Cast iron is an iron-carbon alloy with 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.
Cast iron types are classified mainly based on the shape of the carbon and the morphology of the graphite present. Following are the main types of cast iron:
White Cast Iron: Carbon exists in the form of cementite (Fe3C) and the fracture surface is silvery white. It is fragile and is rarely used alone. White cast iron is an intermediate product for manufacturing malleable cast iron, and cooled cast iron with a surface layer of white cast iron is commonly used for rollers.
Gray cast iron: All or most of the carbon exists in the form of graphite, which is flaky. This type of iron has different applications depending on the shape of the graphite, such as common gray cast iron (flaky graphite) and vermicular cast iron (worm graphite).
Malleable cast iron: Graphite exists in flocculent form, obtained by annealing white cast iron of a certain composition at high temperatures for a long period. Its mechanical properties, mainly toughness and plasticity, are superior to those of gray cast iron.
Ductile cast iron: Graphite exists in spherical form, obtained by spheroidization treatment before casting the cast iron. This type of iron not only has mechanical properties superior to those of gray cast iron and malleable cast iron, but its manufacturing process is simpler than that of malleable cast iron. Furthermore, its mechanical properties can be improved through heat treatment.
Vermicular Cast Iron: Graphite exists in the form of a worm and has good mechanical and processing properties.
Alloy cast iron: To improve the mechanical or physical and chemical properties of cast iron, a certain amount of alloying elements can be added to obtain cast iron alloy. This type of iron includes a variety of special cast iron alloys that are resistant to corrosion, heat and wear.
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 silvery white appearance, which is why it is known as white cast iron.
Currently, white cast iron is mainly used as a raw material in steelmaking and as a basis for the production of malleable cast iron.
2. Gray cast iron
In cast iron, most or all of the carbon exists as flaked graphite and its fracture is dark gray in color. As a result, it is referred to as gray cast iron.
3. M labeled cast iron
In cast iron, some of the carbon exists as graphite, just like gray cast iron, while the other part exists as free cementite, similar to white cast iron.
This causes the fracture surface to display black and white spots, earning it the name “stained cast iron.”
Unfortunately, this type of cast iron is also hard and brittle, which makes it rarely used in industrial applications.
According to different forms of graphite
According to the different forms of graphite in cast iron, cast iron can be divided into:
- Gray Cast Iron
In gray cast iron, carbon exists as flaked 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 flocculent form.
This type of cast iron has improved mechanical properties, mainly in terms of toughness and plasticity, compared to gray cast iron, hence its name “malleable cast iron”.
3. Ductile iron
In cast iron, carbon exists in the form of spherical graphite.
This is achieved through a spheroidization treatment before the casting process.
This type of cast iron has superior mechanical properties compared to gray cast iron and malleable cast iron. Furthermore, 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 use 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 remelting molten pig iron (a component of steelmaking pig iron) in a furnace and adjusting its composition by adding ferroalloys, steel scrap and recycled iron.
The main difference between cast iron and pig iron is that cast iron goes through a secondary processing step and most of it is cast into cast iron.
Iron castings have excellent casting properties and can be molded into complex shapes. They also have good machinability and are known for their wear resistance and shock absorption, in addition to their low cost.
Cast iron designation: (according to GB5612-85)
Cast iron codes are made up of the first letter of the Chinese alphabet, indicating their specific characteristics.
When two cast iron names have the same code letter, they can be differentiated by adding lowercase letters after the uppercase letter.
For cast iron with the same name that requires additional classification, the first letter of Chinese Pinyin representing the characteristics of its subclass is added at the end.
Description of cast iron name, code and brand:
| Cast iron name | Code/Grade | Example of representation method |
| Gray cast iron | HT | HT100 |
| Vermicular graphite cast iron | Routine | RuT400 |
| Nodular cast iron | QT | QT400-17 |
| Malleable cast iron with black heart | KHT | KHT300-06 |
| Malleable cast iron with white heart | 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 | TR | RTCr2 |
| Heat resistant ductile iron | RQT | RQTA16 |
| Austenitic cast iron | AT THE | —- |
Note: A series of numbers following the code in the class indicates 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 by 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 generally not indicated.
Common elements such as C, Si, Mn, S, and P are not normally labeled. The symbols and contents of its elements are only marked if they serve a specific purpose.
Use of various cast irons
White cast iron
In white cast iron, all the carbon exists in the form of permeant carbon (Fe3C), resulting in a bright white fracture surface.
Because of this, it is known 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 in some applications that require non-impact wear resistance, such as drawing dies and iron balls for ball mills.
Instead, it is primarily used as a raw material for steelmaking and malleable cast iron production.
Gray cast iron
In cast iron, most or all of the carbon exists as sheet-form 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 dosing processes and low cost, which makes it widely used for the production of castings with complex structures and weather-resistant parts. wear.
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 graphite, gray cast iron has low density, strength, hardness and plasticity and zero toughness.
The existence of this graphite is similar to the presence of many small notches in the steel substrate, which reduces the bearing area and increases the number of cracks, resulting in low strength and low toughness in gray cast iron, making it unsuitable for processing under pressure. .
To improve its properties, certain inoculants such as ferrosilicon and calcium silicate are added to cast iron before casting to refine the pearlite matrix.
M iron permissible
Malleable iron is made from a white cast iron base cast from a low-carbon, low-silicon iron-carbon alloy. After undergoing prolonged high-temperature annealing, 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 mainly a ferrite (F) base with flocculent graphite, while the structure of pearlitic malleable cast iron is mainly a pearlite matrix (P) with flocculent graphite.
The third type is malleable cast iron with a white core, which has a structure that depends on the size of the section. 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.
Ductile iron
Before casting the cast iron (nodular pig iron), a spheroidizing agent, commonly made from ferrosilicon or magnesium, is added to spheroidize the graphite in the cast iron. The addition of spheroidizing agent greatly improves the tensile strength, yield strength, plasticity and impact resistance of nodular cast iron. This is because the carbon (graphite) in the cast iron matrix exists in a spherical form, improving its splitting effect in 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 automobile crankshafts, connecting rods, rollers, and rear axles.
How do alloying elements improve the performance of cast iron?
Common alloying elements in cast iron alloys include silicon, manganese, phosphorus, nickel, chromium, molybdenum, copper, aluminum, boron, vanadium, titanium, antimony, and tin. These elements improve the performance of cast iron through several mechanisms:
- Silicon (Si): As a beneficial element, it promotes graphitization, improving the mechanical properties and wear resistance of castings.
- Manganese (Mn): Refines the grain structure, thus improving mechanical properties.
- Phosphorus (P) and Sulfur (S): Although normally considered impurities, they can improve machinability under certain circumstances.
- Other alloying elements such as Nickel (Ni), Chromium (Cr), Molybdenum (Mo), Copper (Cu), Aluminum (Al), Vanadium (V), Titanium (Ti), Antimony (Sb) and Tin (Sn): The addition of these elements significantly increases the strength, hardness, wear resistance, oxidation resistance and corrosion resistance of cast iron.
Furthermore, the inclusion of alloying elements can change the internal structure of cast iron, leading to new phase changes, thus improving its process performance, such as thermoplasticity, cold deformability, machinability, hardenability and weldability. For example, silicon and carbon together promote graphitization, increasing the compactness and toughness of castings, reducing the tendency to white mouth, stabilizing austenite, and refining graphite and pearlite.
By improving the mechanical properties, wear resistance, oxidation resistance and corrosion resistance of cast iron, alloying elements improve the overall performance of cast iron alloy.
What are the detailed applications and performance characteristics of cast iron white cast iron?
White cast iron, named for its silvery-white fracture surface, is a type of cast iron that does not precipitate graphite during the crystallization process. This type of cast iron has a large amount of free cementite in its structure, resulting in high hardness (generally above HB500), but it is also very brittle. Due to its high hardness and wear resistance, as well as its low cost, white cast iron is a viable choice for wear-resistant applications, despite being considered too brittle for many structural components.
The main application fields of white cast iron include wear-resistant parts such as agricultural tools, grinding balls, coal mill parts, shot blasting machine blades, slurry pump parts, foundry sand pipes and the outer layer of cold hard laminating rolls. Furthermore, it is used as a raw material for steelmaking and as a blank for the production of malleable cast iron. Specifically, manganese tungsten white cast iron and tungsten chromium white cast iron are used for parts that require mechanical machining and conditions with large impact loads, low stress abrasive wear and high stress grinding abrasive wear , respectively.
In terms of performance characteristics, white cast iron is hard and brittle, not easy to machine, and is rarely used directly for casting parts. Its carbon exists entirely in the form of cementite (Fe3C), making it have mechanical properties superior to those of gray cast iron and malleable cast iron, and its production process is relatively simple. However, due to its fragility, white cast iron cannot withstand cold or hot work and can only be used directly in the molten state.
White cast iron, with its high hardness and wear resistance, plays an important role in specific application scenarios, although its fragility limits its application over a wider range.
What are the specific differences in mechanical properties between gray iron and malleable iron?
The specific differences in mechanical properties between gray iron and malleable iron are mainly reflected in the following aspects:
Graphite Morphology: Graphite in gray iron is flake-shaped, while graphite in malleable iron is worm-like. This difference in graphite morphology leads to differences in mechanical properties. Flaked graphite gives gray iron a certain degree of brittleness, while worm-shaped graphite helps improve the material's toughness.
Mechanical properties: Due to the difference in graphite morphology, the mechanical properties of malleable iron are generally superior to those of gray iron. The mechanical properties of malleable iron fall between ductile iron and gray iron, meaning it is stronger than gray iron but not as strong as ductile iron.
Casting performance: The casting performance of malleable iron is between gray iron and ductile iron. This indicates that malleable iron has good adaptability and flexibility in the casting process, capable of meeting the demands of various application scenarios.
Sensitivity to chemical composition: Compared to gray iron, malleable iron has a smaller impact on mechanical properties when the carbon and silicon content changes from hypoeutectic to eutectic. This implies that malleable iron has greater flexibility in adjusting its chemical composition to optimize its performance.
Heat Treat Capability: Malleable iron can undergo various heat treatments, including isothermal quenching, which offers the possibility of further improving its mechanical properties through heat treatment.
How is the impact of the annealing process on the mechanical properties of malleable cast iron quantified?
The influence of the annealing process on the mechanical properties of malleable cast iron can be quantified in the following ways:
Improvement in strength and plasticity: Through graphitization annealing treatment, malleable cast iron can achieve greater strength, plasticity and impact resistance, allowing it to replace carbon steel to a certain extent. Compared to gray cast iron, malleable cast iron has better resistance and plasticity, especially its impact performance at low temperatures.
Improved wear resistance and vibration damping: The wear resistance and vibration damping of malleable cast iron surpass common carbon steel, a result of its specific microstructure and chemical composition. Optimization during the annealing process can further improve these properties.
Shortening production cycles and reducing energy consumption: Improvements in the annealing process, such as adjusting the carbon and silicon content and adding elements such as bismuth, boron and aluminum for modification treatment, can not only shorten the annealing cycle, but also increase product qualification rates without sacrificing mechanical performance. Furthermore, research on rapid annealing processes has indicated that optimizing annealing conditions can effectively reduce energy consumption and environmental pollution.
Increased degree of graphitization: During the annealing process, the eutectic cementite in white cast iron undergoes graphitization, a crucial process to increase the toughness and plasticity of malleable cast iron. Optimizing the graphitization annealing process helps to improve the mechanical properties of the casting.
Increase in fracture toughness: The preheat treatment process and its microstructure have a significant effect on the fracture toughness of malleable cast iron. By optimizing annealing time and other relevant process parameters, the fracture resistance of malleable cast iron can be effectively improved, which is crucial to increasing its service life and reliability.
What is the spheroidization treatment process of ductile iron and its specific role in improving mechanical properties?
The spheroidization treatment process of ductile iron mainly includes spheroidization and inoculation, through which spherical graphite is obtained. This treatment method effectively reduces the fracturing effect of graphite in the matrix, significantly improving the mechanical properties of cast iron, including plasticity, toughness and strength. Specifically, the spheroidization treatment allows the graphite to exist in a spherical form within the cast iron. This structure, compared to traditional flaky or flocculent graphite, is more conducive to reducing stress concentration in the material, improving overall performance.
The specific role of spheroidization treatment lies in improving the microstructure of cast iron, leading to a more uniform distribution of graphite and reducing the risk of cracks and fractures caused by stress concentration during use. Furthermore, the presence of spherical graphite improves the wear resistance and vibration damping of cast iron, which is especially important for applications that need to withstand high loads and complex stress conditions. For example, in parts such as crankshafts of electrical machines, ductile iron is widely used due to its excellent comprehensive properties.
The spheroidization treatment process of ductile iron, by changing the shape of graphite, not only increases the plasticity, toughness and strength of cast iron, but also helps to improve its wear resistance and vibration damping, thus improving the mechanical performance up to certain point. These improvements make ductile iron a material with high strength, good toughness and plasticity. Its comprehensive performance is close to that of steel, making it suitable for various engineering applications that require complex stresses, high strength and good toughness.
