Re: Hardened Parts
One detail the heat treating article doesn't mention is case hardening where the metal is rapidly heated to a shallow depth for a very short time. The interior of the part doesn't get hot and the surface of the part is quenched because the heat is transmitted very quickly to the interior. This can be done with a very hot flame (wouldn't want to try it), but an induction heating setup if far more controllable because it heats only within the skin depth. The skin depth is a function of the metals electrical resistivity and the frequency of the induction heater. So if you can control the frequency you can, within limits, dial in the depth to which the part is hardened. In some actions, (my memory says Mausers), the locking lugs on the bolt and maybe on the receiver are case hardened.