Have you considered CBN inserts.. not tried them myself but good for 65HRC (shop-apt.co.uk for example)

pgk

It is possible to turn hardened silver steel with carbide inserts or, even better, with CBN inserts. See here for results:

Turning Hardened Silver Steel

I've not tried turning HSS, but it's certainly possible to mill HSS with ordinary carbide cutters.

If a material is really tough then I'll use my cylindrical grinder.

Andrew

Whatever you use to cut the hard steel will give a very shiny finish is what I have found..

If you do a grinding set up protect your lathe bed by laying a cloth or paper.

Thoroughy clean after.

I use my Quorn grinding head sometimes in the lathe ,flooded
with neat cutting oil keeps the grinding wheel clean no special
covering on the machine as the grindings fall away in the coolant
stream onto the normal swarf protection and are then just
washed away.

John

Before using a die grinder check the play in the bearings. They are not intended for precision work so may flop around rather more than the depth of cut you are trying to achieve. Also you might want to look at the lathe bearings and slide movement or it rather defeats the object of the exercise. Doesn't necessarily apply if your only aim is to cut tough materials rather than achivee the precision and finish of normal grinding intentions.