You don't really have to do any more heat treating to your steel, it's done. But be careful if you intend to grind it into a knife blade. Don't let it get hot or you'll lose what little bit of hardness you have. I mean you should be able to touch the blade without it hurting at any point. If you're going to use a file, on the other hand, just be prepared for a lot of work; hardened steel is difficult to cut.
AR-400 is meant to be used for bucket teeth and other hard-wearing parts on heavy earthmoving equipment. However, that 48 HRC hardness won't go far as a knife blade. (You probably already know this, but most knives are in the 55-62 HRC range.) Your edge won't hold for long so you'll be sharpening constantly. Just a warning.
-Allin
