The HRC scale is Rockwell scale C, which covers what we'd regard as hardened steel. It is arbitrarily based on the depth of impression made with a diamond tip under a load of, I think, 150kg. There is also a Rockwell B scale, which finishes where C starts. There are many other scales, eg, Brinell and VIckers, but they are all based on indentation of an anvil under a given load.
It is possible to machine hardened steel (>65Rc) with carbide inserts, but they look a bit sad afterwards. I don't think I'd want to try using a insert parting off tool. Using CBN inserts makes turning hardened steel a breeze, but they're expensive.
Andrew