So if I'm making a knife (something like a buck knife) out of old lawnmower blades, what should I quench it in? Plain water, salt water, or oil... I'm quite confused. Advice would be apreciated.

I hear there made from mild steel, and sometime tool steels.
If you dont know, try a spark test.
If its mild water/salt water would probaly work, if its a higher grade probaly oil
You need to figure out what it is first.

That would be my understanding, someone correct me if im wrong.

You should have a lot more lawn mower blade than you need. So take several samples and test different procedures. Start with a spark test, that will give you an idea where to start.

If played with a couple of lawn mower blades, one was very close to mild steel (I'd guess 1050), one was something that shattered when quenched in water and was red short.

ron