Aluminum Oxide is most commonly used for grinding and cutting ferrous
Silicone carbide is most commonly used for grinding and cutting
I was told by one guy at Norton Abrasives that he was once told that the
reason for that was because when you're grinding ferrous metals, the
carbon in silicone carbide can be absorbed into the iron, increasing
it's carbon content, and thereby making the iron near the cut harder and
more difficult to machine. So, when grinding iron or steel, he was told
they used aluminum oxide to avoid that problem.
But, it turns out that he was misled. Apparantly, the absorbtion of
carbon from silicone carbide abrasives has nothing to do with it. But,
even though the reason given was wrong, he found that to be a good way
to decide what abrasives to use on what.
So, when in doubt, think about the carbon in the silicone carbide, and
know that the only difference between iron, steel and cast iron is the
amount of carbon in the metal, and so you don't want to change the
machining characteristics of your iron based metal by using silicone
carbide on it.
That is, wrong reason, but it still gives the right answer when it comes
to choosing grinding wheels.