This post relates the "Steel for Knife making" post by "rider" to rec.crafts.metalworking on 8/31/06. The thread changed migrated from knives to lawn mower blades, so here I am.
Last week at a threshing show, a guy brought me a brand new mower blade to flatten and then to bend 2 or 3" of each end up at 90 degrees to make a hand pushed sod cutter. I have used and sharpened a lot of mower blades and have seen some past posts on this topic, but opinions and experiences seem to vary widely. Here's my most recent experience: I heated this blade to about 1900 degrees F and forged down the "wings". I let it cool until there was NO color in it all, then quenched it in water. It got hard as glass and shattered off about the same 2 inches that I quenched when I hit it with a hammer. To me, this said that the material was Not water hardening, but rather oil or air hardening stock. So, I reheated it gently,let it soak a few minutes and then slowly moved it from the fire to the coals a few inches away over a period of about 2 minutes, then let it cool to room temp for 10 minutes or so. The stock became soft enough to file easily. I conclude it had to be oil hardening (NOT air hardening), or I would not have been able to anneal it with the above process.
I believe that (if this blade is similar to the blades I use on my Cub Cadet mowers) these blades are left in the annealed condition after forming and sharpening so they don't shatter or crack in service.
Would they make good knives? Who the heck knows, if you don't know the exact analysis of the material. Why spend hours at the grinder or whatever to produce something that might or might not work. The parallel for me is in making blacksmithing tools. I have chosen 3 kinds of material (S1, S7 and W1) for my struck tools and I know how to work each and when to use them. Even worse to me, would be to use mystery metal for a tool that I was going to sell to someone else. When "mystery metal" works is in a "McGyver" situation where you have to do SOMETHING, right now, and getting the exact right stuff is out of the question.
Throwing away old mower blades, I remain Pete Stanaitis.