Cast iron is an iron carbon alloy with a carbon content greater than 2.11% (usually 2.5-4%). It is a multi-element alloy mainly composed of iron, carbon, and silicon, and contains more impurities such as manganese, sulfur, and phosphorus than carbon steel. Sometimes, in order to improve the mechanical, physical, and chemical properties of cast iron, a certain amount of alloying elements can be added to obtain alloy cast iron.
Classification of cast iron
1、 According to the different forms of carbon in cast iron, cast iron can be divided into
1. White cast iron
Except for a small amount dissolved in ferrite, the remaining carbon exists in the form of cementite in cast iron, and its fracture surface is silver white, hence it is called white cast iron. At present, white cast iron is mainly used as a raw material for steelmaking and as a blank for producing malleable cast iron.
2. Grey cast iron
Carbon exists entirely or mostly in the form of flake graphite in cast iron, with a dark gray fracture surface, hence it is called gray cast iron.
3. Hemp cast iron
Part of the carbon exists in the form of graphite, similar to gray cast iron; The other part exists in the form of free carbides, similar to white cast iron. The black and white spots on the fracture surface are called ductile iron. This type of cast iron also has significant hardness and brittleness, so it is rarely used in industry.
2、 According to the different forms of graphite in cast iron, cast iron can be divided into
1. Grey cast iron
Graphite exists in the form of flakes in cast iron.
2. Malleable cast iron
Graphite in cast iron exists in a flocculent form. It is obtained by high-temperature and long-term annealing of white cast iron with a certain composition. Its mechanical properties (especially toughness and plasticity) are higher than gray cast iron, so it is commonly referred to as malleable cast iron.
3. Ductile iron
Graphite exists in spherical form in cast iron. It is obtained after spheroidization treatment before pouring molten iron. 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. Moreover, its mechanical properties can be further improved through heat treatment, so its application in production is becoming increasingly widespread.
The uses of various cast iron
White cast iron
All carbon in white cast iron exists in the form of infiltrated carbon (Fe3c) due to the bright white fracture surface. Therefore, it is called white cast iron. Due to the large amount of hard and brittle Fe3c, white cast iron has high hardness, brittleness, and is difficult to process. Therefore, it is rarely used directly in industrial applications and is only used for a few parts that require wear resistance without impact, such as wire drawing dies, ball mill iron balls, etc. Mostly used as billets for steelmaking and malleable cast iron.
Grey cast iron
Most or all of the carbon in cast iron exists as free graphite flakes. The fracture is gray in color. It has good casting performance, good machinability, wear reduction, and wear resistance. In addition, it has simple melting and batching, low cost, and is widely used in the manufacture of complex castings and wear-resistant parts.
Grey cast iron can be classified into three types based on its matrix structure: ferrite based grey cast iron, pearlite ferrite based grey cast iron, and pearlite based grey cast iron.
Due to the presence of flake graphite in gray cast iron, graphite is a component with low density, low strength, low hardness, and tending towards zero plasticity and toughness. Its existence is like having a large number of small gaps on the steel substrate, which reduces the bearing area and increases the source of cracks. Therefore, gray cast iron has low strength and poor toughness, and cannot be processed under pressure. To improve its performance, a certain amount of silicon iron, silicon calcium and other inoculants are added to the molten iron before pouring to refine the pearlite matrix.
malleable iron
Forged iron is a type of white cast iron made by casting iron carbon alloy with low carbon and silicon content into a blank, and then subjecting it to long-term high-temperature annealing treatment to decompose the carbide into flocculent graphite. It is a graphite treated white cast iron.
Malleable cast iron can be divided into two categories based on the microstructure after heat treatment; One type is black core malleable cast iron and pearl ductile cast iron. The microstructure of Blackheart malleable cast iron is mainly composed of ferrite (F) and flocculent graphite; The microstructure of pearlite malleable cast iron is mainly composed of pearlite (P) matrix and flocculent graphite. Another type is white heart malleable cast iron, whose microstructure is determined by the cross-sectional size. For small cross-sections, ferrite is used as the matrix, while for large cross-sections, the surface area is ferrite, and the core is pearlite and annealed carbon.
Graphite becomes finer and more evenly distributed, resulting in cast iron that has undergone this inoculation treatment. It is called nurturing cast iron.
Ductile iron
Add a certain amount of spheroidizing agent (commonly used include ferrosilicon, magnesium, etc.) to spheroidize graphite in cast iron (ductile iron) before pouring. Due to the spherical existence of carbon (graphite) in the cast iron matrix, its cutting effect on the matrix is improved, and the tensile strength, yield strength, plasticity, and impact toughness of ductile iron are greatly improved. And it has the advantages of wear resistance, shock absorption, good processability, and low cost. It has now widely replaced malleable cast iron and some cast steel and forged steel parts, such as crankshafts, connecting rods, rollers, and automotive rear axles.