Many mower blades of today are not that great as a knife steel. They are more intended to mow by blunt trauma, rather than acually cutting the grass. I would suggest hardening a sample piece, and see how hard it gets. See if it will skate a file when hardened. Also the steel and quality is going to vary from make to make. From hearsay only, one of the worst reported was a Sears blade. Would not even harden in water.