Just an observation Fox. What I'd try to do is work the blade out slowly and keep it cool. I know, you're saying to yourself; who's this jerk and how am I supposed to keep it cool at the forge? I started out knifemaking in stock removal. When you get steel like that, it's best to use it in it's current form. In other words use it for making shop knives or tanto style blades that conform to the shape of the planer blade.
If you grind it on a stone when shaping, dip it in the water as soon as it starts to get warm to the touch and use light pressure. If you luck out, you won't pull the heat treating from it in the process. A dripping rig on your grindstone is even better if you can stand the mess. Yes, it's obvious I had alot of time on my hands once. Now, I don't fool with stuff like that. Free steel can cost you alot sometimes.
It's just a pain to work with heat treated blade material because it takes longer and wears out your sanding belts and grindstones faster. Having said all that I'll say this; It's a big waste of time unless you have lots of time on your hands. Get some factory fresh stuff that's already annealed.
