Garrett Hack replies: The simple answer is O1 is common high carbon tool steel, hardened by quenching in oil. A2 is a more complex alloy that is hardened in air. Both are common in plane and chisel blades. Steel is mysterious stuff, and so much of how it performs depends upon what it is made of (alloys added to the steel), and how it is heat treated (hardened). O1 sharpens more easily of the two, and takes a keener edge, even one as fine as 20°. A2 is tougher, takes longer to sharpen but holds an edge longer. It’s only downside is that it can be a bit brittle unless your cutting edges are closer to 30°.
Bruce Wedlock replies: Basic steel is an alloy of iron and carbon. By adding trace amounts (on the order of 1% or less) of a variety of additional elements to the alloy, the material’s performance (strength, hardness, ductility) can be dramatically improved. A specific steel’s performance can also be altered by heat treatment and tempering. The various alloy combinations and heat treatments are designated by an alpha-numeric code such as O1 and A2 among many others.
O1 and A2 are designated as high carbon steels and contain 1% carbon. O1 contains trace elements manganese, chromium and tungsten. The alloy is heat treated and then quenched in oil, hence the “O” designation.
A2 steel contains 5% chromium and trace elements molybdenum and vanadium. Following heat treatment, it is quenched by cooling in still air, hence the “A” designation.
Both steels are then tempered—heated and held at a specific lower temperature to adjust hardness and toughness.
O1 steel sharpens faster than A2, but dulls quicker, requiring more frequent honing. A2 steel holds an edge longer, but require more effort to sharpen. Veritas now makes premium blades from powdered metal which holds an edge twice as long as A2, but sharpens the same.