Ductile iron is a type of cast iron with characteristics similar to steel. Ductile iron, alternatively called spheroidal graphite iron, spheroidal graphite cast iron, nodular cast iron, or SG iron, is a graphite alloy of iron and carbon. This material possesses ductile properties, as the name implies, whereas other types of cast iron are brittle.
Ductile iron has exceptional strength, durability, flexibility, and deformability due to its unique microstructure. It possesses mechanical attributes similar to steel.
Metallurgy of Ductile Iron
The key distinguishing feature of ductile iron is the shape of graphite in the microstructure. The graphite in ductile iron is present in nodular or spheroidal form, which increases the material’s ductility. Various nodulizing components, including magnesium, ferrosilicon, cerium, tellurium, and others, are added to ductile iron to create nodular graphite.
Ductile Iron Composition
In most cases, ductile iron is composed of 3.2 to 3.6 percent carbon, 2.2 to 2.8 percent silicon, 0.1 to 0.2 percent manganese, 0.03 to 0.04 percent magnesium, 0.005 to 0.04 percent phosphorus, 0.005 to 0.02 percent sulfur, and the remaining (approximately 94 percent) iron. Copper can also be added to ductile iron to boost its strength at the cost of ductility. The corrosion resistance of ductile iron can be enhanced by adding chromium and nickel. Aluminum can also be used instead of silicon to give superior oxidation protection.
ASTM Specifications of Ductile Iron Castings
The ductile iron castings are covered by five ASTM specifications. ASTM A536 is the most extensively used specification, covering the general engineering grades of ductile iron, whereas other specifications cover austenitic and special grades. The ASTM standards that apply to ductile irons are listed below:
- ASTM A395 – Ferritic Ductile Iron Pressure-Retaining Castings
- ASTM A439 – Austenitic Ductile Iron Castings
- ASTM A476 – Ductile Iron Castings for Paper Mill Dryer Rolls
- ASTM A536 – Specification for Ductile Iron Castings
- ASTM A571 – Austenitic Ductile Iron Castings for Pressure-Containing Parts Suitable for Low-Temperature Service
- ASTM A716 – Ductile Iron Culvert Pipe
Ductile iron vs Cast iron: Differences between Ductile Iron and Cast Iron
Ductile iron is superior to cast iron or gray iron. The following are the primary distinctions between ductile iron and cast iron:
- Ductility: Ductile iron, as the name implies, is ductile, whereas cast iron is brittle. As a result, ductile iron does not break abruptly due to impact force. Cast iron cannot be bent, but ductile iron behaves in the other way.
- Microstructure: Ductile iron comprises nodular graphite, whereas cast iron has graphite flakes in the microstructure. These nodules increase ductile iron’s flexibility while decreasing its hardness.
- Cost: Ductile iron is slightly more expensive than cast iron in terms of cost.
- Physical and Mechanical Properties: Ductile iron has superior tensile and yield strength, elongation, and impact resistance than gray iron, but has lower heat conductivity and vibration-damping capability. Cast iron is more corrosive than ductile iron.
Ductile Iron vs Steel: Differences between Carbon Steel and Ductile Iron
There are numerous distinctions between carbon steel and ductile iron. Let’s compare carbon steel and ductile iron.
- Compositions: Carbon steel has a lower carbon content (2%), compared to ductile iron (> 3%). In addition, the carbon in ductile iron is graphite.
- Castability and Welding Characteristics: Carbon steel is more weldable than ductile iron. However, ductile iron offers superior castability over carbon steel.
- Impact and Shock Resistance: Carbon steel is more impact-resistant. However, ductile iron is more resistant to shock and abrasion than carbon steel. Ductile iron has a higher corrosion resistance than unalloyed carbon steel.
- Cost: Carbon steel is more expensive than ductile iron.
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