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What Happens When You Soften a File?

Everyone probably recognizes steel as the most popular tool-metal, but not every knows that steel isn't just one kind of metal; it's really a class of iron-based metal alloys. Pick any metal file and it could be any one of a number of steel alloys. In addition to the variances in the kind of alloy or "recipe" of the metal, the type of heat treatment it has received dramatically affects the file. The effect is determined by what type of steel is being used, how hot it's been, how it was cooled and how quickly it was cooled. Some of the heat treating steps will soften a file.

Hardened Steel
- Files are often hardened, not just tempered. By using hardened steel, a file can stand up to the difficult job of filing other metal. The tradeoff for such hardness is rigidity and brittleness. In fact, some files are so brittle, if you drop them on a hard floor, they'll break. It has occurred to many people that a file may be a good source of tool metal for some other purpose, if it is softened. And they are right, but the process is a little more complex than just softening the metal.
Annealing
- Anneal the file to soften it. That means you heat it up, then cause it to cool very slowly. You may have to place it in insulating material to ensure the metal's heat dissipates very gradually. The annealing process realigns the steel's molecular structure, making it more malleable or soft. The effect is that the steel is more workable, making it much easier for toolmaking and many other kinds of fabrication.
Heat Treating
- Once you enjoy the effect of annealing your file and you rework it into something new, you may not need to heat treat it again; it just depends on what you've made. If you made a chisel, for example, you cannot leave it softened; it won't hold an edge. That's another effect of softening, so you'll have to heat treat it. Heat treating or re-heat treating involves heating the steel until it changes color, then quenching it in an oil bath, causing the molecules to quickly realign, once again. Heat treating is a science and a bit of an art. Not only do you have to know just how hot to get the metal, you have to know -- or guess -- what kind of metal with which you're working to know the effects of heat treatment.
Variables
- If you soften a file, you can narrow down the kind of steel alloy that might have been used. Obviously, it has to at least be a hard enough alloy to be used for a file. While there are matrices to to help approximate how hot to get the steel to reheat treat it -- or to soften it -- just by observing the color while its hot, the alloy content will be a variable in your results. The most important component is the carbon content. Some steel alloys have more exotic hardeners such as tungsten and cobalt. Regardless of the alloy, you'll probably at least have to retemper the steel.

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