String cutter, I am no expert. But I have made a few knives using files, and I didn't temper them. I held them in my hand while grinding, if they get too hot to hold, dip in water. That way you don't hurt the temper. Go slow, it's a very slow process. Final product, is a blade that is very hard, holds an edge for ever. I haven't had chipping problems, but I don't pry or try to hack through hip bones with them.
Don't get it too thin, it gets hot quick when grinding and you'll ruin it. The thinner it gets, the slower you must go.
One thing to keep in mind, like Robtattoo was saying, if you soften it first, much easier to work with. But you have to retemper it later.
For me, the cold slow way worked. I have gutted a couple of deer and still shave hair.
Dan