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Engineering Materials - Ferrous Metals - Cast Iron


Ferrous Metals

Cast Iron

Cast iron is defined as an iron alloy with more than 2% carbon as the main alloying element. In addition to carbon, cast irons must also contain from 1 to 3% silicon which combined with the carbon give them excellent castability. Cast iron has a much lower melting temperature than steel and is more fluid and less reactive with molding materials. However, they do not have enough ductility to be rolled or forged.

The precipitation of carbon (as graphite) during solidification is the key to cast iron's distinctive properties. The graphite provides excellent machinability (even at wear-resisting hardness levels), damps vibration, and aids lubrication on wearing surfaces (even under borderline lubrication conditions).

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.

Range of Compositions for Typical Unalloyed Cast Irons
(Values in 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)

2.0 - 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

 




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