Cast iron is an iron carbon alloy with a carbon content of more than 2.11% (generally 2.5~4%). It is a multi-component alloy with iron, carbon and silicon as the main components and more impurities such as manganese, sulfur and phosphorus than carbon steel. Sometimes, in order to improve the mechanical properties or physical and chemical properties of cast iron, a certain amount of alloy elements can be added to obtain alloy cast iron.
1、 Classification of cast iron
According to the different forms of carbon in cast iron, cast iron can be divided into:
1. White cast iron
Carbon exists in cast iron in the form of cementite except for a few dissolved in ferrite. Its fracture is silver white, so it is called white cast iron. At present, white cast iron is mainly used as raw material for steelmaking and as a blank for producing malleable cast iron.
2. Grey cast iron
All or most of carbon exists in cast iron as sheet graphite, and its fracture is dark gray, so it is called gray cast iron.
3. Ma cast iron
Part of carbon exists in the form of graphite, similar to gray cast iron; The other part exists in the form of free cementite, similar to white cast iron. There are black and white pits in the fracture surface, so it is called hemp cast iron. This kind of cast iron is also hard and brittle, so it is rarely used in industry.
According to different graphite forms in cast iron, cast iron can be divided into:
1. Grey cast iron
Graphite in cast iron exists in sheet form.
2. Malleable cast iron
Graphite in cast iron exists as flocculent. It is obtained from white cast iron of certain composition after high temperature annealing for a long time. Its mechanical properties (especially toughness and plasticity) are higher than that of gray cast iron, so it is customarily called malleable cast iron.
3. Ductile iron
Graphite in cast iron is spherical. It is obtained after spheroidizing treatment before molten iron pouring. 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 increasingly widely used in production.
2、 Classification and designation of cast iron
Cast iron is an iron carbon alloy containing more than 2.1% carbon. It is obtained by remelting cast pig iron (part of steelmaking pig iron) in the furnace and adding ferroalloy, scrap steel and recycled iron to adjust the composition. The difference between cast iron and pig iron is that cast iron is processed twice, and most of them are processed into iron castings. Iron castings have excellent castability, which can be made into complex parts, and generally have good machinability. In addition, it has good wear resistance and shock absorption, and low price.
Designation of cast iron: (according to GB5612-85)
Various cast iron codes are composed of the first capital letter of the Chinese phonetic alphabet representing the characteristics of the cast iron. When the code letters of two kinds of cast iron names are the same, the capital letters can be distinguished by adding the lowercase letters after the capital letters. For cast iron with the same name, if it needs to be subdivided, the first capital letter of Chinese Pinyin with its subdivision characteristics shall be taken and arranged at the back.
Cast iron name, code and designation
Alloy elements are represented by international element symbols. When the content is greater than or equal to 1%, it is represented by an integer. When the content is less than 1%, it is generally not marked. Conventional elements (C, Si, Mn, S, P) are generally not marked, and their element symbols and contents are only marked when they have special functions.
3、 Use of various cast iron
1. White cast iron
All carbon in white cast iron exists in the form of permeating carbon (Fe3c), because the fracture surface is bright white. Therefore, it is called white cast iron. Due to a large number of hard and brittle Fe3c, white cast iron has high hardness, high 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. They are mostly used as billets for steelmaking and malleable cast iron.
2. Grey cast iron
Most or all of the carbon in cast iron exists as sheet graphite in free state. The fracture is gray. It has good casting performance, good machinability, good wear resistance, good wear resistance, simple melting and batching, low cost, and is widely used to manufacture castings with complex structures and wear-resistant parts.
Grey cast iron can be divided into ferrite based grey cast iron, pearlite ferrite based grey cast iron and pearlite based grey cast iron according to different matrix structures.
Because there is flake graphite in gray cast iron, graphite is a component with low density, low strength, low hardness, and zero plasticity and toughness. Its existence is similar to the existence of a large number of small notches on the steel substrate, which reduces the bearing area and increases the crack source, so the gray cast iron has low strength and poor toughness, and cannot be pressure processed. In order to improve its properties, some inoculants such as ferrosilicon and calcium silicate are added into the molten iron before pouring to refine the pearlite matrix.
3. Malleable cast iron
Malleable iron is made of white cast iron blank cast from iron carbon alloy with low carbon and silicon content, and then after long-term high-temperature annealing treatment, the cementite is decomposed into clusters of flocculent graphite. That is to say, malleable iron is a kind of white cast iron after graphitization treatment.
Malleable cast iron can be divided into two types according to different microstructures after heat treatment; One is black core malleable iron and pearlescent malleable iron. The structure of black core malleable cast iron is mainly ferrite (F) basic+flocculent graphite; The structure of pearlitic malleable cast iron is mainly pearlitic (P) matrix+flocculent graphite. The other type is white core malleable cast iron. The structure of white core malleable cast iron depends on the section size. For small section, ferrite is the matrix, while for large section, the surface area is ferrite, and the center is pearlite and annealed carbon.
The graphite becomes fine and evenly distributed, and the cast iron after this inoculation treatment. It is called inoculation cast iron.
4. Ductile iron
A certain amount of spheroidizing agent (commonly used ferrosilicon, magnesium, etc.) is added before molten iron (nodular pig iron) is poured to spheroidize graphite in cast iron. The tensile strength, yield strength, plasticity and impact toughness of nodular cast iron are greatly improved because carbon (graphite) exists in the cast iron matrix in a spherical form, which improves its splitting effect on the matrix. It has the advantages of wear resistance, shock absorption, good process performance and low cost. It has now widely replaced malleable cast iron and some cast steel and forged steel parts, such as crankshaft, connecting rod, roll, automobile rear axle, etc.