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Faulty inoculation causes microstructure anomalies.

Foundrymen depend on two key processing steps to achieve the desired microstructure of ductile cast iron. If the process fails, a number of other types of graphite may develop, causing structural imperfections.

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

Figures 1 and 2 show 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 to add an inoculant. The inoculant is usually a ferrosilicon that contains small amounts of calcium and/or aluminum and/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 covers from the floor-level utility boxes as examples, Figs. 3, 4 and 5 illustrate the presence of primary carbides in a ferritic structure and in structures that contain both ferrite and pearlite.

These utility box covers failed immediately after installation due to the movement of heavy equipment across them. The ductile iron covers that met the A536-80 requirements for 65-45-12 ductile iron grade performed acceptably without failure.

The presence of degenerate graphite illustrated in Figs. 1 and 2 impairs 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 most dramatically reduced. The observed structures can be the consequence of "fade."

Fade occurs when the effects of magnesium treatment and inoculation decrease with time. If the molten metal is held for an extended period after magnesium treatment and inoculation, both degenerate graphite and primary carbides can occur in the structure. Another possibility for the observed structures could be due to high sulfur-base iron contaminated with deleterious trace elements.

Figures 4 and 5 also reveal a third kind of ductile iron anomaly. 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 sulfur.

This structure can become even more pronounced depending upon the magnesium content in the iron versus the contaminant level in the sand.
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Author:Goodrich, George M.
Publication:Modern Casting
Date:Nov 1, 1992
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