I cut some 3/4" hardened steel rods with a 1-1/2" Dremel EZ-lock disk. It is a thin fiber re-enforced disk. It produced a bunch of sparks, but very little heat. I went in a little more than 1/8" deep all the way around to get past the surface hardening then finished it off with a hacksaw. I was probably grinding for nearly 10 minutes to make cuts on 2 rods and the cutoff disks are still nearly full size.

I also tried a 3" cutoff wheel with a cheap HF air tool. I got annoyed at how loud it was so I switched to the Dremel.

Steve