I know this is an old thread but since I've only just joined and am a bit of a "knife knut" the forum I'd like to add to it.
One of the best easily available tutorials on making a knife from an old file is published in the August/September 2006 edition of The Shed magazine. It was by Auckland cutler Pete Caulfield. Check it out your local library may still have it or you can back order it from the publishers.
Here is a link to an excellent web site on making a bushcraft knife. It has been around for a while and you may have already seen it. It has a template for a Ray Mears knife and detailed instructions on all aspects included annealing ,hardening tempering and creating a single bevel edge. In addition to written instructions there are 4 comprehensive videos on youtube which demonstrate the processes.
HI, all there has been some good stuff here about hardening and tempering most of it close to the mark some a little confused and even confusing but good stuff all together the mention of files , a lot of files now on the market are not suitable for making files because the body of the file is not hardenable only the teeth have been case hardened that is the reason they don't last very long doing real work beware cheep files you can make an oven for tempering a hardened blade by using a radiant stove element and a simmer stat from an old stove ...It is probably better to use oil for a hardening quench bath and if you have to use water get it up to body temp before putting the blade in if the blade will not harden id the oil quench then is the time ti use water ... Rob