Difference Between Ductile Iron and Cast Iron

The key difference between ductile iron and cast iron is that we use ductile iron for water pipes due to its durability while we use cast iron usually for engineering and construction structures because of its stability.

Ductile iron and cast iron are useful in the metal industry on an everyday basis. However, the two alloys have different characteristics that result in them being important for various purposes. Both these are alloys of iron. An alloy is a mixture of two or more metals that gives improved properties for different uses.

CONTENTS

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

What is Ductile Iron?

Ductile iron is an alloy of iron, which is rich with graphite. Therefore, it is a form of cast iron. Keith Millis discovered this alloy during the mid-1940’s. He made it through incorporating the ferrous alloy with magnesium treatment. This alloy is important than most of the cast iron forms due to its high impact and fatigue resistance. This property arises due to nodular graphite inclusions.

When considering the composition of this alloy, major constituents are iron, carbon (3.2 to 3.6%), silicon, manganese, magnesium, etc. Sometimes we add chemical elements such as copper or tin in order to increase the tensile and yield strength of the alloy. It also reduces the ductility simultaneously. Apart from that, we can increase the corrosion resistance by adding some nickel or chromium to iron.

When considering the metallurgy of ductile iron, it shows the structural alteration of graphite, since, in the development of this material, the graphite creates spherical nodules that restrict the development of fissures thereby resulting in increased malleability. The major application of this alloy is to produce ductile iron pipes. We use these pipes for water and sewer lines.

What is Cast Iron?

Cast iron is an alloy of iron that we can readily cast in a mould. It is hard and relatively brittle. It contains iron, carbon, silicon, manganese along with trace amounts of sulfur and phosphorous as well. The amount of carbon in this alloy is very high compared to steel. In addition, it has a considerable amount of silicon (1-3%); thus, it is actually an alloy of iron-carbon-silicon. Moreover, it has a relatively low melting temperature than other iron alloys.

In its solidification, this alloy solidifies as a heterogeneous alloy. It is not that much ductile; thus is not suitable for rolling. Apart from that, it does not react with the moulding material when melted and poured. The major reason for the usefulness of this alloy is its low melting point. This low melting point causes good fluidity, castability, excellent machinability, resistance to deformation and wears resistance.

There are several types of cast iron according to the microstructure of the alloy. Those types are as follows;

• Grey cast iron
• Ductile cast iron
• Malleable cast iron
• White cast iron

We use cast iron usually for engineering and construction structures because of its stability. Therefore, it is useful in pipes, machines and automotive industry parts, such as cylinder heads (declining usage), cylinder blocks and gearbox cases. Moreover, it is resistant to destruction via oxidation.

What is the Difference Between Ductile Iron and Cast Iron?

Ductile iron is an alloy of iron, rich with graphite whereas cast iron is an alloy of iron that we can readily cast in a mould. When considering the composition of each iron alloy, the ductile iron contains iron, carbon, silicon, manganese, magnesium, etc. along with copper or tin while the cast iron contains iron, carbon, silicon, manganese along with trace amounts of sulfur and phosphorous as well. Furthermore, their carbon and silicon contents also differ from each other; ductile iron contains about 3.2 to 3.6% carbon, and cast iron contains about 2 to 4% carbon while the silicon contents are 2.5% and 1-3% respectively. Moreover, a significant difference between ductile iron and cast iron is their ductility. Ductile iron is highly ductile whereas cast iron is less ductile.  We can say the key difference between ductile iron and cast iron as their respective uses due this property.

The below infograph presents in detail the difference between ductile iron and cast iron.

Summary – Ductile Iron vs Cast Iron

Ductile iron is a form of cast iron having improved properties. The difference between ductile iron and cast iron is that we use ductile iron for water pipes due to its durability while we use cast iron usually for engineering and construction structures because of its stability.

Reference:

1. “What Is Ductile Iron?” Willman Industries. Available here  
2. “Cast Iron.” Wikipedia, Wikimedia Foundation, 9 Sept. 2018. Available here

Image Courtesy:

1.”DuctileIronConcreteLined”By Yeti Hunter (CC BY-SA 4.0) via Commons Wikimedia 
2.”Cast fe and cu dwv piping”By Achim Hering – Own work, (CC BY 3.0) via Commons Wikimedia