Basic Metallurgy of Cast Iron

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The term 'cast iron', like the term 'steel', identifies a large family of ferrous alloys. Cast irons primarily are alloys of iron that contain more than 2% carbon and form from 1 to 3% silicon. Wide variations in properties can be achieved by varying the balance between carbon and silicon, by alloying with various metallic or nonmetallic elements, and by varying melting, casting and heat treating practices.

Cast irons, as the name implies, are intended to be cast to shape rather than formed in the solid state. Cast irons have low melting temperatures, are very fluid when molten, do not form undesirable surface films when poured, and undergo slight to moderate shrinkage during solidification and cooling. However, cast irons have relatively low impact resistance and ductility, which may limit their use.

Mechanical properties of cast irons - especially strength, ductility, and modulus of elasticity - depend strongly on structure and distribution of microstructural constituents. Physical properties such as thermal conductivity and damping capacity are also strongly influenced by microstructure.

The four basic types of cast iron are white iron, gray iron, ductile iron and malleable iron. White iron and gray iron derive their names from the appearances of their respective fracture surfaces: white iron exhibits a white, crystalline fracture surface, and gray iron exhibits a gray fracture surface with exceedingly tiny facets. Ductile iron derives its name from the fact that, in the as-cast form, it exhibits measurable ductility. By contrast, neither white nor gray iron exhibits significant ductility in a standard tensile test. Malleable iron is cast as white iron, then "malleablized" - that is, heat treated to impart ductility to an otherwise exceedingly brittle material.

Besides the four basic types, there are other specific forms of cast iron to which special names have been applied. Chilled iron is white iron that has been produced by cooling very rapidly through the solidification temperature range. An area of the casting that solidifies at a rate intermediate between those of chilled iron and gray iron, and which exhibits microstructural and fracture-surface features of both types, is known as mottled iron. Compacted graphite cast iron (also known as vermicular iron) has a structure intermediate between those of gray iron and ductile iron.


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