A word of caution and or advice about using unkown steels.With the exception of alot of old sawmill blades, most circular saw saw type blades are mild steel and will not harden enough to make a good blade.Same goes with most lawnmower blades, old files and railroad spikes.That being said, there are some good steels mixed in there.If you are using a file look for old Nicholson Black Diamond Files, they work very well and will harden great, but be careful of modern files and any unmarked files as most are just case hardened and will not hold an edge.If you are looking for railroad spikes look for the ones marked "HC" as they are high carbon and will work fine.If you are not sure, try hardening it and do a file and spark test before going any farther.
After 40 years of working with steels and 20 years of making knives, I have found it is better to buy some good steel and not mess with the unknowns.1095,1080, 1084 and 5160 are all relatively cheap and HT easy with 1095 being the most tricky.Dave