I mow grass for a living, so am quite interested in this. I also like to make a blade or two.
I'm lead to believe that mower blades are made out of a very flexible type of steel, that will bend, rather than shatter, so suspect that the carbon content is indeed very low.
Feral, a chunk of known alloy steel costs $20 shipped to my door and it will have enough metal to make at least two good blades. And because I know the alloy, I know how to heat treat it properly to maximize the blade's potential.
People that use lawnmower blades are simply guessing and hoping. Sure, the blade might come out right, but you're banking on hope. In my opinion, it's just not worth it to save a few pennies on the metal you start with. By ordering a good alloy from a supplier, I can get the right length, width and thickness to start with. This saves me time and money, and makes sure that I don't have to change my blade design to fit the parameters fixed in place by the lawnmower blade.
There is no law dictating what lawnmower blades are made of, and there's certainly no regulatory agency policing the industry to make sure they use XXX alloy. They could be using 5160 for decades and switch all of a sudden to EN11 or A36. Because they're a large operation, they might decide to go with an air-hardening alloy to streamline the manufacturing process. Or they may decide to use mild steel for the body and weld on a tougher edge material. Basically, you can bank on them using the cheapest possible alloy to meet their individual standards.... and it could change at any minute.
Remember, there is a reason why you don't see master bladesmiths buying up all the lawnmower blades at the hardware store.
The cross-section photos of the file above indicate that the steel is at least a medium-carbon and will respond well to becoming a knife. But how did the maker heat treat the file? Does it air-harden? Maybe it requires salt-water quench. What temp range do you temper it at?
I've made a lot of knives from mystery metal. The one thing they all have in common is that they required more work than if I had started from a piece of bar stock that was already the right size. I'll never forget the time I hand sanded through a leaf spring just to find a hidden crack that ruined the knife I was trying to make. Starting out with a .25" thick rust covered spring and sanding it down to a hair over .125" took a lot of my time and sand paper, but the metal was "free" right?
Personally, for a strictly learning experience, I would recommend starting with a chunk of wood or plastic and making a knife-like object. You get to learn all about grinding and filing, and you end up with a great letter opener. All told, you're in it for just a few pennies and it's a lot faster than working metal. I knew one guy that used the plastic sheet from a clipboard to practice making blades, and then he kept the ones he liked for use a templates to mark the outline on steel. It's a good way to learn what does and doesn't work - and not go broke doing it.
Just my thoughts after banging around in the smithy. I hope you find some value in them.
Before you start doing anything to the file, clamp it in a vice with an inch (not the tang) sticking out and **** it with a hammer. If it snaps then all is good, but if it bends over then scrap it. You are also advised to do a dummy heat treating run with the file to check whether it will need to be quenched in oil or water, how hot to harden from, how hot to temper, etc. And that will have to be done for every file since they may not all b the same steel.
