I thought I'd pitch in my 2 cents (I'm a full time smith and I saw the word forge),so here's a couple of things from my experience. Lawnmower blades are not a very good grade of tool steel(in my experience anyway). Leaf springs are a pretty tough steel-they have to withstand flex and hold weight. If you are going to forge tool steel, you need to pay close attention to the proper forging temperatures. If you forge it too hot, you will burn out the alloying material which will make it liable to fail in use. If you forge it too cold (cold can be 1500 degrees depending on the steel)you can build up internal stress which will make the tool fail in use. I forge all my own chisels,punches and drifts. I buy tool steel new from Burgon Tool Steel (1-800-258-7106), they are reasonably priced and do small quantities. The most important thing that you need to do after forging the steel in the proper temperature range is heat treat and temper it properly. I recommend reading "Skills of a Blacksmith Volume 1" by Mark Aspery. He wrote a very thorough chapter on understanding heat treating. The reason why you want to be careful with forging tools is that if they fail in use you can be badly hurt. For instance a turning tool-when the tool snaps while turning where will the pieces go-into your body is a possibility.
I forge a lot of tools that are struck-cold chisel,hot work chisels, wood cutting chisels, etc. Early on in making tools I did not heat treat a chisel and it failed when I was using it. When you are striking a tool with a hammer and it fails, the pieces of steel come off at a great rate of speed and can penetrate through your clothes and stick in you.
I used to use a lot of reclaimed steel in my tools early in my career until it was brought to my attention that it can be a risky proposition. You don't know what the internal stresses are in the piece of steel you are using-ever see a broken leaf spring?
On the other hand if you have no money,reclaimed steel is a bargain. I buy mainly H13 (a hot work steel) and 4340 for my forging tools. Woodworking tools (chisels anyway) I use S1(a good steel that can withstand being struck). I buy a 6 foot length of 4340 3/4" for about 25 bucks or so-pretty cheap considering the safety issue of making tools. IFORGEIRON.com is a great resource also.
Sorry to run on so long, but I thought it might help someone.
If you have any question, please ask-I'd like to be of assistance.
Feel free to send a personal message also.
Last edited by smithingman; 03-02-2010 at 05:40 AM.