Graphite Nucleation in Compacted Graphite Cast Iron

GRAPHITE NUCLEATION IN COMPACTED GRAPHITE CAST IRON G. Alonso and P. Larrañaga AZTERLAN, Basque Research and Technology Alliance (BRTA), Durango, Spain D. M. Stefanescu Ohio State University, Columbus, OH, USA University of Alabama, Tuscaloosa, AL, USA R. Suarez AZTERLAN, Basque Research and Technology Alliance (BRTA), Durango, Spain Veigalan Estudio 2010 S.L.U., Durango, Spain Copyright Ó 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00441-2 Abstract During the last several decades, a multitude of theories have attempted to explain the process of graphite nucleation in lamellar (LG) and spheroidal (SG) graphite iron castings. Nevertheless, the complex 3D morphology of compacted graphite (CG) has hindered significant advances in similar theories for this type of graphite. To bring clarity to this issue, interrupted solidification experiments were conducted on compacted graphite irons at two different levels of titanium content in the melt (0.008% and 0.037%), with and without addition of commercial inoculants (Ce, MnZr). Nucleation sites were characterized through detectors, spectrums, mapping, and line scans utilizing FEG-SEM equipment. It was found that the nature of the inclusions acting as nucleation sites is directly related to the titanium content in the base metal. Nucleation in samples with low level of Ti occurs on Mg–Ca sulfides or Mg–Si–Al nitrides, which usually appear alone and seem to affect the growth of graphite. In the case of high percentage of Ti, double inclusions formed by Ti carbonitrides growing on Mg–Ca sulfides and restricting their growth seem to be the best combination for the nucleation of graphite. This is in line with our earlier findings for spheroidal graphite. Introduction iron. These properties are defined by the chemical composition, shape of graphite, cooling rate and liquid treatment and normally are intermediate between those of flake and spheroidal graphite irons.9,10 The outstanding combination of mechanical and thermal properties of compacted graphite iron and the advances in its manufacture have made it a promising replacement for grey cast iron. CGI may be used for castings with good mechanical properties, especially good vibration damping capacity, good thermal conductivity,11 higher pressures, and relatively low production cost.12 Compacted graphite iron (CGI), also known as vermicular graphite cast iron, is an alloy with attractive features and is widely used in the automotive industry.1–3 Although CGI has been known for more than 40 years,4–8 the mechanical and physical properties of compacted graphite are not yet as well-known as those of nodular or lamellar graphite cast This paper is an invited submission to IJMC selected from presentations at the 6th Keith Millis on Ductile Iron held October 23–26, 2018 at the Sonesta Resort, Hilton Head Island, SC. It is published in the IJMC by permission of the DIS (Ductile Iron Society). International Journal of Metalcasting Keywords: nucleation, compacted graphite, interrupted solidification experiments, FEG-SEM equipment While the manufacture of CGI castings has seen significant expansion over the recent years, the nucleation and growth of compacted graphite during iron solidification is still not fully understood. There is a significant divergence of opinion on the mechanism of formation of CGI. Models describing the growth of CGI altering between the c- and a-direction have been presented.13,14 In contrast, other authors consider that the growth occurs mainly along the c-direction.15 The compacted graphite particles are elongated and randomly oriented as in gray iron; however, they are shorter and thicker and have rounded edges (Fig. 1a). Some investigations16–19 have demonstrated that CGI is interconnected to their nearest neighbors within the eutectic grain (Fig. 1b), quite similar to lamellar graphite, which makes it difficult to locate their nucleation sites. Using polarized light, it has been suggested that a similar crystallographic orientation occurs in SG and in the segmented features of CGI.20 Pan et al.22 performed interrupted solidification experiments on thermal analysis cups to study the solidification of compacted graphite irons. Despite the time lag between sample removal from the stand and quenching, qualitative information on graphite shape evolution can be obtained through this procedure. The authors concluded that initial graphite precipitates in a spherical shape, which degenerates early during subsequent solidification, and then develops into CGI. Detailed observation of the graphite morphology verified that degeneration of small graphite nodules occurs by the extension of several graphite segments out of the graphite spherical surface. These segments grow in contact with the melt and diverge further as growth occurs, resulting in CGI. Similar conclusions were obtained by Alonso et al.23 For these researchers, the transition SGto-CG happened when the spheroidal graphite developed a tail (tadpole graphite) upon further cooling. In the regions where the spheroidal graphite started generating tails that become compacted graphite, branching of graphite appears to occur through twin boundary defects that result in the splitting of the lamellae along the basal plane. This is in line with the work by Qing et al.,24 who stated that branching of graphite may occur by twinning. Figure 1. Microstructures of compacted graphite iron (CGI): (a) in 2D, (b) in 3D (deep-etched SEM micrographs).21 CGI is produced by adding magnesium to the liquid iron to consume sulfur and oxygen. The formation of compacted graphite is a very difficult process to control with only a narrow margin of residual Mg: Too much Mg will give an excess of nodules (CGI nodularity must not exceed 20%), whereas too little Mg will lead to the formation of flake graphite.25 Compacted graphite is only stable at low levels of oxygen and sulfur, since magnesium is a very strong sulfide former, magnesium sulfide (MgS) inclusions are formed preferentially to manganese sulfide (MnS). These MgS inclusions have been identified previously as good nuclei for spheroidal26–31 and compacted graphite.32,33 The deoxidizing effect of magnesium also leads to the presence of magnesium oxide (MgO) and magnesium silicate (xMgO ySiO2) inclusions in CGI. Cerium presents a strong affinity to sulfur, resulting in the formation of highly stable cerium sulfide and oxy-sulfide.34,35 Numerous studies have identified the presence of these nonmetallic inclusions at the center of nodular graphite.36,37 These cerium compounds seem to be beneficial in the inoculation process, resulting in improved nucleation effectiveness throughout the entire solidification range. Titanium is typically present in cast irons in the range of 0.005–0.02% Ti. Higher amounts of Ti (0.1–0.25%) have been used to prevent the formation of spheroidal graphite in cast iron increasing the stable magnesium range for CGI production.38 The concentration limits for Ti on CGI is low, depending on the publication: 0.2 mass% by Thulemann,39 0.08 mass%,39–41 0.04 mass%,42,43 0.017 mass% by Elkem.39 Titanium not only degenerates the graphite in SGI, but also decreases machinability. In CGI, small increases in the titanium content from 0.01 to 0.02% reduce CGI tool life by approximately 50%,40 so titanium must be kept as low as possible, ideally below 0.01%. The compatibility between titanium with carbon and/or nitrogen present in the molten iron promotes the formation of titanium carbonitride inclusions, which form in the liquid state and grow with a cubic shape. These Ti(CN) were found previously by these authors acting as nuclei for spheroidal graphite.31,44 Reactions between titanium and rare earth elements have not been identified yet.45 The addition of Zrbearing inoculants46 produces very fine and dispersed nitrides, which may act as nuclei for graphite during the solidification. These researchers demonstrated the great affinity between Zr and Ti in forming (TiZr)(CN).47 In this work, the effort was expanded to experimentally reveal the type of the nonmetallic inclusions that act as nucleation sites of compacted graphite during early solidification as well as the influence of some trace elements (Ce, Ti, Zr) on these nuclei. To this purpose, interrupted solidification experiments at increasing times were conducted. The investigation was carried out on inoculated and noninoculated irons with two different titanium levels. Advanced field emission gun scanning electron microscope (FEG-SEM) techniques, such as spectrums, mappings, and line scans were used to analyze the nature of these inclusions. International Journal of Metalcasting Experimental Strategy Melting and Casting Two heats with different titanium contents (0.008 and 0.0037 mass%) were produced in a 100 kg medium frequency induction furnace (250 Hz, 100 Kw). The charge for each heat consisted of 20 kg of ductile iron returns and 26 kg of high purity iron. Predetermined amounts of a commercial graphite (98.9% C, 0.03% S), of FeSi alloy (74.6% Si, 0.3% Ca, 0.7%Al) and of FeTi (68.82% Ti, 3.67% Al in the case of the second heat), were also added to the metallic charges. After superheating to 1500 °C, the iron was transferred into the pouring ladle for Mg treatment with 0.55 kg (1.1 mass% of the batch weight) of a FeSiMg alloy (47.2% Si, 6.02% Mg, 1.15% Ca, 0.24% Al, 0.3% Mn and 0.88% RE) by the sandwich method. The FeSiMg alloy was positioned at the bottom of the ladle and then covered with steel scrap, before tapping the melt from the furnace. The chemical compositions of the experimental heats are presented in Table 1. In addition to the elements listed in the table, the alloys contained 0.02% Cr, 0.04% Ni, 0.01% Mo, 0.07% Cu, and less than 0.01% Al. A total of 18 standard thermal analysis (TA) cups were poured from the melts (twelve inoculated and six not inoculated). Inoculation was made directly in the cups through the addition of 0.2% of two different commercial inoculants whose compositions are summarized in Table 2. The liquid treatment of the various TA cups and the amount of Ti in the base iron are shown in Table 3. The solidification of the iron was interrupted by quenching in brine at increasing times for twelve of these TA cups, to obtain information on the microstructure and on the nucleation sites at various stages during the solidification (immediately after pouring and after 60 s). The 3D branching morphology of compacted graphite complicates finding the location of the nucleation sites, although in the Table 1. Chemical Composition (mass%) of Experimental Cast Irons earlier stages of growth, compacted graphite arms are not yet well developed, which helps finding the nuclei. The cooling curves of the not quenched samples were recorded with the ThermolanÒ system48 (Fig. 2a) After cooling to room temperature, the cups were sectioned and prepared for metallographic examination (Fig. 2b and c). Characterization Extensive field emission gun scanning electron microscope (FEG-SEM) examination of the graphite was carried out on non-etched samples. To identify possible nucleation sites an Ultra PLUS Carl Zeiss SMT (0.8 mm resolution at 30 kV) in combination with an X-Max 20 Oxford Instruments EDX detector with a resolution of 127 eV/mm2 was performed. Three different detectors were used for the generation of images: (a) in-lens detector (annular SE detector) for the surface structure; (b) Everhart–Thornleytype detector (SE2) for topography; (c) angular selective backscattered electron detector (AsB) for compositional contrast. In addition, this investigation was complemented with spectrums, mappings and line scans, to analyze the main elements present in the inclusions and to estimate the type of compounds. A demonstration of this procedure is illustrated in Fig. 3 showing a compacted graphite from the sample 1.6 growing around a complex inclusion. Three different zones are identified. Spectrum 1 indicates a clear presence of Mg, Si, Al, and N. The spectrum for position 2 reveals four clear peaks of Mg, Ca, S and O. Spectrum 3 is very similar but with an important increase in the peak of S and a small peak of La. The analysis of the X-ray composition maps Table 3. Liquid Treatment of TA Cups Heat %Ti in the melt 1 0.008 Inoculant None Sample 0s 60 s Unquench. 1.1 1.4 1.7 Ce 1.2 1.5 1.8 ZrMn 1.3 1.6 1.9 C Si Mn P S Mg Ti None 2.1 2.4 2.7 3.57 2.03 0.15 0.012 0.005 0.014 0.008 Ce 2.2 2.5 2.8 3.64 1.97 0.16 0.012 0.005 0.014 0.037 ZrMn 2.3 2.6 2.9 2 0.037 Table 2. Chemical Composition (mass%) of the Inoculants Symbol Si Al Ca Mn Ti Zr Ce Ba Mg Bi \ 0.1% Ce 70.6 0.87 1.08 0.20 0.06 0.10 1.61 \ 0.10 \ 0.10 \ 0.02 Sr, La ZrMn 62.6 1.01 1.79 5.96 0.13 6.77 \ 0.05 0.65 0.22 \ 0.02 Sr, Ce, La International Journal of Metalcasting Figure 2. Thermal cup at room temperature: (a) cooling curves, (b) microstructure at 3400 of a TA cup not quenched, (c) detail of compacted graphite with an oxide and a sulfide acting as nucleation sites. Figure 3. A compacted graphite from sample 1.6. EDX/SEM spectrums at three different positions on the inclusion. Figure 4. X-ray composition maps on the compacted graphite in Fig. 3. presented in Fig. 4 confirms the results suggested by the spectrums, with an important concentration of S, Mg, Ca, and O in the core accompanied in one of the corners by an important concentration of Mg, Si, Al, and N. A more thorough analysis, using X-ray concentration graphs, reveals that Mg, S, Ca, and La show composition peaks at the same position (Fig. 5). Then, Mg, Si, Al, and N also present coincidental maxima. Finally, the O maximum is situated at the same position as the maximum for Mg. Thus, this nucleus appears to be formed by a Mg oxide surrounded by a big rounded Mg–Ca–La sulfide, whose growth was restricted in one direction by a complex (MgSiAl)N. International Journal of Metalcasting Figure 5. X-ray relative concentration graphs (counts) in the compacted graphite in Fig. 3. Table 4. The Probability (Calculated for 20–25 Spheroids for Each Case) of Finding a Certain Inclusion in the Core of the Compacted Graphite Sample %Ti in the melt Inoculant 1.2 0.008 Ce Quenching time (s) Sulfides (%) Nitrides (%) TiCN (%) Oxides (%) Silicates (%) RE (La, Ce) (%) 0 43 57 0 14 0 0 1.4 None 60 40 65 0 25 0 30 1.5 Ce 60 37 75 0 8 4 25 1.6 ZrMn 60 43 74 0 30 13 35 2.1 None 0 100 0 100 0 0 0 2.4 0.037 None 60 100 0 85 19 0 12 2.5 Ce 60 100 0 91 27 0 9 2.6 ZrMn 60 93 0 100 21 0 14 Experimental Results A summary of the different inclusions detected in the compacted graphite as a function of the inoculant, of the level of titanium in the melt, and of the quenching time is shown in Table 4. A classification between low (0.008) and high (0.037) percentage of Ti in the base iron has been carried out. Low Percentage of Ti (0.008%) No Ti(CN) were detected acting as nuclei for compacted graphite from irons produced from low-Ti base iron. Mg– Ca sulfides and complex (MgSiAl)N nitrides were revealed as the predominant inclusions, appearing alone (not combined between them). These (MgSiAl)N can act as direct nucleants (Fig. 6a) despite their disregistry with graphite.49 The sulfides can nucleate on Mg oxides (Fig. 6b). For International Journal of Metalcasting samples inoculated and quenched after 60 s some silicates were found. In these samples, some RE (mainly La and Ce) were detected too, forming sulfides or silicates (Fig. 6c). There was no evidence of Zr and Mn in any of the graphite aggregates studied. High Percentage of Ti (0.037%) Sulfides and Ti carbonitrides were the majority inclusions in this category. Rarely, they appear alone (Fig. 7a), but normally combined with one another (Fig. 7b). No (MgSiAl)N were detected. In this percentage of titanium, some oxides were also found acting as nuclei. Normally, they were MgO or (MgAl)O and they acted as nucleation sites for the sulfides which surrounded them, total or partially (Fig. 7c). Some RE (La and Ce) were detected too, all of them as sulfides and in samples quenched after 60 s. No silicates were found. The majority of the compacted Figure 6. Examples of nuclei from irons with low percentage of Ti in the melt: (a) sample 1.6; (b) sample 1.4; (c) sample 1.5. Figure 7. Examples of nuclei from irons with high percentage of Ti in the melt: (a) sample 2.5 (a tadpole graphite); (b) sample 2.4; (c) sample 2.6. Figure 8. SEM images of two compacted graphite aggregates from irons with 0.008%Ti: (a) nucleating on a very big inclusion made of different compounds; (b) nucleating in contact with an inclusion in the matrix. graphite from sample 2.6 (iron inoculated with ZrMn) exhibited zirconium in the Ti carbides or carbonitrides of their inclusions (Fig. 7c). No evidence of the presence of Mn in the nuclei was detected. The strong thermodynamic affinity of magnesium for sulfur prevented any MnS formation. seems to restrict their growth in one or several directions. In the case of a low percentage of Ti (0.008%), the sulfides lose their dominance in favor of complex (MgSiAl) nitrides, which do not combine with them. In some cases, these sulfides seem to nucleate on Mg oxides. When the level of titanium is high, they can nucleate too on (MgAl) oxides. No (MgSiAl)N were found as nuclei in the 0.037% Ti irons. This is because the high content of titanium does not allow nitrogen to combine with anything else. According to these results, it can be concluded that the concentration of titanium in the base melt modifies the preferred nuclei, as in the case of the spheroidal graphite.31,47 Independently of the level of titanium and of the quenching time, numerous compacted graphite aggregates are observed nucleating on very big and complex inclusions (Fig. 8a) All the silicates detected were found in these types of inclusion. The initial growth of the graphite adopts the shape of the inclusion. The same effect was detected in the incipient growth of lamellar graphite.50 Circumferential growth of the graphite or sectors growing perpendicular to the inclusions faces in the initial stages can explain this behavior. Discussion The Mg–Ca sulfides are the only inclusions detected in all the samples studied in this research, independently of the quantity of titanium present in the base iron. For a 0.037% of Ti, these sulfides appear combined with Ti(CN) which Normally all the inclusions analyzed are surrounded totally or nearly totally by graphite. However, in the case of samples quenched immediately after pouring (0 s), some inclusions (mainly Mg–Si–Al nitrides) can have direct contact with the matrix, with only partial contact with the International Journal of Metalcasting Figure 9. Influence of the type of inclusion in the growth of compacted graphite from the melt with 0.007%Ti: (a) longitudinal growth; (b) longitudinal and transversal growth; (c) circumferential growth. Figure 10. SEM images of duplex in irons quenched after 60 s: (a) bridge thin and long, (b) short bridge, (c) full contact, (d) nuclei in the graphite, (e) nuclei in the connection. graphite (Fig. 8b). The explanation could be the early stage of solidification being enveloped by the graphite during the subsequent cooling. It seems that graphite only nucleates in preferential sites on the inclusion and starts growing perpendicular to the face of the inclusion. In further growing graphite finally surrounds the inclusion. These complex nitrides, with a clear polygonal shape and normally of a big size, are the main inclusions in the nuclei for samples with low percentage of Ti and seem to determine the direction and growth of graphite in one or several orientations (Fig. 9a and b). The nucleation power of the nitride does not seem to be associated with its crystal structure, but with the fact that it is a heterogeneity in the melt. In the case of the sulfides something similar happens, indicating a circumferential growth concentric to the nucleus,28,51,52 with the exception that in this case the growth can be interrupted in some directions by one or more tails. These nucleation sites are characteristics of the tadpole graphite, a precursor to compacted graphite53 (Fig. 9c). For samples with 0.037%Ti where the combination (MgCa)S ? Ti(CN) is the most habitual, this tendency disappears and the relation between the type of inclusion and growth is not so evident, because of difficulties in growing on the edges of the cubic crystals. There is no noticeable difference in the type of inclusions generated between inoculated and non-inoculated samples. On one hand, the use of an inoculant rich in Ce does not International Journal of Metalcasting have a significant influence on the nature of the inclusions detected as nuclei. Because of a smaller interfacial free energy at the nucleating interface,54,55 graphite preferentially nucleates on RE sulfides over RE oxides during solidification. The presence of Mg–Ca–Ce–La sulfides is more related to the quenching time than with the inoculant used. These RE nonmetallic inclusions become heterogeneous nucleation sites for graphite during the last stages of solidification. This statement is supported by the fact that they are rarely found in nuclei at early quenching time while they become usual in nuclei detected at higher solid fractions. They could be considered as a second type of nucleation sites that need more time to become activated. On the other hand, the influence of the ZrMn inoculant is clear but only in the samples with high level of Ti, where the majority of Ti carbonitrides are enriched in zirconium. While the affinity of Zr to Ti is obvious to form nitrides, Zr by itself does not react with N or at least the complex Mg– Si–Al combination is stronger to neutralize N at low Ti levels. In samples quenched after 60 s, independently of the inoculant and of the level of Ti in the melt, a number of duplex graphite were detected (Fig. 10). These types of graphite are formed by two graphite aggregates (normally spherical or quasi-spherical) which are joined by a bridge of carbon, whose length and size can change. In some cases, the graphite particles are wide apart, and connected by protrusions from the two nodules (Fig. 10a). In the other four cases, the graphite aggregates, are in close proximity and have each their own nucleus. The spheroids in Fig. 10c, d have apparently grown into one another and have each their own nucleus. However, the semi-spheroids in Fig. 10b, e are connected by a graphite bridge, and at least those in Fig. 10e originate from a single sulfide inclusion. To the best of our knowledge, there is no proposed mechanism that can explain this kind of growth. Conclusions This research attempts to provide some clarity to the nucleation of compacted graphite through the use of advanced SEM techniques and quenching experiments. With this purpose, a series of TA cups with percentages of Ti in the base iron of 0.008% and 0.0037%, uninoculated or treated with inoculants containing Ce and MnZr were produced. As demonstrated earlier for the spheroidal graphite by these researchers, the role of titanium in the nucleation of compacted graphite appears to be more important than previously thought. Mg–Ca sulfides and Mg–Si–Al nitrides nucleating independently are the main inclusions for the samples with low percentage of Ti. The combination (MgCa)S ? Ti(CN), are the typical nucleation sites for samples with high %Ti. These Ti(CN) seem to restrict the growth of sulfides in some directions. MgO and (MgAl)O were detected as nucleation sites for sulfides. The process of inoculation does not have too much influence on the nature of nuclei. Some RE acting as silicates, but overall, as sulfides, have been found in samples quenched after 60 s, so they could be considered as a second type of sites that are active later during the solidification sequence. In the melt with high %Ti, Zr has a great affinity with titanium to form carbonitrides in iron inoculated with ZrMn. The effect of the nucleus crystallography on the graphite shape is significant. For (MgSiAl)N and sulfides, there is a clear correlation between the growth of graphite and nuclei: in the direction of the inclusion, in the case of nitrides; circumferential for sulfides. This effect disappears for the Ti(CN). Graphite that has the appearance of two nodules linked by a graphite bridge were commonly found during this work. A possible explanation for this kind of graphite is that they can start growing as nodules, then they branch adopting the shape of a tadpole and finally grow again as nodule, or they can start growing as compacted graphite and then both extremes may develop a nodular shape. Acknowledgements The authors would like to acknowledge Diputación Foral de Bizkaia for supporting this research. Also, the authors gratefully acknowledge the valuable comments of Jin Chang Liu on the role of Ti on CGI. REFERENCES 1. S. Dawson, I. Hollinger, M. Robbins, J. Datch, U. Reuter, H. 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