Also referred to as ductile cast iron, spheroidal graphite cast iron, or nodular cast iron, ductile iron is a type of cast iron known for its impact and fatigue resistance, elongation, and wear resistance due to the spherical graphite structures in the metal.
Hardness (Vickers): 183
Tensile Strength, Ultimate: >= 65000 psi (>= 448 MPa)
Tensile Strength, Yield: >= 45000 psi (>= 310 MPa)
Melting Point: 1149 degrees Celsius
In order to create ductile iron, more carbon is added than the iron would normally be able to absorb into the structure. Silicon, Sulphur, manganese, and oxygen mix with the carbon and the spherical graphite nodules form as the iron cools. To cast with ductile iron, the typical processes are then used.
Cast Iron and Ductile iron are very similar until you look at their microstructures under a strong microscope. Both have graphite flakes within, however the graphite flakes in gray iron are shaped like lines and the graphite flakes in ductile iron are more round in shape, hence the alternate names like spheroidal graphite cast iron and nodular cast iron.
There are many unique benefits to using ductile iron over other forms of metal. Most importantly, ductile iron is stronger than gray iron (cast iron). The tensile strength of cast iron is 20,000-60,000 psi while the tensile strength of ductile iron starts at 60,000 psi and can reach 120,000 psi. To better illustrate the strength difference, a gray iron casted part could fall a short distance and crack. However ductile iron could fall from the same distance and be hit with a sledgehammer repeatedly and it would not crack.
Ductile iron also holds the same enhanced lubrication attribute as gray cast iron. Ductile iron has better dry lubrication due to the graphite microstructures within its composition. This allows ductile iron parts to also experience less wear due to the enhanced dry lubrication. While these graphite microstructures can also be a con to gray cast iron, they are only a pro to ductile iron. The microstructures in gray iron are lines and this can cause fractures in gray iron leading to cracking and premature breakage. However, the microstructures in ductile iron are spherical allowing ductile iron to bend rather than fracture as the spherical microstructures work together to stay complete.
Ductile iron dampens vibrations, making ductile iron an excellent choice for use in large machinery. Ductile Iron also dissipates heat well and can be machined fairly easily.
Ductile Iron has many applications across many different industries.