Choosing between ductile iron and traditional cast iron

Ductile iron is slightly more expensive than gray iron but is cheaper and easier to cast than steel. It is usually chosen for its mechanical properties and value. As a less brittle alloy than cast iron, it is used in applications where ductility and impact resistance is useful. It is better than steel at vibration damping and compressive yield, although gray iron is still superior for damping.

Bollards engineered for impact resistance often use ductile iron. It’s the major iron alloy used for pipes, especially those under pressure. Ductile parts can be found in automotive components, pumps, and cable casings where impact is likely.

Gray iron is still an important and well-used alloy for applications where traditional cast iron’s brittleness is not a problem. Items that won’t be exposed to impact in the regular course of wear are often made of gray cast iron. Cast iron frying pans are usually grey iron. Hardscape, like tree grating, trench grating, and manhole covers are also often gray iron. Superior vibration damping makes gray iron an excellent machine base. It’s also the right choice for brakes or engine components that will not be struck but will need to deal with high vibration.

With any project, it’s important to talk to a metallurgist or engineer before choosing an alloy. They will examine the working stress on the component and help choose materials that can safely manage the application. Picking ductile iron vs. cast iron can sometimes just come down to value. Other times, the specific qualities of the alloy are what’s needed.

Ductile iron is a good choice for an item that might deal with the impact force of a vehicle.

Furthermore, what are the differences between ductile iron and cast iron?

Ductile iron is less brittle than other cast iron, even before heat treatment. It does not fracture as easily with impact. Being ductile allows the iron to bend. In comparison, gray cast iron is harder. This hardness means that it manages surface wear well. Gray iron is also better at vibration damping. The differences are due to the microstructures of graphite within these iron alloys.

Gray cast iron is 2.5–4% carbon and 1–3% silicon by weight. The silicon in the alloy is necessary to stabilize the graphite molecules that give gray cast iron its properties. However, this silicon is only effective in supporting and maintaining the graphite if the metal does not undergo thermal shock while cooling. Very rapid cooling will lead to the formation of cementite and make the gray iron into white iron, even with silicon in the mix. In gray cast iron, flakes of graphite are embedded through the surface and are visible in highly-polished gray iron.

Ductile cast iron has 3.2–3.6% carbon, 2.2–2.8% silicon, and some small percentage of a “nodulizing element.” Discovered in 1943, ductile had many of the same properties as malleable iron, only it had these properties right out of the mold. The long and technical heat treatment necessary to make malleable cast iron made it more expensive and prone to error. Ductile was an obvious solution. (Malleable is still used in thin castings where ductile cools too quickly and produces carbides.)

Like gray cast iron, graphite is an important part of ductile iron’s microstructure.  However, the nodulizing element—like magnesium, cerium, or tellurium—shapes the graphite molecules into spheres rather than flakes. These spheres slide past one another rather than creating planes along which the iron can fracture. The nodules make the ductile iron more flexible, and less hard. Where surface hardness is needed, heat treatment can be used.