Two rules of thumb:
1) penetration depth in steel in mm = 400/sqrt(frequency).
3) use at least 3 penetration depths to avoid flux cancellation in the center of the workpiece and avoid the problem of temperature rise limited to the Curie point.
These are general rules of thumb that...
Here is a dissection of the Mini-ductor.
http://www.johndearmond.com/2010/01/08/mini-ductor-induction-heater/
The Mini-ductor II is, to the best of my knowledge, the same as the original but it has a better case than the original's PVC plumbing pipe.
It is a Royer oscillator type heater...