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Ductile iron dilemma: primary carbides.

Ductile cast iron is particularly prone to formation of primary carbides during solidification. A primary reason for this is that the graphite forms into a spherical shape, which is the lowest surface area-to-volume ratio for the graphite. The limited surface area available for graphite precipitation during solidification increases the carbide forming tendency. In addition, the principal element added for nodulzing treatment is magnesium, a known carbide stabilizer.

Another factor is that the sulfur content in ductile iron is purposely lowered to less than 0.02% to facilitate the formation of spherical graphite nodules. Therefore, inoculation is crucial to successfully cast ductile iron without carbides. Even after effective inoculation, fade can occur and result in the formation of primary carbides. Figures 1, 2 and 3 illustrate primary carbides in a ferritic ductile iron. Figures 4, 5 and 6 show primary carbides in a pearlitic ductile iron.

These carbides have several names including: ledeburite, chill, primary carbide, carbides, |Fe.sub.3~C, iron carbide and cementite.
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Author:Goodrich, George M.
Publication:Modern Casting
Date:May 1, 1993
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