Cheshirechappie
Established Member
Having done a bit of rummaging around, I'm fairly sure that induction hardening and impulse hardening are both applications of the same principle. If a suitable workpiece is placed inside a coil carrying a high-frequency alternating current, very fast heating of the surface layers of that workpiece takes place. The workpiece can then either be quenched in a suitable liquid to harden the surface, or in some circumstances quenched in air. The frequency of the coil current (between several hundred Hertz and over a million Hertz) controls the depth of hardening.
It's thus easy to see how very localised and shallow hardening of saw teeth can be effected, given some care in setting up the equipment to do it. I think it would also be possible to heat something like a chisel blade to sufficient depth to ensure adequete hardening on quenching, but I'm not quite sure why the variation in final hardness would occur, except if the process were adopted without sufficient development work being done. I suspect the equipment required would be expensive, but the speed of heating would allow a high throughput of work - much faster than heating with a conventional fired furnace, so for a bulk manufacture operation, it could well be an economical proposition. The failure of quality control may not be down to the process, but to the way in which it was applied - or misapplied.
It's thus easy to see how very localised and shallow hardening of saw teeth can be effected, given some care in setting up the equipment to do it. I think it would also be possible to heat something like a chisel blade to sufficient depth to ensure adequete hardening on quenching, but I'm not quite sure why the variation in final hardness would occur, except if the process were adopted without sufficient development work being done. I suspect the equipment required would be expensive, but the speed of heating would allow a high throughput of work - much faster than heating with a conventional fired furnace, so for a bulk manufacture operation, it could well be an economical proposition. The failure of quality control may not be down to the process, but to the way in which it was applied - or misapplied.
