# LOOKING AT THE GRAPHITE SPHEROIDS UNDER LIGHT MICROSCOPY

Martin Boucherit, Moukrane Dehmas and Jacques Lacaze

CIRIMAT, Université de Toulouse, BP 44362, 31030 Toulouse, France

Copyright © 2025 The Author(s)
https://doi.org/10.1007/s40962-025-01752-y

## Abstract

This brief work corroborates the well-known fact that metallographic preparation is important for obtaining relevant observations, particularly when dealing with graphite in cast irons. This is illustrated here by the correct and then inadequate polishing of a spheroidal graphite iron alloy sample. Under an optical light microscope, the graphite appears essentially compact (no voids) after correct polishing, whereas graphite abrasion and nucleus detachment generated by improper polishing lead to a spatial rearrangement of graphite blocks in the central part of the spheroids. Similar effects can also be observed after

appropriate polishing, when the sample is subsequently subjected to chemical or thermal etch, or else to ionic polishing. The characteristics then observed are not representative of the original arrangement of graphite in the spheroids and might lead to erroneous conclusions on spheroidal graphite growth.

**Keywords:** spheroidal graphite, polishing, metallographic observations

## Introduction

It has always been considered that successful metallographic preparation would reveal the internal structure of graphite by optical microscopy, resulting in an impressive amount of technical and scientific reporting on how graphite spheroids evolve from their nucleus. In their brief report on the metallographic preparation of spheroidal graphite cast iron (SGI) for optical microscope observations, Ahmed and Gawlick¹ gave advice on graphite retention. This advice was extended to lamellar graphite in the Cast Iron Handbook,² where the emphasis was again on graphite retention, but also on revealing the true thickness of lamellae in lamellar graphite cast irons (LGI). After a brief background section, this short note illustrates, with the help of a specific example, the fact that nucleus retention must also be ensured for correct metallographic preparation of SGI if the aim is not only on size distribution of graphite spheroids but also on their internal structure.

## Background

Light optical microscopy (LOM) can be used under a variety of conditions, not just white light. In particular, the property of graphite to reflect or not reflect polarized light depending on its orientation, long known for flake graphite, was used by Morrogh in his 1949 patent application on spheroidal graphite.³ Figure 1 compares a near-diametrical cross section of a spheroid in white and polarized light: a radial structure emanating from the center appears under both illuminations, while polarized light has the added interest of showing that the graphite stack orientation rotates around the center. The spheroids thus appear to be made up of neighboring sectors, whose slightly different contrasts suggest that the orientation of the c-axis is roughly the same in each sector but differs from the orientation of the next sector by rotation around the center.

Based on such LOM observations, a well-known example among the many growth mechanisms that have been proposed is that of Morrogh⁴ illustrated in Figure 2. After metallographic observations such as that in Figure 2-a, Morrogh proposed the mechanism described in Figure 2-b, where the sectors twist continuously from the center outwards to fill the space. This twisting was probably suggested by the rotation of some of the radial lines. Although
<figure><img src="image_1.png" /><figcaption>**Figure 1.** Micrograph under white (a) and polarized (b) light of a graphite spheroid.</figcaption></figure>

<figure><img src="image_2.png" /><figcaption>**Figure 2.** Example of micrograph shown by Morrogh and Williams (a) that led them to suggest the growth mechanism sketched in (b).⁴</figcaption></figure>

this mechanism has never been considered plausible in the casting community, it is still mentioned here and there, for example in the latest review by Stefanescu et al. on the growth mechanism of graphite spheroids.⁵ Morrogh certainly considered that the change in orientation of the radial lines defining the sectors in Figure 1 should have been continuous, whence the supposed twisting.⁴ However, it has been reported by Gruzleski⁶ and Bourdie et al.⁷ that these lines are due to a relief on the metallographic surface issued from polishing, and Bourdie et al. further suggested that their apparent change in direction is associated with the formation of new sub-sectors and their growth competition with previously existing ones. To support their conclusion, Bourdie et al.⁷ complemented light optical microscopy with scanning electron microscopy (SEM) images acquired using a secondary electron detector. At low magnification, such SEM images mostly show the same features as light optical micrographs, with, however, a higher depth of focus and the possibility of tilting the sample to evidence surface relief.⁷

In practice, attention is generally directed on diametrical sections of the spheroids that are expected to show the whole of their growth process. Solidification of SGI in the stable system relies on inoculation to trigger graphite

nucleation, and most current studies on inclusions in SGI focus on their identification in terms of chemistry and of crystallography so as to optimize the process. However, it is worth mentioning that a few past works questioned if every spheroid has a nucleus in its center. Mercier et al.⁸ followed 48 well-formed spheroids by serial polishing of a SGI sample with a step of 1 µm in average. They found that 83% of them had one or two inclusions, and one might suggest that the remaining 17% had an inclusion of size lower than this 1 µm step. Jacobs et al.⁹ evaluated the probability *p* of observing the nucleus on a single metallographic section as $p = \bar{r}/\bar{R}$, where $\bar{r}$ is the average radius of the inclusions and $\bar{R}$ that of the spheroids. Assuming nuclei with an average diameter of 1.5 µm and spheroids averaging 10 µm in diameter, the authors obtained *p* = 12%, which closely matched the 10% they obtained experimentally. Analyzing a population of over 10,000 small spheroids in a quenched sample, Qing et al.¹⁰ found that the nucleus could be observed within 20% of them corresponding to nuclei of 1 µm in diameter inside spheroids of 5 µm in diameter, according to the same estimation. Consequently, it can safely be concluded that every graphite spheroid in cast iron developed from its own nucleus that is expected to appear in a diametrical section.
Thus, investigating diametrical or near-diametrical section of spheroids will be valuable provided that the metallographic procedure ensures retention of the nucleation particle(s). In case spalling off of this particle occurred, a void is left as in the example discussed by Tartera et al.<sup>11,12</sup> that is reproduced in Figure 3. The authors stated that the appearance of the graphite stacking changed because of stress relaxation, and they emphasized the possible confusion that can arise in not considering this change. As a matter of fact, the micrographs in Figure 3 are very similar to that shown by Hunter and Chadwick on a replica of a graphite spheroid (that was obtained by deep etching of a thin sample).<sup>13</sup> From the elongated shape of the center void, these authors suggested that graphite spheroids emanate from a minute graphite flake, an interpretation that is now known to be incorrect.

The present work was intended to clarify the consequences of Tartera's statement. To achieve this aim, a sample was selected that was first properly and then poorly polished. Metallographic observations are presented in the section to follow and discussed thereafter.

## Experimental Details and Observations

The alloy was a Fe–C–Ce sample prepared by introducing in a pure graphite crucible (for analytical chemistry) a piece of pure iron onto a piece of pure cerium (from Elkem company) weighting 2% of the mass of iron.<sup>14</sup> The crucible was then positioned in an alumina tube allowing for setting primary vacuum (10<sup>-3</sup> atm) and located inside a resistance furnace that was heated to 1350 °C. After heating, this temperature was maintained for a 10 min hold during which carbon diffused to the iron piece that eventually melted onto the cerium piece so as to generate the Fe–C–

Ce alloy. At the end of this step of alloy preparation, the furnace was cooled to 1150 °C, i.e., a temperature slightly below the Fe–C eutectic, hold for 60 min and finally quenched to room temperature.

Figure 4-a presents a low-magnification LOM micrograph of the bottom part of the sample. On the bottom right, it is seen that graphite precipitated in contact with the crucible and developed inwards with hemispherical tips. In the bulk of the sample, a twofold distribution of spheroids was observed, strongly suggesting that large spheroids started to develop during the initial cooling from 1350 °C. During the final cooling stage after the hold at 1150 °C, these primary spheroids grew and got encased in an austenite shell while small spheroids appeared and got also enveloped after a while. Because of quenching, a significant amount of ledeburite was also observed (not visible in Figure 4-a that was not etched) and some irregular graphite particles appeared in the last to solidify zones.

For the present study, the sample was polished again following a best practice procedure when dry griding was used, minimizing both the polishing time and the pressure applied on the sample at all polishing steps. Final polishing was carried out using a ¼ µm alumina solution in water. The largest spheroids, such as the one shown in Figure 4-b, showed a duplex structure, evidenced here by a slight change in the contrast (see also Figure 1 that gives another example). Such a duplex structure was associated to primary precipitation (inner zone) and eutectic precipitation (outer zone) by Morrogh and Williams,<sup>4,15</sup> later called first and second stages by Stefanescu (see for example his latest review on the growth mechanism of graphite spheroids<sup>5</sup>). The most relevant observation for the present work is that graphite in the inner zone of all spheroids appeared compact (showing no voids), and that many of the radial

<figure><img src="image_3.png" /><figcaption>**Figure 3.** Light optical micrograph of a graphite spheroid (a) and enlargement of its center part (b). Reproduced from Tartera et al.<sup>12</sup></figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 4.** Low-magnification LOM micrograph of the studied sample (a) and high-magnification LOM micrograph of one large spheroid after best practice polishing (b).</figcaption></figure>

features initiated at the nucleus and showed continuity between the two zones.

The next step in our work was thus to polish again the sample but, now, with the definitive aim of abrading further the graphite precipitates. This was simply achieved by extending the duration of each polishing step and applying higher pressure to the sample. Figure 5-a shows a field with several spheroids and Figure 5-b an example of a quasi-diametrical section of a spheroid. At both magnifications, it is seen that the area around the nucleus no longer appears smooth and that the radial features are blurred. In fact, many if not most of the near-diametrical sections of the sample were such that the nucleus had spalled off.

For further illustration, low-magnification SEM micrographs are better suited than LOM micrographs because of the higher depth of focus of the SEM. Figure 6-a shows a series of spheroid sections that all present a hole in their center because the nucleus was removed. Figure 6-b is a tilted view of one of these spheroids while Figures 6-c and 6-d are enlargements clearly showing there is now space in between the remaining graphite plates of the central part of the spheroid. Thus, the polishing procedure removed some

material in a thick zone around the nucleus, so that the graphite does not anymore appear compact. Note also in Figure 6-d that the arrangement of the remaining graphite plates presents a slight chevron pattern that appears to be due to the poor polishing procedure as it was not observed after the first polishing.

No attempt was made to look for a correlation between the polishing conditions (pressure, time) and the extent of the rearranged zone. In fact, it appeared highly possible that the rearrangement will be limited to the inner zone of the spheroids. An extended investigation using transmission electron microscopy (to be published elsewhere) evidenced a difference in size of the graphite blocks between the inner zone where they are nanometric and the outer zone where they are micrometric. This difference has to do with the change in growth rate and the use of cerium as spheroidizer.

## Discussion and Conclusion

As described above, a modification of the graphite aspect on a metallographic diametrical section of a spheroid can

<figure><img src="image_5.png" /><figcaption>**Figure 5.** LOM micrographs at medium (a) and high (b) magnification of large spheroids after poor polishing.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 6.** SEM secondary electron micrographs (a–d) illustrating the effect of a poor polishing procedure. See text for details.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 7.** SEM micrograph of a spheroid that has been chemically etched, reproduced from Stefanescu et al.⁵</figcaption></figure>

occur after the nucleus had spalled off. Chemical or thermal etching, or else ion polishing, will lead to the same

result even in the case when the nucleus is retained: by removing material to generate relief, these preparation procedures create spaces that allow the graphite remnants to move relative to each other. Figure 7 shows an SEM image from a sample that has been chemically etched and shows the same characteristics as in Figure 6, although the nucleus has been retained.⁵

As given in the preceding section, reorganization of the graphite blocks after some space has been created leads to a biased view of the original graphite stacking. This finding is exacerbated by the appearance of chevrons in Figures 3 and 6-d, which have suggested a growth mechanism for spheroidal graphite based on interfacial breakdown¹³ that has never been corroborated by transmission electron microscopy. An interesting and as yet unanswered question would be whether the stresses that are relieved have accumulated during graphite growth or during cooling of the material after solidification.

## Acknowledgements

Thanks are due to A. Freulon and Y. Thébault for their support in light and electron microscopies.
Funding

Open access funding provided by Institut National Polytechnique de Toulouse.

**Conflict of interest** The authors have no relevant financial or non-financial interests to disclose. The authors have no competing interests to declare that are relevant to the content of this article.

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