Cast iron is an iron-carbon alloy with a carbon content greater than 2.11% (generally 2.5-4%). It is a multi-component alloy with iron, carbon, and silicon as the main components and contains 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 alloying elements can be added to obtain alloy cast iron.
Classification of cast iron:
I. According to the different forms of carbon in cast iron, cast iron can be divided into
1. White cast iron carbon, except for a small amount dissolved in ferrite, the rest of the carbon exists in the form of cementite in cast iron, and its fracture is silvery white, so it is called white cast iron. At present, white cast iron is mainly used as steelmaking raw material and blank for producing malleable cast iron.
2. Gray cast iron carbon exists in cast iron in the form of flake graphite in all or most of the cast iron, and its fracture is dark gray, so it is called gray cast iron.
3. Part of the carbon in pitted cast iron 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 pitting in the fracture, so it is called pitted cast iron. This type of cast iron is also very hard and brittle, so it is rarely used in industry.
According to the different forms of graphite in cast iron, cast iron can be divided into
1. Gray cast iron The graphite in cast iron exists in flakes.
2. Malleable cast iron The graphite in cast iron exists in clusters. 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 spherical form. 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, and its production process is simpler than malleable cast iron, but also can be further improved through heat treatment. Therefore, its application in production is becoming more and more extensive.
Uses of various cast irons
White cast iron
The carbon in white cast iron exists entirely in the form of infiltrated carbon (Fe3c), so the fracture is bright white. Therefore, it is called white cast iron. Due to the presence of a large amount 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 and are not subject to impact, such as wire drawing dies, ball mill iron balls, etc. Most of them are used as billets 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, good cutting processability, anti-friction, good wear resistance, and its melting ingredients are simple, low cost, and widely used in the manufacture of complex castings and wear-resistant parts. Gray cast iron is divided into three categories according to the matrix structure: ferrite-based gray cast iron, pearlite-ferrite-based gray cast iron and pearlite-based gray cast iron. Because there is flake graphite in gray cast iron, and graphite is a component with low density, low strength, low hardness, and plasticity and toughness approaching zero. Its existence is like a large number of small gaps in the steel matrix, which reduces the bearing area and increases the crack source. Therefore, gray cast iron has low strength and poor toughness and cannot be processed by pressure. In order to improve its performance, a certain amount of ferrosilicon, silicon calcium and other inoculants are added to the molten iron before pouring to refine the pearlite matrix.
Malleable cast iron
Mallet cast iron is made of iron-carbon alloy with low carbon and silicon content into white cast iron blanks, and then subjected to long-term high-temperature annealing treatment to decompose cementite into flocculent graphite. Forgeable iron is a graphitized white cast iron. Malleable cast iron is divided into two categories according to the different microstructures after heat treatment; one is blackheart malleable cast iron and pearlite malleable cast iron. The structure of blackheart malleable cast iron is mainly ferrite (F) matrix + flocculent graphite; the structure of pearlite malleable cast iron is mainly pearlite (P) matrix + flocculent graphite. The other is whiteheart malleable cast iron. The structure of whiteheart 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 annealing carbon. The graphite becomes fine and evenly distributed. The cast iron that has undergone this inoculation treatment is called inoculated cast iron.
Ductile cast iron
Before pouring molten iron (ductile iron), a certain amount of spheroidizing agent (commonly used are ferrosilicon, magnesium, etc.) is added to spheroidize the graphite in the cast iron. Since carbon (graphite) exists in the cast iron matrix in spherical form, its cutting effect on the matrix is improved, and the tensile strength, yield strength, plasticity and impact toughness of ductile iron are greatly improved. It also 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 crankshafts, connecting rods, rollers, automobile rear axles, etc.
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