Cast iron is a group of iron-carbon alloys with a carbon content greater than 2%. It is known for its excellent castability and unique properties, making it a valuable material in various industrial applications.
Cast iron has several advantages, including good castability, wear resistance, and excellent thermal conductivity and vibration dampening. However, it can be brittle and prone to cracking if subjected to sudden impacts or stresses.
The choice of cast iron type depends on the specific application and the desired balance of properties, such as strength, ductility, and heat resistance.
In this article, you learn what cast iron is. and different types of cast iron are explained with their composition, application, advantages, and disadvantages.
What is Cast Iron?
Cast iron is an alloy of iron that contains 2 to 4 percent carbon, along with varying amounts of silicon and manganese and traces of impurities such as sulfur and phosphorus. It is made by reducing iron ore in a blast furnace.
The liquid iron is cast, or poured and hardened, into crude ingots called pigs, and the pigs are subsequently remelted along with scrap and alloying elements in cupola furnaces and recast into molds for producing a variety of products.
The Chinese produced cast iron as early as the 6th century bce, and it was produced sporadically in Europe by the 14th century. It was introduced into England about 1500; the first ironworks in America were established on the James River, Virginia, in 1619.
During the 18th and 19th centuries, cast iron was a cheaper engineering material than wrought iron because it did not require intensive refining and working with hammers, but it was more brittle and inferior in tensile strength.
Nevertheless, its load-bearing strength made it the first important structural metal, and it was used in some of the earliest skyscrapers. In the 20th century, steel replaced cast iron in construction, but cast iron continues to have many industrial applications.
Most cast iron is either so-called gray iron or white iron, the colors shown by fracture. Gray iron contains more silicon and is less hard and more machinable than white iron.
Both are brittle, but a malleable cast iron produced by a prolonged heat treatment was developed in France in the 18th century, and a cast iron that is ductile as cast was invented in the United States and Britain in 1948.
Such ductile irons now constitute a major family of metals that are widely used for gears, dies, automobile crankshafts, and many other machine parts.
Types of Cast Iron
Each type of cast iron has unique properties that make it suitable for specific applications, ranging from heavy machinery to intricate architectural details.
Different processing techniques can be used to produce the desired type, which include:
- Grey cast iron
- White cast iron
- Chilled cast iron
- Mottled cast iron
- Malleable cast iron
- Nodular cast iron
- Alloy cast iron
#1. Grey Cast Iron.
Grey Cast iron is a type of cast iron that has been processed to produce free graphite (carbon) molecules in the metal. Its microstructure, characterized by the graphite flakes, is responsible for many of its distinctive physical and mechanical attributes.
The size and structure of the graphite can be controlled by moderating the cooling rate of the iron and by adding silicon to stabilize the graphite. When Grey Cast Iron fractures, it occurs along the graphite flakes and has a grey appearance at the fracture site.
Grey Cast Iron is not as ductile as other cast irons; however, it has excellent thermal conductivity and the best damping capacity of all cast irons. It is also hard-wearing, making it a popular material to work with.
The high wear resistance, high thermal conductivity, and excellent damping capacity of Grey Cast Iron make it ideal for industrial and household settings. Common Grey Cast Iron examples include engine blocks, flywheels, manifolds, and cookware.
#2. White Cast Iron.
White Cast Iron is named based on the appearance of fractures. By tightly controlling the carbon content, reducing the silicon content, and controlling the cooling rate of iron, it is possible to consume all carbon in the iron in the generation of iron carbide.
This ensures there are no free graphite molecules and creates an iron that is hard, brittle, extremely wear-resistant, and has a high compressive strength. As there are no free graphite molecules, any fracture site appears white, giving White Cast Iron its name.
Due to its superior wear-resistant properties, White Cast Iron is highly valued in industrial applications that demand durability under abrasive conditions. White Cast Iron examples include pump housings, mill linings and rods, crushers, and brake shoes.
#3. Ductile Cast Iron
Ductile Cast Iron is produced by adding a small amount of magnesium, approximately 0.2%, which makes the graphite form spherical inclusions that give a more ductile cast iron. This modification in the microstructure significantly enhances the mechanical properties of the material.
Unlike the graphite flakes found in grey cast iron, the spherical graphite inclusions in ductile cast iron prevent stress concentration points, allowing the material to bend or deform under tensile stress rather than fracture. This makes ductile cast iron stronger and more flexible than other types of cast iron.
Ductile cast iron can withstand thermal cycling better than other types of cast iron. This property ensures that ductile cast iron remains stable under fluctuating temperature conditions, making it well-suited for applications requiring long-term performance under thermal stress.
Ductile Cast Iron is predominantly used for its relative ductility and can be found extensively in water and sewerage infrastructure. Its ability to resist deformation under load makes it an ideal material for pipes and fittings in water distribution and waste management systems. The thermal cycling resistance also makes it a popular choice for crankshafts, gears, heavy-duty suspensions, and brakes.
#4. Chilled Cast Iron.
It is white cast iron produced by the quick cooling of molten iron. The quick cooling is generally called chilling, and the iron so produced is known as chilled iron.
All casting molds have contact with molten iron with cool sand on their outer skin. But in most cases, this hardness penetrates to a very small depth (less than a mm).
Sometimes casting is chilled intentionally, and sometimes it becomes accidentally to a considerable depth.
Intentional chilling is done by inserting iron or steel inserts into the mold. When the molten metal comes into contact with the chill, its heat is rapidly conducted away, and the hard surface is formed.
Chills are used on any face of the casting that has to withstand wear and friction. The process of chilling is used in the casting of roles for crushing grains and jaw crusher plates. The running surface of rail-carriage wheels is also chilled.
#5. Mottled Cast Iron.
It is a product between grey cast iron and white cast iron in composition, color, and properties. It is obtained in casting, where certain wearing surfaces have been chilled.
#6. Malleable Cast Iron.
Malleable Cast Iron is manufactured by heat treating White Cast Iron to break down the iron carbide back into free graphite. This process, known as annealing, involves heating the cast iron to approximately 920°C and then cooling it very slowly.
The controlled breakdown of iron carbide into clusters of free graphite, often referred to as temper carbon, transforms the brittle White Cast Iron into a malleable, ductile product with good fracture toughness at low temperatures.
Malleable Cast Iron examples include electrical fittings, mining equipment, and machine parts. These components benefit from Malleable Cast Iron’s ability to endure mechanical stress while maintaining shape and functionality.
#7. Nodular Cast Iron.
It is also known as ductile cast iron, spheroidal graphite (i.e., S.G.) cast iron, or high-strength cast iron.
The nodular cast iron is produced by adding magnesium to the molten cast iron. Magnesium converts graphite of cast iron from a flake form to a spherical or nodular form.
This way, the mechanical properties are considerably improved. The nodular cast iron behaves like steel. It is usually used for pressure-resisting casting, hydraulic cylinders, cylinder heads, rolls for rolling mills, and centrifugally cast products.
#8. Alloy Cast Iron.
The cast iron discussed above is called plain cast iron. The alloy cast iron is produced by adding elements like nickel, chromium, molybdenum, copper, silicon, and manganese.
These alloys give greater strength and result in improved element properties.
The alloy cast iron has,
- Carbon – 0.02%
- Silicon – 0.15%
- Manganese – 0.03%
- Phosphorus – 0.12%
- Sulphur – 0.02%
- Slag – 3% (by weight), and the remaining is iron.
How Cast-Iron Castings Are Made: Detailed Step-by-Step Process
Here, we take you through a comprehensive step-by-step process to explain how cast iron is made:
Step 1: Sourcing High-Quality Raw Materials.
The process begins with sourcing the raw materials. Cast iron is primarily made from pig iron, scrap iron, and steel, combined with alloys such as carbon and silicon. Typically, the composition includes:
- Pig Iron: 70-80%
- Scrap Iron and Steel: 10-20%
- Alloys (Carbon and Silicon): 2-5%
These materials are carefully inspected for quality to ensure the final product meets rigorous standards.
Step 2: Melting in the Furnace.
The raw materials are melted in a high-capacity furnace at temperatures exceeding 1,150°C (2,100°F). The types of furnaces commonly used include:
- Cupola Furnace: Traditional and cost-effective, often used for large-scale production.
- Electric Induction Furnace: Highly efficient and environmentally friendly, suitable for precise control of metal composition.
- Rotary Furnace: Versatile and used for specialized applications requiring uniform heating.
During this process, the materials are mixed to create a uniform molten iron blend. The chemistry of the molten metal is meticulously monitored to maintain precise composition.
Step 3: Moulding.
Once the molten iron is ready, it is poured into Molds made from sand, resin, or metal. The molds are designed based on the desired shape and size of the casting. This step is crucial for ensuring that the final product matches the required specifications.
Step 4: Cooling and Solidification.
The molten iron in the molds is allowed to cool and solidify. This cooling process ensures that the casting takes its final shape while maintaining structural integrity. Proper cooling prevents defects and ensures the casting’s durability.
Step 5: Cleaning and Finishing.
After solidification, the castings are removed from the molds and cleaned. This includes removing excess sand, scale, and other impurities. Additional finishing processes, such as grinding, machining, and polishing, are performed to achieve the desired surface quality and dimensions.
Step 6: Material Grades and Quality Inspection.
Cast iron is available in various grades, each suited for specific applications. Common material grades include:
- Gray Iron: Known for its excellent machinability and thermal conductivity.
- Ductile Iron: Offers superior strength and flexibility.
- White Iron: Highly resistant to abrasion, ideal for wear-resistant applications.
- Malleable Iron: Combines strength and ductility, used in industrial fittings.
Every cast iron casting undergoes rigorous quality inspections. These include dimensional checks, material tests, and performance evaluations to ensure the product meets industry standards and customer requirements.
Step 7: Packaging and Delivery.
The final step involves securely packaging the castings to protect them during transportation. Our logistics team ensures timely delivery to clients, maintaining our reputation as a reliable partner in cast iron manufacturing.
Chemical Composition
The chemical composition of cast iron plays a crucial role in determining its properties. The primary elements in cast iron include:
| Element | Percentage |
|---|---|
| Carbon | 2% – 4% |
| Silicon | 1% – 3% |
| Manganese | 0.5% – 1% |
| Phosphorus | Up to 1% |
| Sulfur | Up to 0.1% |
The carbon and silicon content significantly influence the microstructure and properties of cast iron. Higher carbon levels increase the amount of graphite formed during solidification, while silicon promotes graphite formation and improves castability.
Properties
Cast iron exhibits several distinct physical properties:
- High density: Cast iron has a density of approximately 7.2 g/cm³, making it heavier than steel.
- Low melting point: It melts at around 1150°C to 1250°C, which is lower than steel, making it easier to cast.
- Thermal conductivity: Cast iron has a thermal conductivity of about 50 W/m·K, which is lower than that of steel.
- Electrical conductivity: It has a lower electrical conductivity compared to steel, making it suitable for electrical applications where insulation is required.
Advantages of Cast Iron
- Good casting ability
- Available in large quantities, making production relatively inexpensive.
- It has a high compression strength
- Cast irons have good machinability
- Good anti-vibration property
- It has excellent wear resistance
- Low-stress concentration
- High resistance to deformation
- High durability
Disadvantages of Cast Iron
- Prone to corrosion and rust
- Low tensile strength
- High impact resistance
- High weight-to-strength ratio
- High brittleness
Industrial Applications of Cast Iron
Industrial cast iron is utilized across various sectors due to its exceptional properties:
- Automotive Industry: Engine blocks, brake drums, and components.
- Construction: Pipes, manhole covers, and structural elements.
- Agriculture: Tractor parts, plowshares, and machinery.
- Cookware: Durable pans, pots, and griddles.
- Energy Sector: Wind turbine hubs and generators.
- Manufacturing: Machine bases, tools, and industrial equipment.
Benefits of Using Cast Iron
Cast iron offers several advantages, making it a preferred choice across various industries:
- Durability: Cast iron products have exceptional strength and resistance to wear and tear.
- Heat Retention: Ideal for cookware and industrial applications, cast iron retains heat efficiently.
- Versatility: Used in automotive, construction, and kitchenware industries, among others.
- Cost-Effectiveness: Cast iron provides a long-lasting solution at an affordable cost.