It's funny because I had found myself thinking about case hardening again, last week after re-reading the attempt at a mod-up I did a couple years ago. So I watched the Clickspring video tonight about that case hardening, too. Lots of new and useful information, and he does a really superb job of neatly working on camera, and filming and editing. First class job!
I do find I have some questions about the method for making the files, and perhaps I will try it myself when I can, just to find out the difference.
His method is to cut the files first then case harden them, and finally temper.
I was thinking of case hardening and then annealing, then cutting teeth and hardening followed by tempering.
The tricky point is the heat and exposure to oxygen after cutting the teeth in either procedure. Because that degrades the tooth sharpness. His method would at first seem better that way, as the hardening stage is in an oxygen poor casing of his pack mix. But I noticed he cracks it off in the presence of air. I saw a fair amount of scale (oxidation) as the pack was removed, and during the slow plunge. It also seemed to take a long time to plunge.
I was wondering if the heating to harden in my second proposed method could be done in a slightly reducing atmosphere, and then a quick plunge into water, reducing oxidation and better preserving the teeth? I don't know how well the tooth making would go in annealed case, either. But something to try some day.
I guess the proof is in the pudding. Wish I didn't have so much I have to do at present -- including finishing the lathe.