Cast iron is an iron-carbon alloy with a carbon content above 2.11%. Industrial cast iron generally has a carbon content of 2.5% to 3.5%. In cast iron, carbon mostly exists in the form of graphite, and sometimes in the form of cementite. In addition to carbon, cast iron also contains 1% to 3% silicon, as well as elements such as manganese, phosphorus, and sulfur. Corrosion-resistant cast iron also contains elements such as nickel, chromium, molybdenum, aluminum, copper, boron, and vanadium. Carbon and silicon are the main elements affecting the microstructure and properties of cast iron. Based on the form of carbon present, cast iron can be classified into gray cast iron, white cast iron, and mottled cast iron.

                                                                                   Jimmy Tool End mill for Steel / Cast iron

Gray Cast Iron
Gray cast iron has a high carbon content (2.7% to 4.0%), with carbon mainly existing in the form of flake graphite. Its fracture surface is gray, hence the name "gray iron." It has a low melting point (1145–1250°C), minimal shrinkage during solidification, compressive strength and hardness close to carbon steel, and good vibration damping properties. Due to the presence of flake graphite, it exhibits good wear resistance. It also offers good casting and machining properties. It is used to manufacture structural components such as machine tool beds, cylinders, and housings. Its grade is denoted by "HT" followed by two sets of numbers. For example, HT20-40 (the first number indicates the minimum tensile strength, and the second number indicates the minimum flexural strength). Based on the shape characteristics of graphite, gray cast iron can be classified into ordinary gray cast iron, malleable cast iron, ductile cast iron, and compacted graphite iron.

1. Ordinary Gray Cast Iron
   The graphite in ordinary gray cast iron is flake-like, and it consists of two parts: graphite and the matrix. The matrix can be ferrite, pearlite, or a mixture of ferrite and pearlite, which is similar to the structure of steel.

2. Malleable Cast Iron
   The graphite in malleable cast iron is in a clustered form, obtained through the annealing of white cast iron. The graphite is distributed in clusters, hence the name "malleable iron." It has uniform organizational properties, good wear resistance, and excellent plasticity and toughness. It is used to manufacture complex-shaped parts that can withstand heavy dynamic loads.

3. Ductile Cast Iron
   The graphite in ductile cast iron is spherical. It is obtained by inoculating molten gray cast iron, causing the graphite to precipitate in a spherical form, hence the name "ductile iron." Carbon exists entirely or mostly as free-state spherical graphite, and the fracture surface is silvery-gray. It has higher strength, better toughness, and plasticity compared to ordinary gray cast iron. Its grade is denoted by "QT" followed by two sets of numbers. For example, QT45-5 (the first number indicates the minimum tensile strength, and the second number indicates the minimum elongation). It is used to manufacture internal combustion engines, automotive parts, and agricultural machinery.

4. Compacted Graphite Iron
   Compacted graphite iron is obtained by vermiculizing molten gray cast iron, causing the graphite to precipitate in a vermicular form. Its mechanical properties are similar to those of ductile cast iron, and its casting properties lie between those of gray cast iron and ductile cast iron. It is used to manufacture automotive components.

White Cast Iron
White cast iron has low carbon and silicon content, with carbon mainly existing in the form of cementite. Its fracture surface is bright white. It experiences significant shrinkage during solidification, making it prone to shrinkage cavities and cracks. It has high hardness and brittleness and cannot withstand impact loads. It is often used as a blank for malleable cast iron and to manufacture wear-resistant components.

Mottled Cast Iron
In mottled cast iron, carbon exists in a mixed form of graphite and cementite, and the fracture surface is gray-white. This type of cast iron is relatively brittle and is rarely used in industry.