The first knife I ever made was out of an old file. Try to use flat files (mill or lathe files) rather than half rounds (pipeline files) as you'll have a lot less grinding to do. I've heard of guys grinding hardened files, but your best bet is to anneal, normalize, grind and shape, heat-treat, temper, and finish.
Always good to see another welder on here!
Have a good one,
Nathan