Cast iron is an iron-carbon alloy with a carbon content greater than 2.11% (generally 2.5 to 4%). It is a multi-component alloy with iron, carbon, and silicon as its main elements 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
一、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 of carbon dissolved in the ferrite body, the rest of the carbon exists in the form of cementite in the cast iron. Its fracture surface is silvery white, so 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 the production of malleable iron.
2. gray cast iron
All or most of the carbon exists in cast iron as flake graphite, and its fracture surface is dark gray, so 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 cementite, similar to white cast iron. There are black and white pits 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.
二. According to the different forms of graphite in cast iron, cast iron can be divided into
1. gray cast iron
Graphite is present in cast iron in flake form.
2. malleable cast iron
Graphite in cast iron exists in the form of floccules. It is obtained by annealing white cast iron with a certain composition at high temperature for a long time. Its mechanical properties (especially toughness and plasticity) are higher than gray cast iron, so it is customarily called malleable cast iron.
3． Ductile Iron
Graphite exists in spherical form in cast iron. It is obtained by spheroidizing the molten iron before pouring it. This type of cast iron not only has higher mechanical properties than gray cast iron and malleable cast iron, and the production process is simpler than malleable cast iron, but its mechanical properties can also be further improved through heat treatment, so it is increasingly used in production.
Cast iron classification and grade representation methods
Cast iron is an iron-carbon alloy with a carbon content greater than 2.1%. It is obtained by re-melting casting pig iron (part of steelmaking pig iron) in a furnace, and adding ferroalloy, scrap steel, and returned iron to adjust the composition. The difference from pig iron is that cast iron is processed twice, mostly into cast iron parts. Cast iron parts have excellent castability and can be made into complex parts, and generally have good machinability. In addition, it has the characteristics of good wear resistance and shock absorption, and low price.
Representation method of cast iron grade: (according to GB5612-85)
Various cast iron code names are composed of the first uppercase letter of the Chinese pinyin letters indicating the characteristics of the cast iron. When the code letters of two cast iron names are the same, they can be distinguished by adding lowercase regular letters after the uppercase letters. When cast iron with the same name needs to be subdivided, the first uppercase Chinese Pinyin letter of the subdivision characteristics shall be used and arranged at the back.
Various uses of cast iron
white cast iron
All the carbon in white cast iron exists in the form of penetrating carbon body (Fe3c), so the fracture surface is bright white. Therefore, it is called white cast iron. Due to the 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 in a few parts that require wear resistance without impact, such as wire drawing dies, ball mill iron balls, etc. Mostly used as blanks for steelmaking and malleable iron.
gray cast iron
Most or all of the carbon in cast iron exists as free state flake graphite. The fracture is gray. It has good casting performance, good cutting processability, wear reduction, and good wear resistance. In addition, it has simple melting ingredients and low cost. It is widely used in manufacturing castings with complex structures and wear-resistant parts.
Gray cast iron is divided into three categories according to different matrix structures: ferrite-based gray cast iron, pearlite-ferrite-based gray cast iron, and pearlite-based gray cast iron.
Because gray cast iron contains flake graphite, graphite is a component with low density, low strength, low hardness, and plasticity and toughness that tend to be zero. Its existence is like a large number of small gaps in the steel matrix, which reduces the load-bearing area and increases the source of cracks. Therefore, gray cast iron has low strength and poor toughness, and cannot be pressure processed. In order to improve its performance, a small amount of ferrosilicon, calcium silicate and other inoculants are added to the molten iron before pouring to refine the pearlite matrix.
malleable cast iron
Malleable cast iron is made from an iron-carbon alloy with low carbon and silicon content. Chemically treated white cast iron.
Malleable cast iron is divided into two categories according to different microstructures after heat treatment; one is black-core malleable cast iron and pearlescent malleable cast iron. The structure of black-core malleable cast iron is mainly ferrite (F) base + flocculent graphite; the structure of pearlite malleable cast iron is mainly pearlite (P) matrix + flocculent graphite. The other type is white-center malleable iron. The structure of white-center malleable iron is determined by the cross-section size. Small cross-sections have ferrite as the matrix, and large cross-sections have ferrite in the surface area and pearlite and annealed carbon in the core.
The graphite becomes fine and evenly distributed in the cast iron after this inoculation treatment. Called incubated cast iron.
Ductile Iron
Before pouring molten iron (ductile iron), add a certain amount of spheroidizing agent (commonly used are ferrosilicon, magnesium, etc.) to spheroidize the graphite in the cast iron. Since carbon (graphite) exists in the form of spheroids in the cast iron matrix, its splitting 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 been widely replaced by malleable cast iron and some cast steel and forged steel parts, such as crankshafts, connecting rods, rollers, automobile rear axles, etc.
Post time: Sep-04-2024
