Differences Between Cast Iron and Steel

What are the differences between cast iron and steel? The differences between steel and cast iron vary depending on the type of heat treatment applied and the proportions of elements in their composition.

Differences Between Cast Iron and Steel

Advantages of cast iron over steel:

Melts at lower temperatures,
Low cost,
Good castability,
High compressive strength,
Wear resistance.

Besides this, cast iron has a very important property — it cannot be forged — which should be understood as a downside.

While steels can be shaped hot, and some types cold, the fact that cast iron cannot be forged even when hot is due to its lack of plastic deformation ability while transitioning from liquid to solid state. In a sense, cast iron passes from a molten state to a solid state almost instantly. Cast iron is liquid above 1147°C, and solid below that temperature. This property means the plastic consistency needed for forging never develops in cast iron.

Cast iron is the general name we give to iron-carbon alloys containing between 2.11% and 6.67% carbon. As you know, steels are also obtained by alloying iron with carbon, but unlike cast iron, steels contain a much lower amount of carbon (less than 2.11%).

Another downside of cast iron compared to steel is that its toughness and ductility are lower than steel’s. For this reason, the uses of cast iron are limited.

Steels have a tensile strength ranging from 42 kg/mm² to 192 kg/mm². Steel, which has ductility built into its nature, is quite resistant to impact thanks to this. These properties of steels can be controlled to a large extent by varying the carbon content they contain. In essence, since steel is an iron-carbon alloy, its properties can be controlled by the amount of carbon it carries. Aside from this ability to be controlled in this way, steel can also be brought to the desired properties through another process, namely annealing/tempering.

For example, the difference between cast steel — a type of cast iron — and steel is that cast steel shows the same properties no matter which direction it’s tested in. This property isn’t seen in rolled steel, because the rolling process causes the steel to elongate as it’s compressed. Steel rolled this way, when tested lengthwise, comes out solid; when tested crosswise, it turns out to be in a condition that can easily break. Since there’s no such directionality in cast steel, using cast steel is the more sensible choice for workpieces where using rolled steel would be a mistake.

Difference in Carbon Content

Steel usually contains less than 2% carbon, while cast iron has a higher carbon content, between 2-4%.

Difference in Ductility

Steel, with its forgeable and shapeable structure, is much more ductile than cast iron.

Difference in Manufacturing Method

Cast iron is shaped by pouring into a mold, while steel is generally shaped by rolling or forging.

Related Questions

Cast iron doesn’t exhibit plastic deformation while transitioning from liquid to solid — it essentially solidifies all at once. For this reason, unlike steel, it can’t be shaped by hot forging; it can only be produced by casting.

A lower melting temperature reduces the amount of energy and equipment cost needed for casting. This makes cast iron more economical than cast steel for producing complex-shaped parts.