How does A1, O2 & other cryogenically treated steel compare with respect to hardness & sharpening characteristics?—Mike DiMaggio

Bruce Wedlock replies: O1 is a simple high carbon tool steel with 1.1% manganese added, which is quenched from orange hot in oil. The hardened grain structure is as fine as possible which allows honing to the sharpest possible edge.

A2 differs from O1 with the addition of 5% chromium and 1.1% molybdenum, allowing it to quench in still air. During heat treatment the chromium addition combines with some of the carbon to form chromium carbides—tough, hard particles dispersed through the steel and contributes to A2’s edge retention. But these carbides can pop out under the stress of honing or cutting, leaving small gaps in the edge. To strengthen the A2 edge, try honing plane blades at 30° and chisels and block planes at 35° and see if edge retention is improved.

Cryogenic treatment (-320°F) increases A2’s toughness—treated blades will hold their edge longer. O1, on the other hand, is preferred by many for its relative ease of sharpening and its ability to get sharper.