The heat treating not only hardens the metal, it also makes it a more brittle. If the flat were brittle, you couldn't bend them the way we have to. So, I believe all flats come "soft".
I understand heat treating is pretty easy, though I've never done it. Read up on "quenching" here, in the archives and on the various other builder boards. Then head to Home Despot for a can of MAPP gas and a burner. They're not very expensive, < $35, IIRC.
HTH,
JakeA