I gave the Challenge knife my typical triple normalizing, triple hardening, triple tempering heat treatment that I use on 5160. Given the nature of some of the proposed tasks, I decided to hedge my bets and selectively temper the spine to a tougher spring temper.
To do this I used a trick I learned from Tim Lively: tempering tongs. I have a pair of tongs with 1" square bar welded to the jaws. I heat the jaws up, then use them as a heat reservoir as I pinch the spine of the blade and temper down the length of the blade, drawing the spine to a blue color and the edge to a straw. This method is how I heat treated all of my big blades before getting a kiln large enough to fit them in. Some folks do the same with a torch, but I find that the slower speed gives me more control, as well as letting the heat soak into the core of the blade and not just temper the outside.
I keep a soaking wet rag (one of my old gray T-shirts) handy to control the edge from getting too hot.
I'm getting a haircut tomorrow so I don't scare folks away from my table at the Blade Show next week!
I work my way from the base of the blade to the tip. The jaws have to be pretty hot for this to be effective, and I had to reheat the tongs three times. It takes longest at the base of the blade because you are bringing cold steel up to above 400 degrees. After that it goes relatively quickly.
I drew the very tip back to blue to make sure it didn't snap off under stress. Before that, I held the blade up and dropped it on the cement floor point-first. No damage to the tip.
At this point it's ready to sand down, wrap the handle, seal the cord, and sharpen.