Cast iron, an alloy of iron (Fe) and carbon (C), has gained popularity in the market because of its low cost. It has the ability to build composite structures. The amount of carbon in cast iron is 2 - 4.5% of its weight. Besides iron and carbon, this alloy contains silicon (Si), small amounts of manganese (Mn), sulfur (S), and phosphorus (P). The products made of cast iron exhibit a reasonable amount of resistance to corrosion. It is neither malleable nor ductile, and it cannot be hardened like steel. It melts at temperatures of about 2100-2190ºF, and has a crystalline or a granular fracture. The mechanical properties of this alloy are very much dependent on the morphology of its carbon content.
Carbon is present in the form of plates in gray cast iron, whereas it is incorporated as a compound Fe3C (cementite) in white cast iron. Nodular cast iron which has a better tensile strength than gray cast iron contains carbon in the form of sphere-shaped granite particles.
Low Tensile Strength
Different varieties of cast iron are used in the construction of structures and machinery. Cast iron having a tensile strength of 5 tons per square inch or less, and is of no significant value for purposes where the alloy is required to possess strength. This type may, however, be used as balance weights, foundation blocks, or for purposes where weight alone is the important factor. Some varieties have a tensile strength as high as 19 tons per square inch. However, the average strength is 7 tons per square inch. Addition of vanadium can further increase the strength of cast iron.
High Compressive Strength
Compressive strength is defined as the ability of a material to withstand forces which attempt to squeeze or compress it. Cast iron bears a high compressive strength, which makes it desirable for use in columns and posts of buildings. The compressive strength of gray cast iron can be as high as that of some mild steels.
Low Melting Point
The temperature at which this alloy starts melting ranges from 2100 to 2190°F. Nevertheless, many advanced melting, alloying, and casting methods are being used to form cast iron, which is at par with steel.
Resistance to Deformation
Cast iron structures provide a rigid frame and thus, show resistance to deformation. This can be understood from the fact that when molten iron is poured into molds, a thinner part may separate from a thicker one, and breakdown further. At this point, the problem of the structure breakdown becomes prominent. The reason for this is when the thin part cools first and contracts, the thick part which cools down afterward causes stress in the thin part, offering sufficient force for it to break.
Resistance to Oxidation
Rust is formed by the reaction of iron and oxygen in the presence of water, air, or moisture. It leads to the corrosion of iron and its alloys. In addition to this, any iron masses in the presence of water and oxygen can eventually convert into rust, which is followed by disintegration. Cast iron offers resistance to oxidation, thus, attaining freedom from the problem of rust.
Cast iron has a wide range of applications including its usage in machinery, cookware, pipes, car parts such as cylinder heads, blocks, gearbox cases, etc.