Gray Iron

Grey Iron has a few major differences than ductile

First the chemistry
gray iron has more of the following elements
-manganese
-Sulfur (bonded with pyrite)
-Silicon
-Chrome

Grey Iron

...hmmm
Less Carbon
....and
a dash of titanium

Polished up it looks like this



In a foundry gray iron gets poured much faster than the ductile iron because it can be melted faster and it needs less additions before it gets poured into a cast



As you can clearly see there are no nodules present. The addition of magnesium right before the iron gets poured forces the iron to form into nodules. This makes the iron much stronger in tensile and hardness aspects.
Gray iron is much better for absorbing and redistributing heat...so it still has plenty of uses.