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Dec. 03, 2020
What Is Ductile Iron?
Ductile iron is a type of cast iron that is characterized by impact and fatigue resistance, elongation, and wear resistance due to its spherical (circular) graphite structure in the metal.
Ductile iron and cast iron both contain graphite. If you look closely (using a high magnification microscope of 100 x or higher) on conventional cast iron (gray iron), you will see that the graphite bit looks like curved lines called "flakes." However, when you look at the graphite in ductile iron, they look like little spheres or nodules (hence the name of nodular graphite iron and ductile iron).
However, ductile iron has become one of the most popular types of cast iron. The development of ductile iron continued into the 1950s, leading to better development of the casting process and thus acceptance of ductile iron. The use of engineering materials for commercial use increased nine-fold in the 1960s, demonstrating the acceptability of ductile iron.
Most of the magic of making ductile iron occurs in the furnace with the molten iron. You start with iron (of course) and then add more carbon than iron would normally absorb into the structure. Explaining the iron-carbon relationship in another way is like adding so much salt to the water that no more salt dissolves. This, by the way, is what makes ductile iron different from steel. Steel absorbs only as much carbon as iron can.
Silicon, sulfur, manganese, and oxygen all play a role in the mixture, and as the iron cools, it helps the carbon form a spherical graphite structure.
Ductile iron is very strong compared with ordinary cast iron (gray cast iron). The tensile strength of cast iron is 20,000 to 60,000 pounds, while ductile iron starts at 60,000 pounds and can go up to 120,000 pounds. Ductile iron usually has a yield strength of 40,000-90,000 psi, but the yield strength of cast iron is so low that it cannot be measured.
Let's use our power in a different way. We have seen that gray iron parts can break when they fall ten feet and hit the ground. With ductile iron castings, you can hit parts with an eight-pound octagon hammer all day and never break.
The cause of the gray iron problem is that the graphite scales contribute to the fracture of the graphite sheets, while nodular nodules in ductile iron hold the iron together. Given the exact same case of the same part made of two different metals, brittle gray cast iron wants to crack and ductile cast iron wants to bend.
Because of the graphite in ductile iron, ductile iron also has excellent wear resistance. When you rub on ductile iron, ductile iron wears away much more slowly than many other metals. The wear resistance comes in part from the graphite structure, which acts on iron like a dry lubricant.
Ductile iron also dissipates heat very well and can be machined easily, although ductile iron is more difficult to process than ordinary gray cast iron. Ductile iron has better damping vibration and sound effects than steel, making it ideal for use on large machines.
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