How many kinds of cast iron are there? What is the purpose?
Cast iron can be divided into:
1. White cast iron
Carbon exists in cast iron in the form of cementite except a few dissolved in iron. The fracture is silver white, so it is called white cast iron. At present, white cast iron is mainly used as raw materials for steelmaking to produce malleable iron billets.
All carbon in white cast iron exists in the form of permeating carbon (Fe3c), and the fracture of ball cast iron is bright white. Due to a large number of hard and brittle Fe3c, white cast iron is hard, brittle and difficult to machine. Therefore, it is rarely used directly in industrial applications, and only a few parts that require wear resistance without impact, such as wire drawing dies, iron balls for ball mills, etc. They are mostly used as billets for steelmaking and malleable cast iron.
2. Grey cast iron
All or most of the carbon layer exists in cast iron as sheet graphite, and its fracture is dark gray, so it is called gray iron.
Most or all of the carbon in cast iron exists as free flake graphite. It has good casting performance, good machinability, good wear reduction, good wear resistance, simple melting ingredients, low cost, and is widely used to manufacture castings and wear-resistant parts with complex structures.
3. Ma cast iron
The carbon part is graphite, similar to gray cast iron; Some are 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 also has a greater hardness and brittleness characteristics, 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. After high temperature annealing for a long time, white cast iron with some components can be obtained. Its mechanical properties (especially toughness and plasticity) are higher than that of gray cast iron, so it is customarily called 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 black center structure of malleable cast iron is mainly ferrite (F) matrix+flocculent graphite; Pearlitic malleable cast iron matrix+pearlitic 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.
3. Ductile iron
Graphite in cast iron is spherical. The product is obtained by spheroidizing the molten iron before pouring. This kind of cast iron not only has higher mechanical properties than gray 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 has been widely used in production.
A spheroidizing agent (usually ferrosilicon, magnesium, etc.) is added to the cast iron, and then poured into the cast iron to spheroidize it. Carbon (graphite) exists in the cast iron matrix as a ball, which improves its cutting effect on the matrix, and greatly improves its tensile strength, yield strength, plasticity and impact toughness. 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 for crankshaft, connecting rod, roll, automobile rear axle and other forged steel parts.