Thread: How sharp is sharp?

In the metallurgical sense, the limit for "sharpness" has to do with the types of carbides (of course including composition), and their sizes. Or their size in the metallic structure, if it does not contain Carbon, or Iron.
Generally, a martensite rich structure can offer a sharper edge than perlite, ferrite austenite etc. For that, carbon content and heat treatment are the most important factors.
A piece of plain iron, no matter how you sharpen it, WILL NOT shave. I have tried it many times. It will feel sharp but won't shave.
For the elements that are added to the steel, the smallest the crystals they form with Carbon, the sharper and less brittle the edge will be, generally. Chromium forms large crystals. Vanadium forms one of the smallest, if not the smallest.
And, the purer the steel from unwanted elements it is, like Sulfur and Phosphorus, the better. On everything.
I'm sure there are people who know more about the details, but that's my understanding on how things work.