Iron casters discuss keys to optimizing inoculation.

Industry experts share insights into graphite nucleation, thermal analysis for inoculation efficiency and inoculation of thin-wall castings.

The last AFS International Inoculation Conference was held 20 years ago, and, based on the enthusiastic response to this year's event, it was long overdue. Nearly 230 attended the 1998 program held April 6-8 in Rosemont, Illinois. Attendees hailed from five countries, including four Canadian provinces and 23 states.

The conference gathered 33 speakers representing six countries, six foundries, eight universities, eight suppliers, three consultants and two research organizations. The presenters discussed 19 papers covering a range of topics from the basics of iron inoculation to more advanced methods of controlling graphite nucleation and optimizing inoculation benefits. The program was sponsored by the Molten Metal Processing Committee of the AFS Cast Iron Div.

SULFUR'S ROLE IN GRAPHITE NUCLEATION OF IRON

In their paper titled "The Importance of Sulfur to Control Graphite Nucleation in Cast Irons," Julien Riposan and Mihail Chisamera, Univ. Politechnica Bucharest, discussed the results of their experiments with sulfur (S)-based compounds during inoculation.

[TABULAR DATA FOR TABLE 1 OMITTED]

[TABULAR DATA FOR TABLE 2 OMITTED]

The inoculation effect depends on the occurrence of compounds - oxides, sulfides, nitrides, carbonitrides - which promote graphite nucleation as a result of their stability in cast iron. After magnesium (Mg) treatment, liquid iron becomes low in oxygen (O), S and nitrogen (N), so the effect of inoculation is limited. Chisamera and Riposan performed experiments to determine the connection between a S addition after the Mg addition and inoculation effectiveness. The experiment was designed to assess the influence of inoculant materials on graphite nucleation capacity and chill tendency of cast irons.

Experimental Procedures

Ductile iron having slight hypereutectic or hypoeutectic compositions was produced in a 22 lb-capacity, 8000 Hz-frequency induction crucible furnace. The spheroidizing treatment was accomplished with the tundish cover process using Iron-Silicon-Calcium-Magnesium (FeSiCaMg) alloy. Inoculation was performed using different treatments, including ladle, pouring basin and reaction chamber, with conventional inoculants in combination with S. FeS was added to Si-based inoculants by mechanically mixing, pressing for use as an insert inoculant or during manufacture.

Results

The experiments led to the following conclusions:

* the S action as FeS in Mg-treated cast iron has a low inoculant influence, and MgS has a low nucleation capacity;

* adding a SiCa+FeS mixture leads to an increased nucleation potential, evidenced by an increased nodule count (smaller nodule diameter) and a reduction in chill depth (Tables 1 and 2);

* adding cerium (Ce) with SiCa+FeSlowers the nodule count and increases the chill tendency of cast iron due to the link of S with excess Ca, Ce and Mg;

* in the case of S associated with FeSi, there is a lower inoculation effect when compared with FeSi+bismuth (Bi), due to the reduced potential to make stable compounds;

* inoculation with Bi leads to a similar graphitizing capability as SiCa+FeS, but has a lesser effect on chill control;

* S inoculation of ductile iron (up to 0.01%) can increase the efficiency of SiCa inoculant, because it does not become an alloying contaminant of the charge material (compared to Bi);

* S inoculation by different procedures can improve the graphite nucleation capacity when added to the ladle or as a late addition to either the pouring basin or within the mold.

THERMAL ANALYSIS

In his paper titled "Optimizing Inoculation Practice by Means of Thermal Analysis," Rudolf V. Sillen, Nova Cast AB, discussed how thermal analysis can be used to test the efficiency of various inoculants.

Methodology

Sillen recommended the following testing method:

1. Take samples of base iron approximately every hour during at least one shift. Use a small ladle made of special refractory material or steel covered with shell sand. In addition, use test cups that do not contain tellurium (Te), which causes C to precipitate as iron carbide ([Fe.sub.3]C) instead of the desired graphite. Take samples at the same interval after inoculation to minimize variations due to fading.

2. Test the inoculated iron, and plot your results as a cooling curve, recording eutectic temperatures [ILLUSTRATION FOR FIGURE 1 OMITTED]). Because the inoculation acts as a deoxidizer, the liquidus temperature of an iron that contains a high amount of O can be reduced 14.4-18F (8-10C). Inoculation also raises both the maximum and minimum eutectic temperatures. Other parts of the cooling curve that respond to inoculation are the start of eutectic freezing, maximum recalescence rate (the difference between eutectic extremes) and the first derivative at solidus.

Make a series of tests, varying factors such as amount of inoculant, grain size and method of addition. Both maximum and minimum eutectic temperatures should be as high as possible, and the maximum recalescence rate should be reduced.

3. Evaluate the results, and test the preferred method on your castings. Categorize the castings based on their significant modulus.

Twin Testing

Compare the efficiency of inoculants by pouring two cups at the same time. The first cup should contain no additives, and the second cup should contain 4, 8 or 12 grams of inoculant, corresponding to 0.1, 0.2 and 0.3%. Compare the cooling curves for the two cups.

When increasing inoculation, the low eutectic temperature increases along with the amount of nucleation sites and eutectic solidification can occur at lower undercooling. The increase gradually falls or remains constant when more inoculant is added [ILLUSTRATION FOR FIGURE 2 OMITTED].

If the inoculant is fine meshed, it can be added directly to the test cup, but if the crashing is coarse, it must be added to a ladle before pouring.

Dynamic Inoculation

Tests in more than 20 foundries show that base iron is not constant over a day, even though the chemistry is constant. If a foundry is unaware of this and continues to add inoculant at a consistent rate, some ladles will be underinoculated while others are overinoculated.

The twin cup testing method can determine the properties of a base iron, and the inoculant can be varied accordingly. Add 0.2% of a good stream inoculant to cup two. Pour both cups with the base iron at regular intervals to get a quantitative measure of the nucleation status. Determine the value of the gray eutectic temperature (1153+6.7 Si%) and subtract the low eutectic temperature of uninoculated iron. This value should fall between 68-95F (20-35C) and, if higher, may indicate the need for inoculation. Divide this value by the value of the gray eutectic temperature minus the low eutectic temperature of the inoculated iron. If this total is 1 or lower, the iron has been underinoculated, and if it is 2.5 or higher, the iron has been overinoculated.

UNDERSTANDING NUCLEATION

Although it is 50 years since the invention of ductile iron, the understanding of its graphite nucleation mechanisms remains incomplete, according to Richard Harding, John Campbell and Nigel Saunders, Univ. of Birmingham, Edgbaston, Birmingham, England. The three authors discussed the influence of the environment on a dissolving inoculant particle and its nuclei in their paper titled "An Assessment of Our Current Understanding of the Inoculation of Ductile Iron." Trace elements can be vital to controlling the structure, as illustrated by the role of 0.03-0.05% Mg in changing the graphite morphology of iron from flake to spheroidal graphite. However, due to the lack of research, most of the other factors affecting graphite nucleation and growth mechanisms remain poorly understood.

The first goal of ductile iron production is to promote the formation of spheroidal graphite during solidification. Although various elements such as Mg, Ce and Ca are suitable nodulizers, only Mg is used industrially, usually added as part of an alloy.

The Mg deoxidizes and desulfurizes the melt. When impurities such as S and O are present, they poison the graphite deposition sites on the growth front and allow less symmetrical forms of graphite to form. The role of Mg is to gather the impurities in the molten iron, enabling the preferred spheroidal growth mode to occur.

The second production goal is to promote graphitic rather than carbidic solidification. Although various materials are used to inoculate flake graphite irons, ductile iron is almost exclusively inoculated with retro-silicon (FeSi) alloys containing 65-75% Si. Pure FeSi is not effective, and it must contain a small amount of trace elements, including aluminum, barium, Bi, strontium (St) and rare earths.

Through inoculation, the graphite nucleates heterogeneously on the foreign particles. Several theories have been proposed to explain the nucleation, including the Mg Bubble, Salt-like Carbides and Silicon Carbide theories, with the overriding conclusion that a wide range of particles have been found to act as nuclei. This implies that graphite nucleation may be easier than previously thought and that the environment within which the nuclei operate may be more important than the nature of the nuclei.

Simulation of Nucleation

The melting of a FeSi inoculant produces a region that is initially free from C. However, C will diffuse into the center of these liquid regions within 1 sec. The rate at which Si is lost by diffusion is expected to yield a life for such regions that can be measured in minutes. By the time the iron has solidified, the Si-rich region may or may not have disappeared. It is possible that incomplete homogenization of the melt on a less severe scale (resulting in a heterogeneous distribution of Si on a scale which might be difficult to detect) may be an essential feature of successful inoculation.

As the melted inoculant particles dissolve, regions of roughly concentric rings of graded composition will form around them. Computer modeling can be used to assess the changes in phase equilibrium that occur when the inoculant dissolves. Computer simulation was used to construct a vertical section between a simple ductile iron cast part and a FeSi inoculant. As the inoculant dissolves, its composition traverses this vertical section and increases the stability of graphite and silicon carbides (SIC). Therefore, if dissolution of the inoculant is not complete, then the regions with high Si will become prone to the formation of SiC, graphite and FeSi intermetallics.

Furthermore, the formation of graphite will occur above the eutectic temperature with a high degree of supercooling for graphite nucleation. Graphite is likely to precipitate as a primary phase, even if the iron has a hypoeutectic composition. The presence of potentially high undercooling would mean that precipitation will occur on many different types of substrates. In contrast, in most other systems, nucleation is found to be very specific, requiring similar crystal structures. low disregistry and similar electronic compatibility, so that good "wetability" of the solid on the substrate can be achieved.

THIN-WALL INOCULATION

With the emphasis of the automotive industry on reducing the weight of casting components, a shift has begun in the U.S. to aluminum, cast parts, according to Doru M. Stefanescu, Univ. of Alabama-Tuscaloosa. He said that, despite this shift, European automakers have made significant progress in replacing aluminum parts with cast iron parts. For iron to regain its lost market it must be engineered to transpose its potential resources, he said, stressing that new technologies able to produce high-quality, thin-wall castings (less than 3 mm wall thickness) must be developed. In his paper titled "Inoculation of Thin Wall Castings," Stefanescu discussed the importance of microstructure control in thin-wall ductile iron castings.

The main effects of decreased section size on microstructure include: higher tendency for carbide formation and an increased propensity for the gray-to-white structural transition (GWT), and graphite morphology transitions such as A-to-D for gray iron and compacted to spheroidal for compacted graphite (CG) iron. Both of these are influenced by a large number of variables including metal properties, mold properties, casting design and inoculation.

Inoculation Particularities

Avoiding carbide formation in thin gray iron castings isn't difficult as long as the base chemistry and inoculation practices are carefully selected. Attention must be paid to the Mn/S ratio influence on the nucleation potential. As shown in Fig. 3, an optimum Mn/S ratio of 20 must be used to minimize chill formation. As the Mn/S ratio decreases, more sulfides are available for nucleation. If the Mn/S ratio becomes too small, the excess free O inhibits eutectic grain growth and the chilling tendency increases.

Thin ductile iron sections can be produced carbide-free without special inoculant. AS shown in Fig. 4, if the addition of Fe75%Si as post-inoculant is increased to 1.25%, carbide-free 3 mm plates can be cast. However, at this amount of post-inoculant, dirty iron is produced, diminishing mechanical properties. Thus, more sophisticated composition was developed for inoculant. In a Ductile Iron Society study, a Sr-containing FeSi outperformed all other commercial inoculant tested when avoidance of carbides in the 3.18-mm section was set as the criterion. In research performed in France, carbide-free 3-mm thin sections were best cast with post-inoculant of FeSiBi and FeSiSr. However, in all these tests, it was emphasized that inoculation can't solve the problem without an appropriate base melt chemistry and residual Mg.

In terms of casting CG iron, the problem is to avoid CG-SG transition that chill promotes. In principle, the chemistry of the base metal and of the treatment alloys can be adjusted to match a specific section size, but producing CG iron in thin and thick section castings is a problem. An alternative is the Fe-C-Al type of CG iron. Treating an iron having 2.68-4.48% Al and less than 1% Si with 1.4% FeSi-5%Mg resulted in carbide-free CG structure in pins having diameters as small as 3.2 mm without post inoculation.