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The Difference between Cast Iron and Ductile Iron

Some customers asked us about the difference between cast iron and ductile iron, including their advantages and disadvantages. Herein, we listed their main properties and hope to give you a reference for material selection.

Actually, before comparison, we need to clarify a definition. The cast iron is meaning a category of iron material, whose carbon content is higher than 2.1%. Therefore, it includes many different types according to different graphite shapes and microstructures, such as grey iron, ductile iron, white iron, malleable iron, vermicular iron, austenitic iron etc.

Therefore, strictly speaking, you can not compare cast iron with ductile iron, because ductile iron is just a type of cast iron. However, on the drawings, you may find the material requirements just written as "Cast Iron", and no other requirements. Under this circumstances, according to our experience, the designer's meaning was "grey iron".

Why we say so? Because grey iron is the most common cast iron material, and its production cost is the lowest one compared with all other cast iron materials. So, if the customers just required to use "cast iron", then the grey iron will be the best choice.

Ok, so let us compare ductile iron with all other cast irons.

Advantages:

Ductile iron is also very common and good material worldwide. It has the following advantages:

1. Mature production technology.

Many iron foundries could produce ductile iron very well, their physical properties could be controlled strictly. Grey iron also has this advantages, most of iron foundries could produce grey iron very well too, but as for other cast iron materials, not many foundries could produce them.

2. Low prices.

Prices are always an important issues for customers. Ductile iron just needs some extra nodular agent materials than grey iron, so its production costs are only some higher than cheapest grey iron, but far lower than malleable iron and austenitic iron. This is also the main reason why ductile iron is replacing malleable iron.

3. Excellent mechanical properties.

Ductile iron has some good mechanical properties than other cast irons. For example, its highest tensile strength could reach 900 Mpa, the largest elongation could reach 18%, moreover, its has good impact property, even good impact property in low temperature condition. Moreover, after suitable heat treatment, its physical properties could be improved significantly.

Disadvantages:

No any material is perfect, ductile iron also has some disadvantages.

1. Comparatively high production costs.

Although we said ductile iron is not expensive, its prices are not the lowest. It is still more costly compared with grey iron, because of extra spheroidizing agent alloys, and pig iron of high quality. So, if grey iron can meet your requirements and application, then you should not choose ductile iron.

2. Comparatively high requirements to quality control.

As for grey cast iron, the main inspection to material quality is just tensile strength, only if you have special requirements to the hardness. So, even the traditional cupola furnace could produce this material very well.

However, ductile iron needs to inspect tensile strength, yield strength, elongation, hardness, nodular rate (spheroidization rate). Therefore, it requires the ductile casting manufacturers to have enough experience and equipments to produce the castings, and inspect their quality in-house.

3. Can not meet some special physical requirements.

Although ductile iron has excellent mechanical properties, but it can not cover all aspects. For example, compared with ductile iron, grey cast iron has better resistance to deformation during heating, so most stove and boiler parts were made by grey iron. Moreover, as for some alloy irons, they will have better properties, such as anti-heating, wear-resisting, corrosion-resisting. Malleable iron has better ductility, and austenitic iron has higher tensile strength.

Herein, let us briefly introduce the main differences between ductile iron and grey iron.

  • Ductile iron has higher tensile strength, yield strength and elongation.

  • Grey iron has lower prices, and easier to be produced.

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