Induction heating internal splines

[Campeze]

TL;DR

Using induction heating to surface harden the hollow internal part of a metal cylinder workpiece that has splines on the inside.

I want to surface heat internal splines with induction heating with an internal diameter coil. It is a cylinder, uniform shape, and the teeth are 1mm wide and 2mm deep. The inside diameter of the hollow section is 19mm and the length is 55cm.
I was wondering when calculating power/resistance, do I take into consideration the whole workpiece or just the area of workpiece I want to heat?

 

[Baluncore]

I was wondering when calculating power/resistance, do I take into consideration the whole workpiece or just the area of workpiece I want to heat?

Welcome to PF.

It is easy to generate a current that runs around the outside of a workpiece. It is more difficult to induce a current inside a sleeve. To heat the inside you need to arrange the coil(s) to induce currents in the internal surface only. You must also quench that surface immediately after heating because an induction heated surface is shallow and will be chilled quickly. You may need to spin the workpiece as it is heated because of the induction loop polarisation.

For a long workpiece you will need to arrange the two processes to occur in one movement, say as the workpiece is removed vertically downwards from the induction coils past the quenching jets.

 

[Campeze]

Thanks for your reply. I am just struggling with calculating the power and current required in the coil.

It weighs 0.7 kgs and the ideal penetration depth is 1mm. With the formula Pworkpiece = mc(Tf-Tin/sec), would m be the mass of the whole object or just the zone I want the eddy currents to penetrate?

 

[Baluncore]

That equation is a generalisation. It applies only to the mass of the volume of metal heated. That may be over the whole workpiece, or only a limited part.

In this case it is only the internal part of the workpiece that is being heat treated. Any unwanted or stray heating that occurs must be included in the total, and will increase distortion. If you heated the entire workpiece to full depth, distortion would take the workpiece out of tolerance.

With well designed coils you can minimise the heated mass to the area needing heat treatment. Thermal energy change = mass * thermal capacity * temperature change. Think of it as an energy budget. Minimise the heated mass.

The heated mass will be the material density multiplied by the heated volume. The heated volume will be the area being heated multiplied by the depth of the heating. The depth of heating will be a function of skin effect and the heating time.