This study demonstrates the use of simulation in the design of induction hardening coils. It compares three coil geometries, two of which leverage the flexibility of 3D printing. The paper explains how to set up and run the simulations in order to predict temperature fields, hardness profiles, and microstructure distributions in the workpiece. Based on the simulations, the conventionally manufactured coil and one of the two 3D-printed coils do not achieve the desired martensitic microstructure everywhere along the surface of the workpiece. In the case of the 3D-printed coil, the simulations show that the workpiece overheats in an area where its diameter abruptly changes. To fix the problem, the coil was adapted with an additional winding that carries current in the opposite direction. Simulations show that the redesign reduces hot spot temperature by more than 200 °C, producing the desired microstructure in that area of the workpiece and a more uniform hardness profile.