Gray iron

Gray iron or grey iron was the original "cast iron". It is an alloy of carbon, silicon, and iron, containing from 1.7 to 4.5% C and 1 to 3% Si. It is relatively easy and inexpensive to make. Compared to the more modern engineered irons, gray iron has a lower tensile strength and lower ductility. In other words, it will fail more easily, and its mode of failure will be sudden fracture (it will not bend). It is used for housings where tensile strength is non-critical, such as engine blocks, pump housings, valve bodies, electrical boxes and decorative castings.

In grey cast iron, a large part or all of the carbon is in the form of flakes or nodules of graphites. The presence of graphite flakes gives capacity to damp vibrations caused by internal friction and, consequently, the ability to dissipate energy. Graphitic cast iron has a dark gray or almost black fracture. Upon small degree of super cooling, graphite is formed when the cast iron solidifies from its liquid state. Slow cooling promotes graphitisation. Rapid cooling partly or completely suppresses graphitisation and leads to formation of cementite. Rapidly cooled gray iron in which the carbon does not form graphite flakes, but rather combines with iron to form cementite is called white iron.