With old files, it's really hard to say....simply because it's difficult to know what you're dealing with (steel type). Many files in the past 10+ years have been made by "case hardening"....what that means is the steel is a type that is soft, with the "hardness" being only a couple thousandths of an inch on the surface. It might not make sense, but these files will still break.
The problem with this type of file/steel is that you'll work hard at creating a knife from it, TRY to heat treat (harden and temper), and it simply will not harden.
When you "after getting it hot in the fire".....was that a wood fire? If so, it likely never got hot enough. Here's an over simplified "How to anneal"........heat the steel to 1525F+ and place it in a media that will force the steel to cool SLOWLY.....dry ashes, garden lime, and sand are some things folks use. Personally I recommend vermiculite (you can find it at nursery/garden supply stores). Generally it will take the steel 8-12 hours to cool down enough to handle.
That's just a brief input....there's a lot to learn when you build a knife. Again, personally, I highly recommend buying a bar of "knife quality" steel such as 1080, 1084, 5160, etc. That way it will come in a condition that you can work with, and you will know exactly what you have......and will be able to look up how to heat treat it.
