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Well if it was me I would replace them... HOWEVER, you can anneal the old file (take the temper out of it) by bringing it to a dull cherry red (just till you see a deep, dark, even red tint in the metal). When it reaches the dull red color place the file in a bucket of dry wood ash (completely covered with 3 or more inches of the ashes). Leave the file in the ashes for about 2 to 4 hours and let it cool slowly. After you remove the file from the ashes, the metal will not have the hard temper in it, and you will be able to work the metal (form it, shape it, whatever. This is also a way to prepare old files for making knives). If you wish to cut or freshen the cutting teeth of the file, you must lay the file on a bar of lead or wooden block and use a hammer and sharp cold chisel that is wide enough to shape the teeth the full width of the file. If you are forming a knife blade or tool you can heat the steel for forging, but do not heat it more than a dull cherry red color. Do not work the steel (other than forming small teeth for a file) while the steel is cool. Pounding with a hammer will pack the fibres of the cold metal and will lead to breaking or splitting of the steel. After re-forming the teeth to the desired shape. You will have to re-harden the file. Again bring the heat of the forge or rosebud tipped torch along the file till it causes an even deep red tint in the steel file (I know that many folks advocate a "straw color" for tempering, but I like the dark cherry-red color for tempering). Do not heat to the "bright" red color (at that temp you start to burn off the carbon in the steel). When you can just make out the deep red tint of the steel, thrust the file into a bucket of salt water (point first) and swish it around in a "figure-eight" pattern till it is totally cooled. At this point the file is very brittle and may break in usage, if not "relieved" of the stresses in the steel. I usually bring the steel to a slight temp (some folks like to place the steel in an oven at about 250+ degrees for 20 minutes). But,I usually use a torch and just pass the flames over the steel, with the torch tip at a slight distance from the steel, only till it is too hot to touch without a thick glove (yea, I know that is not precise). Then I let the file cool back down slowly. I have used this same process to make my own coil springs from teeth on a combine reel, and to form knife blades, from files and thin leaf springs. A little playing around trying this and you catch on quick to what works best. Again as I said at the beginning, it is cheaper to just buy a new file {or chemically treat them}. But I like to play with metal and try new things so this is just another little skill that is handy to know and practice. It ain't precise like some folks like to be.... but it does work well!
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