Inside diameters less than ¾ inch are extremely hard to fit a coil into and heat efficiently. Tiny coils will blow up if the power they are conducting exceeds their ability to remain cool. Sharp radii profiles are difficult to harden into.
It is a copper loop that when energized and placed in close proximity to the work piece that causes it to heat. Induction coils are made from copper tubing and have water circulating through them to keep them cool.