I. Cast iron can be classified according to the different forms of carbon in it:

1. White cast iron

Except for a small amount of carbon dissolved in ferrite, the rest of the carbon exists in the form of cementite in cast iron. Its fracture is silvery white, hence the name white cast iron. At present, white cast iron is mainly used as raw material for steelmaking and blanks for producing malleable cast iron.

2. Gray cast iron

All or most of the carbon exists in the form of flake graphite in cast iron, and its fracture is dark gray, hence the name gray cast iron.

3. Mottled cast iron

Part of the carbon exists in the form of graphite, similar to gray cast iron; another part exists in the form of free cementite, similar to white cast iron. The fracture shows mottled spots of black and white, hence the name mottled cast iron. This type of cast iron also has high hardness and brittleness, so it is rarely used in industry.

II. According to the different forms of graphite in cast iron, cast iron can be classified as:

1. Gray cast iron

The graphite in cast iron exists in flake form.

2. Malleable cast iron

The graphite in cast iron exists in a flocculent form. It is obtained by annealing white cast iron of a certain composition at high temperature for a long time. Its mechanical properties (especially toughness and plasticity) are higher than those of gray cast iron, so it is usually called malleable cast iron.

3. Ductile cast iron

The graphite in cast iron exists in a spherical form. It is obtained after spheroidization treatment before the molten iron is poured. This type of cast iron not only has higher mechanical properties than gray cast iron and malleable cast iron, but also has a simpler production process than malleable cast iron, and its mechanical properties can be further improved through heat treatment, so its application in production is becoming increasingly widespread.

Uses of various cast irons

White cast iron

The carbon in white cast iron exists entirely in the form of cementite (Fe3c), and the fracture is bright white. Therefore, it is called white cast iron. Due to the large amount of hard and brittle Fe3c, white cast iron has high hardness and brittleness and is difficult to process. Therefore, it is rarely used directly in industrial applications, only for a few parts that require wear resistance without impact, such as wire drawing dies and ball mill balls. Most of it is used as raw material for steelmaking and malleable cast iron.

Gray cast iron

Most or all of the carbon in cast iron exists in the form of free flake graphite. The fracture is gray. It has good casting performance, machinability, wear resistance, and wear resistance. In addition, its melting and mixing are simple and the cost is low. It is widely used in manufacturing complex castings and wear-resistant parts.

According to the different matrix structures, gray cast iron is divided into three types: ferritic gray cast iron, pearlitic-ferritic gray cast iron, and pearlitic gray cast iron. Because flake graphite exists in gray cast iron, and graphite is a component with low density, low strength, low hardness, and zero plasticity and toughness. Its existence is like a large number of small notches on the steel matrix, which reduces the bearing area and increases the crack source, so gray cast iron has low strength and poor toughness and cannot be pressure processed. In order to improve its performance, a small amount of silicon iron, calcium silicon, etc., are added to the molten iron before pouring to refine the pearlitic matrix.

Malleable cast iron is made by casting a white cast iron blank with a lower carbon and silicon content, and then subjecting it to high-temperature annealing for a long time to decompose the cementite into flocculent graphite. Malleable iron is a type of white cast iron that has undergone graphitization treatment. Malleable cast iron is divided into two categories according to the microstructure after heat treatment: one is black heart malleable cast iron and pearlitic malleable cast iron. The microstructure of black heart malleable iron is mainly ferrite (F) base + flocculent graphite; the microstructure of pearlitic malleable cast iron is mainly pearlite (P) base flocculent graphite. The other type is white heart malleable cast iron. The microstructure of white heart malleable cast iron is determined by the cross-sectional size. The small cross-section is based on ferrite, and the surface area of the large cross-section is ferrite, and the core is pearlite and annealed carbon. The graphite becomes finer and more evenly distributed, and the cast iron treated in this way is called inoculated cast iron.

Ductile cast iron

A certain amount of spheroidizing agent (commonly used are silicon iron, magnesium, etc.) is added before the molten iron (ductile iron) is poured to spheroidize the graphite in the cast iron. Because carbon (graphite) exists in a spherical shape in the cast iron matrix, it improves its cutting effect on the matrix, and the tensile strength, yield strength, plasticity, and impact toughness of ductile cast iron are greatly improved. It also has the advantages of wear resistance, shock absorption, good process performance, and low cost, and has now widely replaced malleable cast iron and some cast steel and forged steel parts, such as crankshafts, connecting rods, rolls, and automobile rear axles.