Cast iron is an iron-carbon alloy with a carbon content ranging from 2.5% to 4%, typically exceeding 2.11%.
It is composed of several components, including iron, carbon, and silicon, and may also contain impurities such as manganese, sulfur, and phosphorus, which are more prevalent than in carbon steel.
To enhance its mechanical properties or physical and chemical characteristics, certain alloy elements can be added to produce alloy cast iron.
Types of cast iron
According to the different forms of carbon
According to the different forms of carbon in cast iron, cast iron can be divided into:
1. White cast iron
In cast iron, carbon exists primarily as cementite, with only a small amount dissolved in ferrite.
Its fracture has a silver-white appearance, hence it is known as white cast iron.
Currently, white cast iron is mainly utilized as a raw material in steelmaking and as a base for producing malleable cast iron.
2. Grey cast iron
In cast iron, most or all of the carbon exists as flake-like graphite, and its fracture displays a dark gray color. As a result, it is referred to as gray cast iron.
3. Mottled cast iron
In cast iron, part of the carbon exists as graphite, much like gray cast iron, while the other part exists as free cementite, similar to white cast iron.
This results in the fracture surface displaying both black and white spots, earning it the name “mottled cast iron.”
Unfortunately, this type of cast iron is also hard and brittle, making it rarely used in industrial applications.
According to different graphite forms
According to different graphite forms in cast iron, cast iron can be divided into:
- Grey Cast Iron
In grey cast iron, the carbon exists as flake-like graphite.
- Malleable Cast Iron
Malleable cast iron is obtained by annealing white cast iron of a specific composition at high temperatures for an extended period. As a result, the carbon in malleable cast iron exists in a flocculent form.
This type of cast iron boasts improved mechanical properties, particularly in terms of toughness and plasticity, compared to grey cast iron, hence its name “malleable cast iron.”
3. Ductile iron
In cast iron, the carbon exists in the form of spherical graphite.
This is achieved through a spheroidizing treatment prior to the casting process.
This type of cast iron boasts superior mechanical properties compared to both grey cast iron and malleable cast iron. Additionally, its production process is simpler than that of malleable cast iron, and its mechanical properties can be further improved through heat treatment. As a result, its usage in production is becoming increasingly widespread.
Classification and designation of cast iron
Cast iron is an iron-carbon alloy that contains more than 2.1% carbon.
It is produced by re-melting cast pig iron (a component of steelmaking pig iron) in a furnace, and adjusting its composition by adding ferroalloys, scrap steel, and recycled iron.
The key difference between cast iron and pig iron is that cast iron undergoes a secondary processing step, and the majority of it is cast into iron castings.
Iron castings possess excellent casting properties and can be molded into complex shapes. They also have good machinability and are known for their resistance to wear and shock absorption, as well as their low cost.
Designation of cast iron: (according to GB5612-85)
Cast iron codes are comprised of the first letter of the Chinese alphabet, indicating its specific characteristics.
When two cast iron names have the same code letter, they can be differentiated by adding lowercase letters after the capital letter.
For cast iron with the same name that requires further classification, the first letter of the Chinese Pinyin representing its subclass characteristics is added to the end.
Description of name, code and brand of cast iron:
|Cast iron name||Code /Grade||Example of representation method|
|Grey cast iron||HT||HT100|
|Vermicular graphite cast iron||RuT||RuT400|
|Nodular cast iron||QT||QT400-17|
|Black heart malleable cast iron||KHT||KHT300-06|
|White heart malleable cast iron||KBT||KBT350-04|
|Pearlitic malleable cast iron||KZT||KZT450-06|
|Wear resistant cast iron||MT||MT Cu1PTi-150|
|Wear resistant white cast iron||KmBT||KmBTMn5Mo2Cu|
|Wear-resistant ductile iron||KmQT||KmQTMn6|
|Chilled cast iron||LT||LTCrMoR|
|Corrosion-resistant cast iron||ST||STSi15R|
|Corrosion resistant ductile iron||SQT||SQTAl15Si5|
|Heat-resistant cast iron||RT||RTCr2|
|Heat resistant ductile iron||RQT||RQTA16|
|Austenitic cast iron||AT||—-|
Note: A series of numbers following the code in the grade denote the tensile strength value.
In cases where there are two sets of numbers, the first set represents the tensile strength value, and the second set represents the elongation value.
These two sets of numbers are separated by a “one”.
Alloy elements are represented using international element symbols. If the content is equal to or greater than 1%, it is represented as an integer. If the content is less than 1%, it is usually not indicated.
Common elements such as C, Si, Mn, S, and P are typically not marked. Their element symbols and content are only marked if they serve a specific purpose.
Use of various cast iron
White cast iron
In white cast iron, all the carbon exists in the form of permeating carbon (Fe3C), resulting in a bright white fracture surface.
Because of this, it is referred to as white cast iron.
However, due to the high concentration of hard and brittle Fe3C, white cast iron has a high level of hardness but is also highly brittle and difficult to process.
As a result, it is not commonly used directly in industrial applications, except for a few applications that require wear resistance without impact, such as wire drawing dies and iron balls for ball mills.
Instead, it is primarily utilized as a raw material for steelmaking and malleable cast iron production.
Grey cast iron
In cast iron, most or all of the carbon exists as sheet-like graphite in a free state, resulting in a gray fracture surface.
Gray cast iron has good casting properties, is easy to machine, has good wear resistance, simple melting and batching processes, and a low cost, making it widely used for the production of castings with complex structures and wear-resistant parts.
Gray cast iron can be divided into ferrite-based gray cast iron, pearlite-ferrite based gray cast iron, and pearlite-based gray cast iron based on its matrix structure.
Due to the presence of flake-like graphite, gray cast iron has low density, strength, hardness, and zero plasticity and toughness.
The existence of this graphite is similar to the presence of many small notches on the steel substrate, which reduces the bearing area and increases the number of cracks, resulting in low strength and poor toughness in gray cast iron, and making it unsuitable for pressure processing.
To improve its properties, certain inoculants such as ferrosilicon and calcium silicate are added to the molten iron before casting to refine the pearlite matrix.
Malleable iron is made from a white cast iron base that is cast from an iron-carbon alloy with low carbon and silicon content. After undergoing long-term high-temperature annealing, the cementite decomposes into clusters of flocculent graphite, resulting in a type of graphitized white cast iron.
Malleable cast iron can be divided into two types based on its microstructure after heat treatment: black core malleable iron and pearlescent malleable iron. The structure of black core malleable cast iron is primarily ferrite (F) base with flocculent graphite, while the structure of pearlitic malleable cast iron is primarily pearlitic (P) matrix with flocculent graphite.
The third type is white core malleable cast iron, which has a structure that depends on the section size. For small sections, the matrix is ferrite, while for larger sections, the surface area is ferrite with the center being pearlite and annealed carbon.
Inoculation cast iron is produced when the graphite becomes fine and evenly distributed after inoculation treatment.
Before pouring the molten iron (nodular pig iron), a spheroidizing agent, commonly made of ferrosilicon or magnesium, is added to spheroidize the graphite in the cast iron. The addition of the spheroidizing agent greatly improves the tensile strength, yield strength, plasticity, and impact toughness of nodular cast iron. This is because the carbon (graphite) in the cast iron matrix exists in a spherical form, improving its splitting effect on the matrix.
Nodular cast iron has several advantages, including wear resistance, shock absorption, good process performance, and low cost. These advantages have led to its widespread use in replacing malleable cast iron, as well as some cast steel and forged steel parts, such as crankshafts, connecting rods, rolls, and automobile rear axles.