If you have any control of the plasma cutting process for these blanks....make sure it is a high definition class plasma using oxygen as the plasma gas and air as the shield. This combination of a high energy density arc, along with oxygen as the plasma gas virtually eliminates the chemically induced hardening that many air and oxygen plasma systems produce.
Air plasma, and conventional oxygen plasma systems allow nitrogen to impregnate the molten cut face.....this puts a nitride finish on the edge....which is extremely hard. The Hypertherm HPR series plasma is designed to minimize the plasma hardening effect....by keeping the N2 away from the edge. Attached is a picture of a 5/16" hole cut with an HPR260 plasma on 3/8" A-36 steel.....the hole was then tapped using a $3 Craftsmen tap.....
Jim Colt Hypertherm