This is another example of dealing with "Spec Manufacturing"...... it's a crapshoot as to what the specific alloy of any given "saw blade" is (setting aside the sawmill BANDSAW type blades, which can USUALLY be counted on as 15N20). Even blades from the same brand, same size, same everything, can be, and often are, different alloys from production run to production run. Yes, some will harden enough to preform as a knife blade, but then again, then next one used, even if the exact same everything, may not. You can pretty much discount blades with carbide teeth right out of the gate. Those are the most likely to NOT make decent knife blades. In the end, it's just a better overall policy to use new, known materials/steel. I see where some folks like the romance of repurposing something, but a knife blade is not one of those things.