Explaining the peculiar: cast iron anomalies and their causes.

Examining numerous cases of cast iron 'anomalies,' this AFS Cast Iron Div. project uncovers the causes of these odd microstructural conditions.

Millions of tons of quality cast iron parts are produced by U.S. foundries each year. Nevertheless, on occasion, foundrymen can be perplexed upon encountering unusual microstructures not usually associated with good foundry practice.

When these peculiarities occur, examining the casting microstructure can reveal important clues. As such, foundries can identify the problem and develop a solution for the microstructure anomaly.

For nine years, the AFS Cast Iron Quality Control Committee (5-J) has collected examples of microstructures that are considered anomalies. The purpose of the report, which was presented at the 1997 AFS Casting Congress, is to share these oddities and their causes so other foundries can benefit.

In the complete report, 56 micro-structure anomalies were divided into three basic categories: anomalies associated with solidification; anomalies associated with cooling after solidification and heat treatment; and the catch-all category, "other" anomalies. Following are examples gleaned from the report. (Note: all photos were reduced to 75% for publication.)

SOLIDIFICATION ANOMALIES

Faulty Inoculation

Foundrymen rely on two key processing steps - nodularization and inoculation - to achieve the desired microstructure of ductile iron. If these processes fail, several other types of graphite may develop, causing structural imperfections.

The first step introduces a nodularizing agent [such as magnesium (Mg)] that creates the condition for the graphite to precipitate and grow in a nodular shape. If insufficient Mg is added, or if the molten metal is held for an extended period after the Mg has been added, the graphite will not precipitate in a round shape.

Figure 1 shows unacceptable graphite nodularity that was identified in the cover plate for a floor-level utility box in a major convention center. The designated material for this cover was ASTM A536-80, Grade 65-45-12 ductile cast iron.

The second critical processing step is the addition of inoculant. The inoculant is usually a ferrosilicon (FeSi) that contains small amounts of calcium (Ca) and/or aluminum (Al) or other special-purpose elements. The principal purpose of the inoculant is to prevent chill. More specifically, the inoculant enhances graphite nucleation, preventing the formation of primary carbides.

Using the cover from the floor-level utility boxes as an example, Fig. 2 illustrates the presence of primary carbides in a ferritic structure and in structures that contain both ferrite and pearlite. The utility box cover failed immediately after installation, due to the movement of heavy equipment across it. The ductile iron covers that met the A536-80 requirements for 65-45-12 grade ductile iron performed acceptably without failure.

The presence of the degenerate graphite illustrated in Fig. 1 will impair a part's mechanical properties. The presence of primary carbides in the structure also can reduce mechanical properties. In both instances, the ductility, as measured by the percent elongation, is dramatically reduced. The observed structures can be the consequence of fade.

Fade occurs when the effects of Mg treatment and inoculation decrease with time. If the molten metal is held for an extended period after Mg treatment and inoculation, both degenerate graphite and primary carbides can occur in the structure. Another possible cause for the observed structures could be that the high sulfur (S) base iron was contaminated with deleterious trace elements.

A third kind of ductile iron anomaly also is found in Fig. 2. In some instances, the graphite structure at the surface of ductile iron castings is flake graphite, more commonly associated with gray iron. Flake graphite structures at the surface can occur in ductile iron as the consequence of surface reactions with contaminants in the sand, usually S.

This structure can become even more pronounced, depending upon the Mg content in the iron vs. the contaminant level in the sand. High contaminants and/or low Mg will produce relatively more flakes.

Primary Carbides and Steadite in Gray Iron

Two microstructure constituents in gray iron can cause hard spots - a condition that aggravates machinists. These two constituents are iron carbides and steadite (iron phosphides). Figure 3 shows a typical example of iron carbides while Fig. 4 shows a typical example of steadite.

Iron carbide and steadite are eutectic phases between iron (Fe) and carbon (C), and Fe and phosphorus (P), respectively. Because they are eutectic, they are the last to solidify. The solidification temperature for iron carbide is 2066F (1130C). For steadite, the solidification temperature is 1920F (1049C).

When these two eutectics combine, a tertiary Fe-C-P eutectic, with a still lower melting point, will occur in the micro-structure. An example of this combined eutectic structure is shown in Fig. 5. Although the melting point for this constituent is not published, it is believed to be lower than the melting points for the individual eutectics.

Since these eutectics are the last to solidify, they can be present in the cast iron structure as liquid surrounded by solid and can be drawn from thin sections to feed thick sections. The consequence can be microscopic shrinkage voids in thin sections. These voids have a shape that is similar to the carbide and steadite constituents that would be found in the structure. Figure 6 shows an example of the microscopic voids that form from drawing the liquid eutectic phases from thin sections.

As with ductile iron, inoculation in gray iron is primarily used to control the occurrence of primary carbides. Ladle, mold or late-stream inoculation (or combinations of the various techniques) have all proven effective. Control of tramp elements that are known carbide stabilizers, such as chromium (Cr), vanadium (V) and molybdenum (Mo), and other less common elements in gray iron such as antimony (Sb), tellurium (Te) and hydrogen (H), also are essential.

POST-SOLIDIFICATION ANOMALIES

Widmanstatten Graphite

Widmanstatten graphite can occur in cast iron as the result of lead (Pb) contamination, among other elements. Pb levels as low as 0.005% have been known to create the Widmanstatten graphite. Widmanstatten graphite occurs after solidification with the precipitation of C atoms on crystallographic planes, creating a spiky appearance to existing graphite flakes. If the condition becomes significant, the precipitation onto crystallographic planes can occur aside from the primary graphite flakes, creating hatch marks in the structure. Figure 7 shows Widmanstatten graphite in the unetched structure.

Research has shown that this graphite type can be controlled with the addition of rare earth elements, primarily cerium (Ce). As a consequence, the condition does not often occur in ductile iron because of the presence of rare earth elements in the treatment alloy. If the condition occurs in gray iron, it can be controlled by eliminating the Pb. A Ce-bearing inoculant also can reduce the effect.

The presence of this graphite form greatly reduces the mechanical properties of the resulting iron. For example, a normal Class 30 gray iron with a Pb concentration of 0.05% without the benefit of Ce or other rare earths can actually have a tensile strength of less than 15,000 psi as a result of the presence of the Widmanstatten graphite. This graphite form will become Type F in the soon-to-be-published revised ASTM specification A247.

OTHER ANOMALIES

Lustrous Carbon Defects in Cast Iron

Lustrous carbon defects generally appear on the surface or just under the surface formed by the cope mold or top of the core. This defect often appears as adherent, shiny, "wrinkled" deposits of C, and also is known as resin, kish or a soot defect. It can be found on castings made in urethane-bonded sands, and in shell, lost foam or green sand molds. Lustrous carbon is caused by high levels of volatile gases trapped at the mold or core surface. The volatile gases are released as the organic binders (especially urethane-based cold-set and shell mold systems) break down during the pouring process, releasing the hydrocarbons.

In lesser amounts, the carbonaceous material provides a reducing atmosphere in the mold, which minimizes casting surface oxidation and improves casting surface quality or peel. It is often removed from the casting surface by casting cleaning operations. As the level of these volatile gases increases, the severity of the defect increases and the lustrous carbon folds into solidifying metal causing unacceptable cold shuts and laps.

Figure 8 shows a typical example of a lustrous carbon defect. This figure illustrates the depth of the discontinuities and their microstructural differences. It also illustrates a lap defect resulting from folding the graphite layer into the metal.

The frequency and severity of the defect can be reduced and controlled by the following means:

* lowering the binder content, especially the isocyanate component in urethane bonded systems;

* increasing the mold/core mechanical venting and permeability;

* increasing the pouring temperature;

* reducing the fill time and pouring turbulence;

* applying a low-carbon coating to the core/mold coating;

* adding 0.5%-1.0% oxidizing materials, such as iron oxide, to the core sand.

Nitrogen in Gray Iron

The nitrogen (N) level in gray iron normally has an equilibrium of 70 ppm. Occasionally, however, high N can occur. When the dissolved N increases, the graphite is affected, producing "fat" graphite, as shown in Fig. 9. This type of graphite is generated when N exceeds 150 ppm. N is generally controlled with the use of titanium (Ti). Ordinarily, at high N content, if Ti is present, the graphite structure will be normal [ILLUSTRATION FOR FIGURE 10 OMITTED].

This article was excerpted from a much larger 1997 AFS Transaction Paper (97-30). A copy of the complete report is available through the AFS Library at 800/537-4237.