The special inner hole heating inductor for induction heating furnace is shown in the figure. Generally, a multi-turn inductor is used, the diameter of which can reach Φ13mm, but high pressure water must be passed through, and the water pressure should not be less than 0.6MPa. It should be noted that the water inlet pipe of this type of sensor must be connected to a pipe passing through the center of the coil. For easy identification, the inlet pipe joint can be designed to be larger than the outlet pipe joint. When the size of the special inner hole heating inductor of the induction heating furnace allows, it is better to add a magnet sleeve or fill with a formable magnet powder (a paste-like magnet). For smaller inner holes, loop-type sensors can be used, and the workpiece must be rotated at this time. The current research and development direction for induction heating of small inner holes is to switch to ultra-high frequency power heating. For example, the time to heat the small inner holes with a 50kW/2.5MHz power supply is 0.2s, and the thickness of the hardened layer is 0.1 ~ 0.3mm.