Not everyone who can harden a knife blade correctly, can be counted on to do the same with a razor. The use of A-2, air hardened, is so attractive for avoiding distortion that it begs the question "Why aren't they all doing it?" I think Mike Blue is correct in saying it becomes adequately hard, but this is something I would advise checking from specifications before tying up time and trouble in one.

The best air-hardening comes from cold, moving air. It is sometimes hardened in oil, and a possibility worth trying on a quickly made test piece is dipping the thin edge in oil while holding the spine out of it. I don't believe this could cause a frowning curve of the spine, but it will put the edge under tension, and reduce any tendency for it to become wavy.. The only thing I can imagine going wrong is the edge cracking with a sharp spink.

Japanese swords have a hamon pattern because they need the edge to be hard, but the body of the blade to be tough and resilient, and resist chopping, parrying and waving around. I can't see any functional reason for it in a razor.