What is Cast Iron?

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 term used to describe a family of metal alloys whose primary alloying element is iron. Because of its name, some people think cast iron is 100% iron. This is not true; in fact, carbon steels have higher amounts of iron than cast irons.

A predominantly iron alloy must have over 2% carbon for the final alloy to be considered cast iron. Cast irons also contain smaller amounts of other alloys, with manganese and silicon being two of the most popular. These additional alloying elements are used to further modify the properties of cast iron and result in specific cast iron alloy designations.

Types of Cast Iron

There also is not just one group of cast irons. Rather, there are four major subtypes of cast iron alloys. These are:

• Ductile cast iron: Contains nodules of graphite, making it more ductile than other cast irons while still having excellent strength properties
• Gray cast iron: Has flakes of graphite in it, which improves its machinability relative to other cast irons
• White cast iron: Has high amounts of iron carbides, making it very brittle but with a high degree of wear resistance
• Malleable cast iron: Essentially white cast iron that has been specially heat treated to transform the iron carbides into graphite nodules; it has similar properties to ductile cast iron

Properties of Cast Iron

The properties of cast iron have made it a material of choice for centuries in both industrial and domestic applications:

Strength

Despite its brittleness, cast iron boasts excellent compressive strength. It can support heavy loads without deforming, making it ideal for structural applications, heavy machinery, and supports in construction.

Hardness

The presence of carbon in the form of graphite or iron carbide contributes to the overall hardness of cast iron. This hardness makes it resistant to wear and abrasion, which is particularly beneficial for surfaces that experience frequent mechanical contact, such as gears and pistons.

Compressive Strength

Cast iron exhibits superior compressive strength, which is why it is often used for columns and posts in building construction and other infrastructure.

Heat Retention

Cast iron is highly effective at absorbing and retaining heat, making it a preferred material for cookware such as frying pans and baking dishes.

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 mould. During this subsequent remelting, the cast iron may undergo several metallurgical modifications by introducing alloying elements or heat-treating processes. Often, this is where cast iron falls into the specifications of one of the four groups mentioned above.

When molten, cast iron metal has better fluidity than steel and a low melting temperature. These two attributes make cast iron an ideal candidate for the casting process, which is partially why it is so popular. However, with improved technology for manufacturing and forming steel, the use of cast iron has diminished over the past several centuries.

What is Cast Iron Made Of?

Cast iron is an alloy primarily made of iron, with significant amounts of carbon and silicon. Other elements like manganese, sulphur, and phosphorus might also be present in smaller amounts. Here’s a more detailed look at its composition and characteristics:

• Iron: The base metal in cast iron is iron. It forms the bulk of the alloy and imparts strength and magnetic properties.
• Carbon: Cast iron typically contains between 2% and 4% carbon, which is much higher than in steel. The form of the carbon significantly influences the material’s properties. Graphite provides lubrication and makes the cast iron softer and more machinable, while iron carbide makes it harder but also more brittle.
• Silicon: Usually ranging from 1% to 3%, silicon is added to cast iron to help control the crystallization of graphite. It promotes the growth of graphite flakes during the cooling process, influencing the material’s strength and ductility.
• Manganese: Present in smaller quantities, typically around 0.5% to 1%, manganese helps to counteract the formation of iron sulphides, which can weaken the cast iron.
• Sulphur and Phosphorus: These elements are generally kept to a minimum in cast iron, but they can be present in amounts up to around 0.08% and 1%, respectively. Sulphur can make the iron more brittle, whereas phosphorus can increase fluidity and stiffness.

Where is Cast Iron Used?

Cast iron is used abundantly in applications that require high hardness and abrasion resistance and are less concerned about structural properties. This is because steel can generally be modified to have more desirable structural characteristics than cast iron for many projects. Although, before steel became more common, cast iron was somewhat popular for structural purposes. Cast iron is also quite affordable because of its basic alloying element requirements and low-cost production. Therefore, it is still quite common to see cast iron used around the world today. Here are some examples of cast iron uses, although it is important to know that these will vary depending on what one of the four types of cast iron metal mentioned above is selected:

• Brake discs
• Gears and gear plates
• Sprockets
• Chains
• Machinery
• Cookware
• Decorative architectural pieces
• Engine blocks
• Shafts and rods of various types
• Housings
• Mining equipment