Ductile iron hasn’t been around nearly as long as its cousin, cast iron. It was developed as recently as 1943 by Keith Millis. Since then, ductile iron unique properties have allowed it to be used in a number of different applications, making it the most popular type of iron in the world.
What are those properties? What about ductile iron makes it different? First, it is important to understand that ductile iron is not solid metal, it’s an alloy of several different metals.
Those metals are:
For our purposes, carbon is the metal to focus on. Ductile iron gets its unique properties (more on those below) from the way carbon behaves within the iron. Carbon actually takes it’s crystalline form of graphite, organizing itself is spherical nodule inclusions. That’s a fancy way of saying that the carbon forms little graphite balls embedded throughout the iron. In cast iron, the graphite organizes itself in layers, which are observable as flakes. The result is that cast iron is brittle and can more easily fracture along those flakes. Ductile iron, on the other hand, thanks to those tiny graphite balls, is more likely to bend than to break.
That brings us to those important properties, the ones that made ductile iron such a big deal in the first place. Those properties are:
High tensile strength – Tensile strength is the degree of stress a material can withstand before breaking. This makes it useful for applications such as crankshafts or building materials.
Good elongation – Elongation refers to the amount a material can actually be bent and stretched. While tensile strength is a measure of the amount of stress a material can sustain, elongation is a measure of how much the material itself can actually be stretched.
High elastic limit – The elastic limit of a material refers to how much a material can be stretched before it undergoes plastic deformation. Think of it as an elastic waistband. Stretch it a little bit and it returns to its original shape and size. Stretch it too far and it simply stays stretched out, losing the ability to snap back.
Impact-resistant – Because of the above properties, ductile iron is extremely resilient. Drop it, run a car into it, hit it, it’ll be fine. Like Rocky, it can take a ton of punishment and keep coming back for more.
Normally in life, there is always a trade-off. To get one thing, you have to give up something else. Not so with ductile iron. In gaining all the above properties, ductile iron sacrifices none of the attributes of cast iron. It is still highly compression-resistant, easy to machine into a variety of shapes and sizes, still able to cast in a mold, very resistant to abrasion and can undergo vast amounts of repetitive stress without failing (fatigue strength).
This perfect storm of properties has to make ductile iron ideal for a variety of industries, from construction to agricultural; in applications as from pulleys to pipes. Should specific characteristics (even greater strength, elasticity, etc.) be required it is easy to customize the alloy by varying the percentages of its other components, ensuring that ductile iron will form the backbone of several industries for many years to come.