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PURE, carbon-free iron is practically never used as a cast metal because it is soft and weak. As carbon content increases, so does hardness and strength. The beneficial effect of carbon, although creating production problems, is advantageous to about 0.9% carbon. These alloys are called STEELS. Additional problems are encountered with carbon contents to about 2.0%, and these "semi-steels" are seldom used. WHITE IRONS, containing 2 to about 3.3% carbon, can be used for highly abrasive service, but the application is limited due to the brittle nature of the alloy. It is a true cast iron, but most of the contained carbon is present as iron carbide Fe3C, a hard and brittle compound.

The carbon contents of white and MALLEABLE irons overlap. Indeed, malleable iron must solidify as white iron. (Hence, its production is limited to relatively thin

Ductile Iron X50

Gray Iron X50

White Iron X50

castings.) A lengthy heat treatment of the white iron castings decomposes the Fe3C into iron and nodules of graphite. In this condition, malleable iron exhibits strength/elongation combinations from 40,000 p.s.i. (280 mPa) with 18% elongation to 116,000 p.s.i. (800 mPa) with about 2% elongation.

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