Concept and classification of cast iron 

Cast iron is an iron carbon alloy with a carbon content of more than 2.11% (generally 2.5-4%). It is a multicomponent alloy with iron, carbon and silicon as the main elements and more manganese, sulfur, phosphorus and other impurities than carbon steel. Sometimes, in order to improve the mechanical properties or physical and chemical properties of cast iron, a certain amount of alloying elements can be added to obtain alloyed cast iron.
As early as the spring and Autumn Period in the sixth century BC, China began to use cast iron, nearly 2000 years earlier than European countries. Up to now, cast iron is still one of the most important materials in industrial production. 

Classification of cast iron 

1、 According to the different forms of carbon in cast iron, cast iron can be divided into three types
1. Except a few of the carbon in white cast iron is dissolved in ferrite, the rest of the carbon exists in cast iron in the form of cementite, and its fracture is silvery white, so it is called white cast iron. At present, white cast iron is mainly used as raw material for steelmaking and blank for producing malleable cast iron.
2. All or most of the carbon in gray cast iron exists in the form of flake graphite, and its fracture is dark gray, so it is called gray cast iron.
3. A part of carbon in pockmarked cast iron exists in the form of graphite, similar to that in gray cast iron; The other part is in the form of free cementite, similar to white cast iron. There are black and white pitting in the fracture, so it is called pockmarked cast iron. This kind of cast iron is also hard and brittle, so it is rarely used in industry. 

2、 According to the different forms of graphite in cast iron, cast iron can be divided into three types
1. Graphite in gray cast iron exists as flake.
2. The graphite in malleable cast iron is flocculent. It is obtained from white cast iron with certain composition after high temperature and long time annealing. Its mechanical properties (especially toughness and plasticity) are higher than those of gray cast iron, so it is traditionally called malleable cast iron.
3. The graphite in nodular cast iron is spherical. It is obtained by spheroidizing before hot metal casting. This kind of cast iron not only has higher mechanical properties than gray cast iron and malleable cast iron, but also has simpler production process than malleable cast iron, and can further improve its mechanical properties through heat treatment, so it is widely used in production.

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