Difference Between Steel and Cast Iron

The key difference between steel and cast iron is that the steel is ductile and malleable whereas the cast iron is hard and has high compressive strength.

Steel and Cast iron are alloys or iron in which the main alloying element is carbon. These alloys are useful in many applications due to their increased desirable properties. One of the increased properties of steel and cast iron is that they are harder than iron. Because, the presence of carbon causes high hardness. Moreover, these alloys undergo heat treatments to impart desired properties. In iron-carbon alloys, carbon can exist in the forms of iron carbide and graphite. Hence, these forms and the different percentages of carbon vary the properties of the alloy.

CONTENTS

1. Overview and Key Difference
2. What is Steel
3. What is Cast Iron
4. Side by Side Comparison – Steel vs Cast Iron in Tabular Form
5. Summary

What is Steel?

In steel, the main alloying element is carbon, and the other elements are Manganese, Silicon and Copper. In fact, the steel contains up to 2% carbon, up to 1.65% Manganese, up to 0.6% Silicon, and up to 0.6% copper by weight.

We can classify steel as follows depending on the percentage of carbon in steel.;

• low carbon steel
• Medium carbon steel
• High carbon steel
• Tool steel

In steel, carbon is present as iron carbide. Steel is harder than iron, but because of the ductility in steel, it has the ability to change into different shapes by the application of forces. Moreover, steel melts at temperatures between 1325^oC and 1530^oC.

What is Cast Iron?

Cast iron is an alloy of iron that contains 2- 4% of carbon by weight. In this alloy, a higher silicon concentration (1-3% by weight) and a greater concentration of impurities are present. As a result, we can refer the cast iron alloys as Fe-C-Si alloys.

Further, we can cast this alloy easily into desired shapes on account of their higher fluidity, but it cannot work so due to brittleness. In this alloy, the presence of carbon is in the form of graphite or iron carbide or both. We can determine the form of carbon that it acquires by the cooling rate during solidification, the influence of other alloying elements, and heat treatments.

The melting point of cast iron ranges between 1130- 1250^oC. Furthermore, we can classify this alloy into different groups as follows depending on their composition and structure:

• White cast iron
• Grey cast iron
• Malleable cast iron
• Nodular cast iron
• High alloy cast iron

What is the Difference Between Steel and Cast Iron?

Both steel and cast iron are two forms of iron alloys. Cast iron is cheaper than most steels. Also, the melting temperature of cast iron is lower compared to steel, but it has high compressive strength, high hardness, and high wear resistance. Therefore, the key difference between steel and cast iron is that the steel is ductile and malleable whereas the cast iron is hard 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 the carbon in cast iron is in the form of graphite or iron carbide or both. Furthermore, cast iron has excellent fluidity, which steel does not have.

More details are shown in the below infographic on difference between steel and cast iron.

Summary – Steel vs Cast Iron

Both steel and cast iron are two forms of alloys of iron. However, there are several differences between the two forms. Aboveall, the key difference between steel and cast iron is that the steel is ductile and malleable whereas the cast iron is hard and has high compressive strength.

Reference:

1. Wondris, E.F., et al. “Steel.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 14 Dec. 2017. Available here 
2. Britannica, The Editors of Encyclopaedia. “Cast Iron.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 4 Aug. 2016. Available here   

Image Courtesy:

1.”safty-pins-fastener-restraint-steel-object-metal” by pics_pd  (CC0) via pixnio
2.”Cast-Iron-Pan” By Evan-Amos – Own work, (Public Domain) via Commons Wikimedia