im just a hobby knife maker ive seen a few dozen how to forge weld videos i would like to try a rasp hatchet but i have no idea if i can with charcoil and forced air i dont know how hot it needs to be google image ...
First, let's qualify this....what type of charcoal are we talking about? If it's BBQ briquettes, then it's a waste trying, all you'll do is shower yourself with cinders and ash. If it's "lump" charcoal, then yes, I used lump charcoal, that I made myself, for several years.
Unless you have a valid way to measure the temp, a specific number would be useless, however, it will be bright yellows range as far as color. That's another that needs qualifying....the heat could be higher or lower, depending on the material/steel.
Rasps, such as in the pic are notoriously difficult to forge weld, especially is you don't remove the teeth on the mating surfaces. There's also the fact that it's rare to find a rasp these days that isn't case hardened (mild steel with a surface hardening treatment). If you happen to use a case hardened rasp, it likely won't weld, and the end product would be pretty much useless....... but hey.....you could consider it "practice"
I've personally never tried the commercially available lump charcoal (always made my own), but if it's NOT the compressed briquettes, it should work. Be aware that it takes a LOT of charcoal. When I was using it, I would go through approx 3/4 of a 55 gallon barrel in a day of forging.