If you want a knife ground from a file, great, get one and enjoy it. But, I think that most established custom knife makers know more about knife steel than did your shop teacher. And, I don't know any who would make a blade from steel with 1.5% carbon; they would go bankrupt from all of the broken returns and grinding them to shape would take months. Perhaps your shop teacher had had experience with older files that had lower carbon steel. These days, those who make blades of carbon steel use saw steel, not file steel.

If you look, you'll see that most files are 1/8" to >1/4" thick; they need this thickness for the strength to withstand the flexing to which files are prone when in use. Most knives are no thicker than 3/32" and are often 3/64" thick or less; if you had a knife this thin made of heat-treated file steel it would snap like a potato chip the first time it was even mildly stressed. To have any strength at all, a knife of file steel would need to be thick and heavy.

Check it out; Google knife steel. You will find that most knife makers who use carbon steel use something like 1095 steel; 1095 is 0.95% carbon. It will heat-treat and harden quite well and take a long-lasting edge but is still tough enough to withstand some flexing. Ka-Bar knives are made of 1095. You are right that a knife is not a pry-bar or screwdriver but the blade has to be able to bend a bit. And, brittle does matter in a knife. Hardened file steel is more like glass in its flexing ability; ie none.
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