What Kevin said with one caveat, with any file or rasp always do a snap test on the tang before you even start. Just heat it to non magnetic and quench in warm oil, heat it up with a scrap piece of steel, after the quench put part of the tang in a vice and if it snapped off it got hard. You can also see the grain. If it just bends heat it to non magnetic again and quench in warm water, if it won't snap off in a water quench it probably isn't a good choice for a cutting tool