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Ductile Iron vs. Cast Iron

How engineers choose an alloy for metal castings

July 31, 2019

A glowing crucible of molten iron is being cast in a foundry
When choosing a cast iron alloy, designers may choose gray, white, ductile, or malleable iron.

When choosing an iron alloy for a casting, a metallurgist considers the budget, required mechanical properties, and post-casting steps like machining and heat treatment. These requirements decide what alloy to use. These days, the biggest choice is between ductile iron vs. cast iron.

Technically, “cast iron” suggests iron alloys that are cast in a foundry. Ductile iron is one such alloy. However, ductile is a relative newcomer on the scene, with unique mechanical properties that set it apart from the other iron alloys. This difference means that “cast iron” usually specifies gray or white iron alloys that have been part of metalsmithing for centuries.

What types of iron are cast in the foundry?

  • Ductile Cast Iron: This type of iron was developed in 1948 and has become a very important alloy for ferrous foundries. It is much less brittle than other cast irons.
  • Malleable Cast Iron: Before the invention of ductile iron, this alloy was more popular. It is a white cast iron that is heat treated for a very long period. Conditions in the heat treater must be very controlled. When the malleable iron is finished, it is a lot less brittle. Malleable is still used for small, thin castings where ductile is less successful.
  • White Cast Iron: This alloy is cooled more quickly than other cast irons, producing a molecule called cementite in its lattice. It is a brittle alloy but has excellent hardness and abrasion resistance. It’s often used in bearings and other high-friction applications.
  • Gray Cast Iron: Most standard “cast iron” items are made from gray cast iron, so when people use the term, this is likely the alloy that is meant. Unlike white cast iron, gray iron has a graphitic microstructure. It has excellent vibration damping capacity and great machinability.

White and gray cast irons create very similar looking castings. Their coloring is only evident when they fracture.

A row of bollards sits outside a beautiful stone home
Ductile iron is a good choice for an item that might deal with the impact force of a vehicle.

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.

A cast iron detectable warning plate warns of an impending slope
Gray irons are hard and wear-resistant: a great choice where impact forces are human, not mechanical.

 

Comparing ductile iron, gray iron, white iron, and malleable iron
Ductile Iron (ASTM A536)
Gray Iron (ASTM 40)
White Iron
Malleable Cast Iron (cast white iron, annealed)

Castability

Standard castability: much easier than steel casting

Yield Strength

50k psi

33k psi

Elongation %

18

0.5

0

12

Tensile Strength

72k psi

40-50 k psi

25 k psi

52 k psi

Hardness [Brinell]

130-217

260

450

130

Corrosion Resistance

Will rust, but can develop a protective patina in outdoor environments.

Thermal expansion 20C

6.46*10-6 in/(in* ºF)

6.46*10-6 in/(in* ºF)

5.0*10-6 in/(in* ºF)

6.6*10-6 in/(in* ºF)

Relative damping (log ratio of successive amplitude)

5-20*104

20-500*104

2-4*104

8-15*104

Cost

$$

$

$

$$$

Machinability

Medium

Good

Good

Medium

Weldability

Low-Medium depending on procedure

Medium

Not weldable

Welding is possible but converts some of the malleable iron to grey iron

A close-up photograph of the ornate side of a weathered cast iron bench
A bench with details made from traditional gray cast iron.

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.

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