Cast Iron: Definition, Properties, And Uses

Iron is found in the composition of many different metal alloys. Without it, carbon steel, stainless steel, superalloys, and several other metal alloy groups could not be manufactured.

However, cast iron, ironically the metal alloy group with “iron” in its name, is often misunderstood as to what it is, how it is made, and what it can be used for.

This article takes an in-depth look at cast iron.

What is Cast Iron?

Cast iron is a class of iron-carbon alloys with a carbon content of more than 2% and a silicon content of around 1–3%. Its usefulness derives from its relatively low melting temperature.

Cast iron is one of the oldest ferrous metals used in construction and outdoor ornament. It has a relatively high carbon content of 2% to 5%. It is primarily composed of iron (Fe), carbon (C) and silicon (Si), but may also contain traces of sulphur (S), manganese (Mn), and phosphorus (P).

It is hard, brittle, nonmalleable (i.e. it cannot be bent, stretched or hammered into shape) and more fusible than steel.

Its structure is crystalline and it fractures under excessive tensile loading with little prior distortion. Cast iron is, however, very good in compression.

The composition of cast iron and the method of manufacture are critical in determining its characteristics.

Cast iron is used in pipes, machinery, and automotive parts such as cylinder heads, cylinder blocks, and gearboxes. It is resistant to oxidation damage but difficult to weld.

How Is Cast Iron Made?

The manufacturing process of cast iron is how the metal gets its name. To make cast iron, iron ore is heated in a furnace until it becomes molten. Then the molten metal is cast (poured out and allowed to harden, in the shape of an ingot).

The cast iron ingots are melted again into a final mold. During this subsequent remelting, the cast iron may have several metallurgical modifications made to it through the introduction of alloying elements or heat-treating processes.

Oftentimes, this is where a cast iron falls into the specifications of one of the four groups mentioned above.

Cast iron, when molten, has better fluidity than steel. Cast iron also has a low melting temperature. These two attributes make cast iron an ideal candidate for the casting process. This is partially why it is so popular.

However, with improved technology for manufacturing and forming steel, use of cast iron has diminished over the past several centuries.

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, and 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?

The main difference between the two elements is that steel is produced from iron ore and scrap metals, and is called an alloy of iron, with controlled carbon. Whereas around 4% of carbon in iron makes it cast iron, and less than 2% of carbon makes it steel.

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 of cast iron include:

• Hardness. Cast iron is hard and it can be hardened by heating and sudden cooling. This makes it quite durable. Mild steel can be hardened and tempered by using relevant processes.
• 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
• Melting Point. Cast iron has a lower melting point of 12000°C compared to the melting point of mild steel, which ranges between 13000°C and 14000°C.
• Castability. Cast iron is easier to work with when it comes to casting shapes out of the material. Due to the extra carbon present in cast iron, it’s molten form is more fluid and this makes it easier to cast the material into complex shapes.
• Machinability. Cast iron is almost elastic up to ultimate tensile strength and produces discontinuous chips which break away from the sample easily. This helps to improve the cutting ability. Due to this, cast iron is the preferred material when it comes to high machinability and strength.

Types of Cast Iron

There are four basic types of cast iron:

• Gray cast iron.
• Ductile cast iron.
• Malleable cast iron.
• White cast iron.
• Alloy cast Iron

#1. Gray Cast Iron.

Grey Cast iron refers to a type of cast iron that has been processed to produce free graphite (carbon) molecules in the metal. 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 fractures 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 an 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 the excellent damping capacity of Grey Cast Iron makes it ideal for 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.

White Cast Iron is used primarily for its wear resistant properties in 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. It can also withstand thermal cycling better than other cast iron products.

Ductile Cast Iron is predominantly used for its relative ductility and can be found extensively in water and sewerage infrastructure. The thermal cycling resistance also makes it a popular choice for crankshafts, gears, heavy-duty suspensions, and brakes.

#4. Malleable Cast Iron.

Malleable Cast Iron is a type of cast iron that is manufactured by heat treating White Cast Iron to break down the iron carbide back into free graphite. This produces a malleable and ductile product that has good fracture toughness at low temperatures.

Malleable Cast Iron is used for electrical fittings, mining equipment and machine parts.

#5. Alloy Cast Iron.

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

Use of Cast Iron

Cast iron can be used for many types of materials and for 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, and cisterns are manufactured using cast iron.
• Column base and metal columns can be made using cast iron
• Casting mold is 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 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 Non-machinable (white cast iron).

FAQs.