Ah yes, small stuff then :-).
Of course, things become less critical if all you want to do is warm up to hardening and tempering temperatures and you could even arrange for water quench down through the centre of the coil, which I think is how the pro gear works.
The really interesting property is the utilisation of skin effect, which means you can selectively harden and temper outer surfaces only, but operating frequency needs to be much higher than a few Khz to benefit from this...
Chris