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.
What makes Ductile Iron…“Ductile Iron?”
Both ductile iron and cast iron have graphite within. If you look closely (with a high-power microscope with 100x magnification or more) at regular cast iron (gray iron), you will see the graphite bits look like squiggly lines called “flakes.” When you look at the graphite in ductile cast iron, however, they look like little spheres or nodules (thus the spheroidal graphite iron and nodular iron names).
We give Keith Millis credit for creating ductile iron way back in 1943. He and his buddies Albert Gagnebin and Norman Pilling received US patent 2,485,760 and US Patent 2,485,761 for making ductile iron using magnesium (Mg) in the metallurgy (the metal composition or what was in the secret recipe) to get the graphite to line up into spheres.
Millis was not the first to strengthen normal cast iron. We still cast Meehanite® castings today. Augustus Meehan patented the Meehanite process back in January of 1931. Meehan used calcium silicide to also produce similar nodules to what is in ductile iron.
Yet it is ductile iron that has become one of the most popular types of iron casting. Development of ductile iron continued into the 1950s, making the process of ductile iron casting better leading to acceptance of ductile iron, acceptance proven out by the nine-fold increase in use during the 1960s as an engineered material for commercial applications.
How Ductile Iron is Made?
Most of the magic in making ductile iron happens in the furnace with the molten iron. You start with the iron (of course) and then add more carbon than the iron would normally be able to absorb into the structure. Explaining the iron-carbon relationship in another way, it is like adding so much salt to your water that you reach a point no more salt will dissolve. By the way, this is what makes ductile iron different than steel. Steel only has as much carbon as the iron can absorb.
Silicon, Sulphur, manganese, and oxygen all do their part in the mix to help the carbon to form into spherical graphite structures as the iron cools. 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? (Composition)
If you were to do a chemical analysis of ductile iron, this is what you would normally find:
Carbon 3.2 – 3.60%
Silicon 2.2 – 2.8%
Manganese 0.1 – 0.2%
Magnesium 0.03 – 0.04%
Phosphorus 0.005 – 0.04%
Sulfur 0.005 – 0.02%
Copper <= 0.40%
To improve the strength of ductile iron, additional tin or copper can be added. To improve corrosion resistance, copper, nickel, or chromium can replace anywhere from 15-30% of the iron.
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 a ductile iron casting, you can hit the part all day long with an eight-pound sledge hammer and it is not likely to crack.
What causes the problem for gray iron is those graphite flakes which encourage fractures along the flakes, while the nodules in ductile iron work at keeping the iron together. Given the exact same scenario of the same part made from the two different metals, while the brittle gray iron wants to crack, the ductile iron wants 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 like a dry lubricant on the iron.
Ductile iron also dissipates (gets rid of) 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 sound much better than steel would making ductile iron a good match for use on large machines.
What is Ductile Iron Used For? (Applications)
Ductile iron is great for use where you need strong metal with wear resistance.
Here is a sample list of things made from or contain ductile iron:
Pipe and pipe fittings (almost 50% of ductile iron sold in the US is for pipe and fittings)
Connecting rods (like in engines)
Disc brake calipers
Gears and Gear Boxes
Housings and manifolds
Piano Harps (the part that holds the strings of a piano)
Suspension system parts
Valves (especially high-pressure valves)
Yokes for power transmission
What is the difference between Ductile Iron and Cast Iron?
Cast iron refers to all iron parts that are cast and have a high carbon content, but in normal usage, “cast iron” refers to gray iron, the cast parts with weaker iron structures containing graphite flakes. Ductile iron must specifically have the spherical graphite structure in the metal.
Regular cast iron can be cast cheaply, machined very easily, and strength can be added through heat treating. Gray iron lacks the strength and durability of ductile cast iron. Ductile iron castings are not much more expensive than gray iron castings yet if there is no need for the benefits of ductile iron you should probably stick with the gray iron castings.
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