Cast irons are a large group of ferrous alloys which contain a large amount of carbon and solidify with a eutectic (chemical carbide which solidifies at a lower temperature). They are derived from pig iron and are mainly alloyed with carbon and silicon.
Their melting temperatures are appreciably lower than for steel. Molten iron is more fluid than molten steel and less reactive with molding materials making it excellent for casting. Often dismissed as a cheap, dirty, and brittle metal, cast iron is getting much more attention and use today because of its machinability, light weight, strength, wear resistance, and damping properties.
There are four basic types of cast irons: white iron, gray iron, ductile iron, and malleable iron.
- White iron contains a large percentage of eutectic carbides which increase hardness at the expense of toughness. The presence of different carbides, produced by alloying, makes white iron extremely hard and abrasion resistant but very brittle.
- Gray iron is characterized by a graphite microstructure which causes fractures of the material to have a grey appearance. It is the most commonly used cast iron. The flake-like shape of graphite in gray iron influences its mechanical properties. The graphite flakes can act as stress raisers which may prematurely cause localized plastic and initiate fracture in the matrix. As a result, gray iron exhibits no elastic behavior and fails in tension without significant plastic deformation but gives it excellent machinability, damping characteristics, and self-lubricating properties.
- Ductile iron, also called nodular cast iron, contains tiny amounts of magnesium or cerium added to these alloys to slow down the growth of graphite precipitates. Through timing and control, this allows the carbon to separate as spheroidal particles during solidification. The properties are similar to malleable iron, but parts can be cast with larger sections.
- Malleable iron is cast as a carbidic iron and annealing heat treatment is used to convert the carbide to graphite. Graphite exists in a more spherical form in malleable iron, giving it ductility and strength similar to cast, low-carbon steel. Carbide formation causes shrinkage of the iron, and thus the casting process requires a larger field than the desired output.
Specifications and Properties
Selecting metal alloys requires an analysis of the desired dimensions and specifications. Dimensions to consider include outer diameter (OD), inner diameter (ID), overall length, and overall thickness. Other specifications of importance (based on application) include product shape, tensile strength, yield strength, melting point, conductivity, corrosion resistance, ductility, and malleability. These properties differ based on the forming method and alloy composition.
Historical and Modern Applications
Cast irons historically have been used for building structures such as bridges, textile mills, and other buildings. Most common uses of cast irons today are in specialized industrial applications and in home appliances.