Well the obvious suggestion is to make knives from them but I use them for heat treating experiments and demonstrations. The grain of a freshly broken file is an excellent guideline for what the grain of our blades made from simple steels should look like afer heat treating. Don Fogg did a demonstraiton on how thermal cycling refines the grain with a file at a demo once that has had a lasting impression on me to this day. Start by breaking an old file near the end and examine the grain, then way overheat the file and quench it, then break it near the end again to see or to exhibit the coarse grain that looks like rock salt. Then run that same file through three normalizing heats watching for decalsecense on heating and recalescense on cooling to arrow in on the color of the steel at this point in the lighting conditions you are working in and then using that reference point to heat it again do another quench at the proper temperature and break the file again near the end. If everything went well, you should be right back to a grain similar to what the file had when you first broke it.
