particles were found MgO, but most of them were MgS. Their smaller sizes depicts that they have nucleated at the end of solidification. The steeper is the formation energy line, the quicker it will reach the limit of homogeneous nucleation, as shown for MgS and SrS (Fig. 6(a,b)). Detailed formulations and calculations have been earlier reported by Muhmond et. al [4] for hypo eutectic iron. By adding pure Mg, it will form MgO due to its higher affinity towards oxygen, with or without silicates, which together have served as nuclei for the graphite nodule to grow on it. Primary graphite nodules formed will float up, causing a change in the remaining liquid composition towards hypo eutectic. At the inter-dendritic areas, new MgS particles will form (Fig. 6(a,b)), due to enrichment of sulfur in the melt, since oxygen was already reduced to lower level in the melt prior to solidification. SrS might be formed as well; however in the presence of Ce, Mg and Ca it is difficult to nucleate SrS due to its lower affinity to Sr. If the newly sulfide/oxides particles are formed prior to, or at the beginning of eutectic reaction, they could serve as nuclei for graphite. The morphology of the graphite growth will depend on the concentration of dissolved trace elements such as O and S concentration around the growing graphite. The oxide/sulfide particles which nucleates close the end of solidification, may or may not serve as nuclei for graphite. Many MgS particles were found connected to flake graphite which proves that the dissolved content of sulfur was above the critical limit for transition of graphite. The formation of new particles is important because they reduces S and O concentration, however in the absence of such inclusions, the concentration of the mentioned dissolved elements would continue to increase and the graphite morphology will be significantly modified. From some other results often we found very few MgS particles in the undercooled graphite region compared to the other forms of graphite. An example is shown in Fig. 2b marked area A, larger graphite particles are round but the smaller ones are under transition towards compacted graphite but in the marked area B , the graphite are elongated, shows the difference in composition (S and O) of the melt locally. In the marked area C, a nodule being trapped by the growing eutectic graphite is found.

The Ca treated sample was completely free from undercooled fine graphite or normal flake graphite, due to the effective scavenging of oxygen and sulfur from the liquid. A pore that might have been formed during cooling of melt, was later filled by C diffusion towards it (Fig. 4d).

Fig. 5: A2-3, InLense detector images of the Sr treated sample, after Ion etching. White marked regions in d, e, f and g indicates

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Fig. 6: (a,b) Homogeneous nucleation of new MgS and SrS particles during solidification, while

assuming hyper eutectic composition, KMg part. = KSr part. = 0.031 and start and end temperature = 1200

and 1120 oC. (c) Ellinghams Updated diagram [15] ,Thermodynamic energy of formation of different

oxides and sulfides, (except for SrS [16]).

Increasing oxygen content have changed the graphite morphology drastically from flake graphite towards extremely fine vermicular type of graphite, in the re-melted Ca treated sample (Fig. 4f). Due to the presence of about 2 ppm oxygen in the argon gas and melting in the alumina crucible would have allowed the melt to dissolve some oxygen in it and Ca might have been faded out, thus causing the formation of undercooled type of graphite. The opposite happened in the Sr treated sample where due to higher affinity of Sr towards O, it reduced oxygen content in the sample while sulfur remained less affected which played a role in modifying the growth morphology to normal flake graphite.

If one carefully observe different types of graphite morphology shown in Fig. 2(a,b) and Fig. 5(a,b), the transition of graphite morphology from the centre of the austenite dendrite towards the eutectic changes from nodular to compacted/vermicular and then undercooled fine graphite, or even flake graphite finally. The explanation can be given by the O/S content dissolved in the liquid, which enriches due to micro-segregation from the edges of the dendrite towards the last solidified region. By getting closer to the end of solidification, the O/S content increases exponentially. In the absence of de-oxidizers and de-sulphurizer, it could easily be above the critical limit and thus causing transition towards either flake graphite or undercooled graphite growth morphology, depending on the type of trace element and its concentration.

Concluding remarks

The growth of graphite in a dendritic or star like manner, at the centre of nodules, depicts certain relationship of the graphite growth morphology with the lattice structure of the graphite and/or the presence of nano sized C-C or C-Si and even C-Fe clusters in the melt which changes the kinetics of the graphite growth, is under investigations. MgO and MgS being present in the centre of a nodule as well as connected to/or close to undercooled/vermicular graphite gives an indication that the concentration of trace elements around the growing graphite during cooling and solidification could affect the graphite growth morphology. The possibility of trace elements (O, S etc.) being co-valent bonded in the graphite lattice, stabilizing either flake or compacted/vermicular growth of graphite, will be carefully analyzed in future.

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Acknowledgment

The authors would like to thank Elkem AS Foundary Technology Products R&D for providing the financial support.

References

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