ive already built a zvs induction heater, but i noticed the frequency when heating steel is always around 24khz and 37khz. I looked online and saw a video of someone heating steel with a zvs induction heater running at 400khz. My question is: What part of the circuit determins the frequency of a zvs induction heater?

heres an online schematic of the zvs induction heater circuit: http://diymania.hv4all.com/zvs ih.PNG

my circuit is generally the same but instead of having the CT winding as my work coil, i wound it to a toroidal ferrite core as the primary and wound 4 turns of thicker wire as the secondary. the capacitor is also moved from the CT winding part to the new work coil. Doing this reduces the heat caused by the mosfets significantly. The input voltage is 48v and it comes from a 900W power supply.

 

The coil and capacitor form the resonant circuit, so once the system is built, it's a bit difficult to change the frequency as you may have to shorten the coil, change the capacitor and work out the new impedance matching inductor.
http://www.richieburnett.co.uk/indheat.html goes into it in reasonable detail.