Cast Iron: Definition, Properties, and Uses

What is Cast Iron?

Cast iron is a group of iron-carbon alloys with a carbon content of more than 2%. Its usefulness derives from its relatively low melting temperature.

The alloy constituents affect its color when fractured: white cast iron has carbide impurities which allow cracks to pass straight through, grey cast iron has graphite flakes which deflect a passing crack and initiate countless new cracks as the material breaks, and ductile cast iron has spherical graphite “nodules” which stop the crack from further progressing.

Carbon (C) ranging from 1.8 to 4%, and silicon (Si) 1–3%, are the main alloying elements of cast iron. Iron alloys with lower carbon content are known as steel.

Cast iron tends to be brittle, except for malleable cast irons. With its relatively low melting point, good fluidity, castability, excellent machinability, resistance to deformation, and wear resistance.

Cast irons have become an engineering material with a wide range of applications and are used in pipes, machines, and automotive industry parts, such as cylinder heads, cylinder blocks, and gearbox cases. It is resistant to damage by oxidation but is difficult to weld.

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The earliest cast-iron artifacts date to the 5th century BC and were discovered by archaeologists in what is now Jiangsu in China. Cast iron was used in ancient China for warfare, agriculture, and architecture.

During the 15th century, cast iron became utilized for cannon in Burgundy, France, and England during the Reformation. The amounts of cast iron used for cannon required large-scale production. The first cast-iron bridge was built during the 1770s by Abraham Darby III and is known as The Iron Bridge in Shropshire, England. Cast iron was also used in the construction of buildings.

How to Make Cast Iron?

Cast iron is made from pig iron, which is the product of melting iron ore in a blast furnace. Cast iron can be made directly from the molten pig iron or by re-melting pig iron, often along with substantial quantities of iron, steel, limestone, carbon (coke), and taking various steps to remove undesirable contaminants.

Phosphorus and sulfur may be burnt out of the molten iron, but this also burns out the carbon, which must be replaced. Depending on the application, carbon and silicon content are adjusted to the desired levels, which may be anywhere from 2–3.5% and 1–3%, respectively.

If desired, other elements are then added to the melt before the final form is produced by casting.

Cast iron is sometimes melted in a special type of blast furnace known as a cupola, but in modern applications, it is more often melted in electric induction furnaces or electric arc furnaces. After melting is complete, the molten cast iron is poured into a holding furnace or ladle.

What is the Difference Between Cast Iron & Steel?

Cast iron is actually an alloy of iron and carbon, just like steel, with the main difference being that cast iron has more carbon in it. Whereas steel requires a carbon content of no more than 2 percent, cast iron usually contains 2 to 3.5 percent carbon.

Cast iron is cheaper than most steel. Also, the cast iron melting temperature is lower than that of steel, but it has high compressive strength, high hardness, and high wear resistance. Therefore, the important difference between steel and cast iron is that steel is ductile and malleable, whereas cast iron is hardened and has high compressive strength.

As another important difference between steel and cast iron, we can say that carbon in steel is in the form of iron carbide while cast iron has carbon as graphite or iron carbide or both. In addition, cast iron has excellent fluidity, with no steel.

Properties of Cast iron

A few common mechanical properties for cast iron include:

  • Hardness. Material’s resistance to abrasion and indentation
  • Toughness.  Material’s ability to absorb energy
  • Ductility. Material’s ability to deform without fracture
  • Elasticity. Material’s ability to return to its original dimensions after it has been deformed
  • Malleability. Material’s ability to deform under compression without rupturing
  • Tensile strength. The greatest longitudinal stress a material can bear without tearing apart
  • Fatigue strength. The highest stress that a material can withstand for a given number of cycles without breaking

Composite of Cast iron

Cast iron also contains a small number of impurities such as silicon, sulfur, manganese and phosphorus, copper, nickel, chromium, which affect its properties on a small or large scale. The effect of these properties is as follows.

Composite of Cast iron
  • Silicon: In cast iron, silicon particles have up to 4 percent presence. It promotes graphite formation which makes it soft and easily machinable and Increases hardenability and electrical resistivity.
  • Sulfur:  It is present up to 0.1 percent. It makes cast iron hard and brittle.
  • Manganese: It promotes carbide formation of cast iron which makes it elite, hard and increases resistance to wear and abrasions. It is present up to 0.75 percent.
  • Chromium: It also promotes carbide formation which Increases hardenability, wear resistance, corrosion, and oxidation resistance.
  • Nickle: Increases tensile strength.
  • Tungsten: It increases hot hardness and hot strength.
  • Molybdenum: Increases hardenability.
  • Vanadium: Increases hardenability and hot hardness.
  • Aluminum: Works as deoxidizer in steel.
  • Titanium: Works as deoxidizer in steel.
  • Niobium: It reduces hardenability and increases ductility, which results in increased impact strength.
  • Cobalt: It reduces hardenability and resists softening at elevated temperatures.

Types of Cast Iron

There are four types of main cast Iron as explain below:

  • Gray Cast iron
  • White Cast iron
  • Malleable Cast Iron
  • Ductile Cast iron
Types of Cast iron

1. Gray Cast Iron

Gray iron, or grey cast iron, is a type of cast iron that has a graphitic microstructure. It is named after the gray color of the fracture it forms, which is due to the presence of graphite. It is the most common cast iron and the most widely used cast material based on weight.

The First Type of Cast iron contains carbon in free form. It is also known as commercial cast iron due to mostly being used for commercial purposes.

It contains 3 to 3.5 percentile carbon and rests iron. This cast iron has low tensile strength, high compressive strength, and no ductility. It has excellent machining property. This metal is designate by FG followed by the digit show maximum tensile stress.

2. White Cast Iron

White cast iron is a type of carbon-iron alloy that contains carbon content greater than 2% in the form of cementite.

The name white cast is derived from its white surface, which is caused by carbide impurities that allow cracks throughout the metal. When fractured it exhibits a silver-like (white) fracture.

Cast iron in which carbon is present in a combined form or in carbide form is known as white cast iron. The white color is due to carbide formation. It is hard and not suitable for machining. This iron has high tensile strength and low compressive strength.

3. Malleable Cast Iron

Malleable cast irons are produced from white cast iron by heating it and holding it at 1,500–1,750°F (816–954°C) and then slowly cooling through a temperature range of 1,300–1,400°F (704–760°C).

This cast iron is obtained from white cast iron by the heat treatment process. It is mostly obtained by annealing of white cast iron.

It has improved strength and ductility like steel but can be easily cast. It is often used for small tensile, with good tensile strength and the ability to flex without breaking (ductility). Uses include electrical fittings, hand tools, pipe fittings, washers, brackets, fence fittings, etc.

4. Ductile cast iron

It is also known as Nodular or high strength cast iron. It has high strength. It is obtained from Gray cast iron by adding a small amount of magnesium to the molten stage.

It is designated by SG followed by the numbers which indicate tensile strength and percentage elongation.

5. Mottled Cast Iron

Cast iron has an equal amount of free carbon and carbide known as mottled cast iron. It has intermediate property and color of Gray cast iron and white cast iron.

6. Chilled Cast Iron

If the white cast iron quickly cooled from the molten stage, the iron produced is known as chilled cast iron.

7. Alloy Cast Iron

Alloy cast iron is produced by adding some alloying elements in cast iron like nickel, chromium, copper, etc. It has increase properties according to the alloying element. This cast iron is produced to get the desire properties of cast iron.

Use of Cast Iron

Cast iron can be use for many types of materials and making different tools etc.

  • Grey cast iron: Engine cylinder blocks, flywheels, gearbox cases, machine-tool bases.
  • White cast iron: Bearing surfaces.
  • Ductile cast iron: Gears, camshafts, crankshafts, piston ring.
  • Many types of sanitary fittings like sewer pipes, manholes, water pipes, cisterns are manufacturing using cast iron.
  • Column base and metal columns can be made using cast iron
  • Casting mold used for making lamp posts, metal staircases, gates,s, etc.
  • Carriage wheels and rail chairs are made from cast iron.
  • Various types of agriculture equipment and implements can be made from it.
  • Various machinery parts can be made from cast iron
  • It is used in making automotive parts
  • It is used in making pots pans and utensils
  • It is used in making anchors for ships

Advantages and Disadvantages of Cast Iron.

Advantages of cast iron

  • It has Good casting properties
  • It has good Sensibility
  • It has excellent resistance to wear
  • It has good machinability.
  • It has very low notch sensitivity
  • It has a Low-stress concentration
  • It bears Low cost
  • It has Durability
  • It has Resistance to deformation
  • It has three to five times higher compression strength than steel.
  • It has excellent anti-vibration (or damping) properties so it is used to make machine frames.
  • It has constant mechanical properties between 20 and 350 ° C.
  • It is available in large quantities, hence produced on a mass scale. Tools required for the casting process are relatively cheap and inexpensive. This results in a low cost of its products.
  • It can be given any complex shape and size without using costly machining operations

Disadvantages of cast iron

  • It is Prone to rust
  • It has a poor tensile strength
  • It has a high weight to strength ratio
  • It has High brittleness
  • It has poor impact resistance
  • Compared to steel it has poor machinability
  • Its parts are section sensitive; this is due to the slow cooling of thick sections.
  • The failure of its parts is sudden and in total, it does not reflect the yield point.
  • It is Nonmachinable (white cast iron).

FAQs.

What is Cast Iron?

Cast iron, 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.

What is cast iron made of?

Cast iron is made from pig iron, which is the product of melting iron ore in a blast furnace. Cast iron can be made directly from the molten pig iron or by re-melting pig iron, often along with substantial quantities of iron, steel, limestone, carbon (coke), and taking various steps to remove undesirable contaminants.

What is cast iron used for?

With its relatively low melting point, good fluidity, castability, excellent machinability, resistance to deformation and wear resistance, cast irons have become an engineering material with a wide range of applications and are used in pipes, machines, and automotive industry parts, such as cylinder heads, cylinder.

What is the Difference Between Cast Iron & Steel?

Cast iron is actually an alloy of iron and carbon, just like steel, with the main difference being that cast iron has more carbon in it. Whereas steel requires a carbon content of no more than 2 percent, cast iron usually contains 2 to 3.5 percent carbon.

What are the properties of cast iron?

Properties for cast iron include:
1. Hardness – material’s resistance to abrasion and indentation.
2. Toughness – material’s ability to absorb energy.
3. Ductility – material’s ability to deform without fracture.
4. Elasticity – material’s ability to return to its original dimensions after it has been deformed.

What are the Uses of Cast Iron?

Applications of cast iron:
It is used in making pipes, to carry suitable fluids.
It is used in making different machines.
It is used in making automotive parts.
It is used in making pots pans and utensils.
It is used in making anchors for ships.

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