A while ago I asked an engineering forum a simple question about hardening a gear shaft - that is a shaft with gears cut into it. The answer was allong the lines of screwdrivers answer above, and it threw me a bit, so I did some research. It seems it's true, you have to know what the metal has been alloyed to become, so you can heat it to the correct temp. Also the quenching is important - air quench, oil quench, water etc.
However, my take on it - suck it and see if you can afford the time and loss of material (ie you have a few more to try).
My example was grinding a machined gear off a shaft, cutting the shaft off another gear, boring it out, tig welding the hollow gear onto the shaft then heat reating it. You guessed it, did the machining and tigging but not the hardening and testing. Project for another time.
However, my take on it - suck it and see if you can afford the time and loss of material (ie you have a few more to try).
My example was grinding a machined gear off a shaft, cutting the shaft off another gear, boring it out, tig welding the hollow gear onto the shaft then heat reating it. You guessed it, did the machining and tigging but not the hardening and testing. Project for another time.