Yes, they are already heat-treated and hardened to RC 58+, depending on blade type (plain carbon blades, laminated blades, or stainless). The cores of the laminated blades are apparently taken up to RC 60-61 hardness, with the plain carbon and stainless (Sandvik 12C27) blades at RC ~58-59 or so. Should all be very good, regardless of which blade you choose.
David