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Understanding Cast Irons

The term "cast iron" designates an entire family of metals with a wide variety of properties. It is a generic term like steel which also designates a family of metals. Steels and cast irons are both primarily iron with carbon (C) as the main alloying element. Steels contain less than 2% and usually less than 1% C, while all cast irons contain more than 2% C. About 2% is the maximum C content at which iron can solidify as a single phase alloy with all of the C in solution in austenite. Thus, the cast irons by definition solidify as heterogeneous alloys and always have more than one constituent in their microstructure.

In addition to C, cast irons also must contain appreciable silicon (Si), usually from 1–3%, and thus they are actually iron-carbon-silicon alloys. The high C content and the Si in cast irons make them excellent casting alloys. Their melting temperatures are appreciably lower than for steel. Molten iron is more fluid than molten steel and less reactive with molding materials. Formation of lower density graphite in the iron during solidification reduces the change in volume of the metal from liquid to solid and makes production of more complex castings possible. Cast irons, however, do not have sufficient ductility to be rolled or forged.

The various types of cast iron cannot be designated by chemical composition because of similarities between the types. Table 1 lists typical composition ranges for the most frequently determined elements in the five generic types of cast iron.

Table 1. Range of Compositions for Typical Unalloyed Cast Irons

Percent (%)

Type of
Iron

Carbon

Silicon

Manganese

Sulfur

Phosphorus

Gray

2.5-4.0

1.0-3.0

0.2-1.0

0.02-0.25

0.02-1.0

Ductile

3.0-4.0

1.8-2.8

0.1-1.0

0.01-0.03

0.01-0.1

Compacted Graphite

2.5-4.0

1.0-3.0

0.2-1.0

0.01-0.03

0.01-0.1

Malleable
(Cast White)

2. -2.9

0.9-1.9

0.15-1.2

0.02-0.2

0.02-0.2

White

1.8-3.6

0.5-1.9

0.25-0.8

0.06-0.2

0.06-0.2

There is a sixth classification for commercial purposes—the high-alloy irons. These have a wide range in base composition and also contain major quantities of other elements.

The presence of certain minor elements also is vital to the successful production of each type of iron. For example, nucleating agents, called inoculants, are used in the production of gray iron to control the graphite type and size. Trace amounts of bismuth and tellurium are used in the production of malleable iron, and the presence of a few hundredths of a percent magnesium (Mg) causes the formation of the spheroidal graphite in ductile iron.

In addition, the composition of an iron must be adjusted to suit particular castings. Small castings and large castings of the same grade of iron cannot be made from the same composition of metal. For this reason, most iron castings are purchased on the basis of mechanical properties rather than composition. The common exception is for castings that require special properties such as corrosion resistance or elevated temperature strength.

The various types of cast iron can be classified by their microstructure. This classification is based on the form and shape in which the major portion of the C occurs in the iron. This system provides for five basic types: gray iron, ductile iron, malleable iron, compacted graphite iron (CGI) and white iron. Each of these types may be moderately alloyed or heat treated without changing its basic classification. The high-alloy irons, generally containing over 3% of added alloy, also can be individually classified as gray or ductile iron or white, but the high-alloy irons are classified commercially as a separate group.

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