The thermocouple reading relies on three things to measure temperature correctly.
The thermocouple reading relies on three things to measure temperature correctly. If you put a thermocouple inside an induction healing coil and try to heat it, it may get warm, but it's not going to get glowing red for typical frequencies. That's because the diameter of that wire of the thermocouple is usually much smaller than what can be heated effectively with induction. It does heat a little but not a lot.
The second thing in this equation is the characteristics of the measuring instrument. When you put the thermocouple in the magnetic field, there is electromagnetic noise that gets transmitted down the thermocouple wire to the instrument. You have to make sure that your temperature measuring instrument has sufficient isolation built in so as not to get junk reading from the thermocouple, The thermocouple needs to be attached really well.
The third thing is that the thermocouple has to be attached extremely well to the metal part for it to read the metal part temperature. You can either screw it in With a small, eyeglass tool or you can weld it onto the metal part. And if you do any of those two things, you will get an accurate reading from the thermocouple. Given that your measuring device is capable of isolating the noise.