If I think for a while off and on, it comes back - it's in text on here somewhere with near certainty, but years ago. 

I think I got tired of throwing out old files and asked if they were through hardened. Several people told me with certainty that they were only case hardened and no good for anything else (that's false, though they can be case hardened above their through hardness and probably were surface hardened with marginally hardenable steel long ago). 

I heated one, hammered it and made a thin knife out of it to make a marking knife and then started cutting all of my boxes (for the mandatory recycling required here) with them and they had the same crispness that the chisels do. 

A little knife was a good choice because very thin cross sections of water hardening steel will still near fully harden in vegetable oil. 

I also had a fascination at the time of making edged tools that would have very little burr after a finishing stone but not be chippy, which led to rolling the tips just a little bit on a fine slow to thin a burr as much as possible, which led to other things in combination with making a couple of rosewood plane bodies (which also favored rolling the tip of the edge on a very fine stone  -so much so that a cheap pexto firmer well outperformed a high cost harder chisel that was set up without rolling the edge). 

Walking into curiosities is never really deliberate from the start. 

The little thin knives do help to learn a whole lot about edge crispness, though, and geometry. Cutting boxes with a softer knife is noticeable vs. one that's harder, and cutting boxes with an undertempered overhard knife, same. 

The payoff for much of this stuff that's really not talked about on forums because it's not woodworking is just how many other things are quickly understood without thinking about figuring them out in the first place. Like why razors are a higher carbon plain steel, and why alloying attempts with tungsten didn't work out, and why the stainless razors (friodur) aren't as good and attempts at ultra hard razors also aren't. or pocket knives, or kitchen knives, or plane irons, or on and on.