Temperature is typically controlled by using welded-on temperature probes, connected to the induction source.

Thanks Kingnero,

kindly advice how does the probe functions if I use two material grade axles of different austenitising temperature or two distinct diameter 20mm bars and 40mm bars? Does it position itself to that austenisation temperature automatically or forces us to change any of power source input parameters to land us in austenisation temperature?

You will want to change power supply settings for different size bars, and maybe different alloys (temperatures).
Part of this is to tune in order to get more efficient coupling and part of it is to control depth of heating.
After all if you are going from 20mm to 40mm you will likely be changing coils, and the settings to go with it.

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P.E. Metallurgy, consulting work welcomed

Thanks @Edstainless:

We do change coil for 20mm to 40mm and the point is we keep same power and respective part to coil gap maintained 2mm for both dia 2mm. The question is how it does can able to attain austenisation temperature for all diameters for same power keeping coil to part gap same. We dont even alter the frequency tapping as well. We are getting martensitic structure, i just want to know the empirical relation which constantly producing the required austenisation temperature irrespective of diameter of bars we used?

If you want an empirical relationship you will need to develop it.
You should be changing taps (freq and/or capacitors) in order to balance with each coil.
The balance between the req, power, coil fit, and time has a lot of variables that depend on your own setup.

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P.E. Metallurgy, consulting work welcomed

@Edstainless thanks for your feedback,

I believe i didnt convey my question properly.

How does my coil produces a austenising temperature in the part exactly why not more or less than austenisation temperature?

Which factor decides in power source that when temperature raises it is sufficient enough to be austenisation temperature and not to create skin effect or insufficient austenisation.

Because when i do validation i have done trials for 40% power 50% power and 60% power on all cases just case depth reduces/increases but microstructure remains martensite which means on all 3 power ranges austenisation did occured.

Hope my query makes sense.

Your induction machine has no intelligence regarding austenising temperature or whatever.
You set a temperature, there is a closed loop PID regulating device by means of the surface temp probes, and that's it. Probably with some whistles and bells like upslope, downlope, equalising, ...
But you have to (empirically) set the temps and times for the material to behave as you want it to.

It seems that either you don't know exactly what your goal is (relating final microstructure/depth/hardness/...), or how to achieve that goal. Or at least that's the impression I get from this conversation.
Can you explain either (or both) of those? That'll help us helping you.