Cast iron is a four-element alloy containing iron, carbon (between 2 and 4 percent), silicon and manganese. Sometimes additional elements are added to the alloy. The physical properties of an iron casting are strongly influenced by its cooling rate during solidification. This, in turn, depends on the size and shape of the casting and other details of the foundry practice. For this reason (and unlike other engineering materials), cast iron is usually specified by its mechanical properties rather than its chemical analysis.
The distinctive properties of cast iron derive largely from its carbon content.
- The high carbon content makes molten iron very fluid, so that it can be poured into even very intricate shapes;
- Carbon precipitation during solidification counteracts normal shrinkage, producing precise sections;
- The presence of graphite in the metal provides excellent workability (even at wear-resistant hardness levels), dampens vibrations and helps contour lubrication on wear surfaces.
When “chilled”, that is, when heat is quickly removed from the surface during solidification, all carbon near the surface remains combined in the form of iron carbides, resulting in an extremely hard and wear-resistant surface.
Grey Iron. The appearance of gray iron comes from carbon that is precipitated in the form of graphite flakes. Even in softer gradations, it has good wear resistance.
The increase in hardness (which offers even better wear resistance) can be achieved using special foundry techniques, heat treatment or additional elements in the alloy.
Since graphite flakes significantly weaken the cast iron in tension, the compression force is three to five times higher. This force differential is often taken advantage of, with the incorporation of ribs on the compression side of a bending loaded member.
Typical applications of gray iron include gasoline and diesel engine blocks, machine bases and frames, gears, flywheels, and brake disks and drums.
Ductile (Nodular) Iron. Ductile iron is an alloy with magnesium, which causes
excess carbon precipitation in the form of small spheres or nodules. These nodules interrupt the structure less than the graphite flakes of gray iron, thus giving substantial ductility along with better tensile strength, rigidity and impact resistance.
Ductile iron is specified by three numbers, such as 60-40-18, which denote tensile strength (60 ksi), yield strength (40 ksi) and elongation (18 percent).
Typical applications include engine crankshafts, heavy-duty gears, and hardware items such as automobile door hinges.
White Iron. White iron (so-called due to the white appearance of fracture surfaces) is produced in outer portions of gray and ductile iron castings, refrigerating selected mold surfaces, thus preventing carbon precipitation. The resulting structure is extremely hard, wear-resistant and brittle.
Typical applications are found in ball mills, extrusion dies, cement mixer liners, railroad brake shoes, rolling mill rolls, crushers, and pulverizers.
Malleable iron. Typical uses are for heavy parts with load-bearing surfaces,
which are needed in trucks, railway equipment, construction machinery and agricultural machinery.