Originally Posted by
Patrick Chase
Here's a VERY simplified version: A few different things can happen to the Carbon in steel, depending on the composition of the steel and how it's heat-treated.
In a low-alloy high-carbon steel like O1 or Japanese "white steel" the Carbon ends up mixed with the Iron in a few different ways, forming Martensite, Cementite, or Ferrite. The Carbon content and specific heat treatment determines the ratios. Martensite is what makes steel uniformly hard, so heat treatment generally strives to convert as much of the iron to Martensite as possible. Cementite tends to form from excess Carbon at higher Carbon concentrations and is in fact iron carbide, so even low-alloy HCS can contain "carbides", but they tend to be small and well dispersed and not so much of an issue for sharpening.
When you have a higher-alloy steel like A2 the excess Carbon tends to combine with those alloyants (mostly Chromium in A2) to form their respective Carbides (Chromium Carbide). It isn't an "impurity" inasmuch as it's very much done on purpose. Chromium Carbides are (mostly) what gives A2 its well known wear resistance.
Very nice summary and explanation. I'll just add a couple nerdy bits for Luke et al about the low alloy stuff, which is what I have direct experience with (the higher-alloys not so much).
The steel used in a lot of late 19th c. – early 20th c. tools, particularly stuff marked "cast steel" as well as most laminated tools, is what's called "eutectic" steel, which means it has only as much carbon (about 0.7 - 0.8%) as can be eventually converted to martensite during heat treating. This means there's very few carbides in the final product, so such steel tends to be very fine-grained, takes an incredible edge, but doesn't have the wear resistance of steels with more carbides. A modern example of eutectic steel is 1075, which is approx. 0.75 % carbon.
On the other hand, steels like 1095 (0.95% carbon), ) O1 (1%), or Japanese white and blue (1.1 - 1.3%), are "hypereutectic" steels. They have more carbon than can be converted, so some of the excess carbon ends up as carbides, as Patrick explains, which is why these steels have greater wear resistance than eutectic steel. They are still very fine-grained, but not as fine as eutectic steel.
Then, as soon as you start adding stuff like chromium, manganese, etc., you start trading fine grain for wear resistance. This is barely noticeable in something like O1 but becomes more of an issue in steels like A2, and even more in something like D2, as previously mentioned in this thread.
"For me, chairs and chairmaking are a means to an end. My real goal is to spend my days in a quiet, dustless shop doing hand work on an object that is beautiful, useful and fun to make." --Peter Galbert