Knowledge

What Is Ductile Iron?

Ductile Iron is a type of cast iron known for its impact and fatigue resistance, elongation, and wear resistance due to the spherical (round) graphite structures in the metal. Ductile Iron is also called ductile cast iron, spheroidal graphite cast iron, or nodular cast iron.

How Ductile Iron Is Made?

Carbon is added to a molten cast iron bath during the smelting process. More carbon than the iron would normally be able to absorb into the structure is added. Explaining the iron-carbon relationship in another way is like adding so much salt to your water that you reach a point no more salt will dissolve. This is what makes ductile iron different than steel. Steel only has as much carbon as iron can absorb. In addition to the carbon, various other elements are added to create the desired solution. Silicon, Sulphur, manganese, and oxygen all do their part in the mix to help the carbon form into spherical graphite structures as the iron cools.

The most important element in the process, however, is Magnesium, which assists in the production of spherical graphite nodules instead of flakes. According to Millis, this was a surprising interaction. In addition to effectively promoting this process, it is also very effective in lowering the sulfur content of iron. Reducing the sulfur content is important since sulfur makes iron and steel more brittle.

This, admittedly, is an oversimplification of the process (after all, we are not training you for a metallurgical degree, though if you want one, we like the Engineering Degrees found in Wisconsin, don’t mind our geographic bias).

What Is In Ductile Iron?

As stated above, there are a variety of elements that are found in ductile iron. Metallurgists study the effects of different proportions of these elements to determine their effect on the qualities of the iron they produce. Each foundry has slightly different combinations that they provide. For instance, to improve the strength of ductile iron, additional tin or copper can be added. Additionally, if you wish to improve corrosion resistance in iron, then copper, nickel, and/or chromium can replace anywhere from 15-30% of the iron.

There are a few mainstays, however. If you were to do a chemical analysis of ductile iron, this is what you would normally find:

What Are The Benefits Of Ductile Iron?

Ductile iron castings are very strong compared to regular cast iron (gray iron). The tensile strength of cast iron is 20,000 – 60,000 psi while ductile iron starts at 60,000 psi and can go to 120,000 psi. The yield strength for ductile iron is generally 40,000 – 90,000 psi but the yield strength of cast iron is so low it is considered not measurable.

Let us put strength in a different way. We have seen gray iron parts break when they hit the ground after falling ten feet. With ductile iron casting, you can hit the part all day long with an eight-pound sledgehammer and it is not likely to crack.

What causes the problem for gray iron is graphite flakes which encourage fractures along the flakes, while the nodules in ductile iron prevent it from fracturing along those lines. Given the exact same scenario of the same part made from the two different metals, the brittle gray iron is more likely to crack, while the ductile iron is more likely to bend.

Ductile iron also has what is called excellent wear resistance because of the graphite in the iron. When you have something rubbing against ductile iron, the ductile iron wears away much slower than many other metals. The wear resistance is partly from the graphite structures that can act similarly to dry lubricant on the iron.

Ductile iron dissipates heat very well and can be machined fairly easily, though ductile iron is harder to work with than regular gray cast iron. Ductile cast iron dampens vibration and sounds much better than steel would make ductile iron a good match for use on large machines.

What Is Ductile Iron Used For?

Ductile iron is great for use where you need strong metal with wear resistance. Here is a sample list of things made from or containing ductile iron:

Pipe and pipe fittings (almost 50% of ductile iron sold in the US is for pipe and fittings)

Axles

Connecting rods (like in engines)

Crankshafts

Cylinders

Disc brake calipers

Gears and Gear Boxes

Housings and manifolds

Hydrostatic Barrels

Idler arms

Large Machines

Machine Tooling

Military uses

Piano Harps (the part that holds the strings of a piano)

Spindle supports

Steering knuckles

Suspension system parts

Truck axles

Valves (especially high-pressure valves)

Wheel hubs

Yokes for power transmission