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 casting starts at 60,000 psi and can go to 120,000 psi. The yield strength for ductile iron casting 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 casting 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 casting wants to bend.
Ductile iron casting also has what is called excellent wear resistance because of the graphite in the iron. When you have something rubbing against ductile iron casting, the ductile iron casting 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 casting also dissipates (gets rid of) heat very well and can be machined fairly easily, though ductile iron casting is harder to work with than regular gray cast iron. Ductile iron casting dampens vibration and sound much better than steel would making ductile iron casting a good match for use on large machines.