[Jeff Vicars]

I am in the final stages of making a knife from a annealed file. Any suggestions on putting the proper hardness back?

 

[Joel]

I started out making knives out of files, but I always ground them cool to keep the original heat treat intact. Basically though, you need a source of heat and a magnet. The steel is heated(in a semi-dark area) until it is no longer magnetic(the magnet won't "stick"). The source of heat can vary, but some are not usable such as a plain wood fire(contains too many impurities which will surely screw up your steel). Forger's use coal, but not the coal itself since it is high in sulfur and THAT will surely destroy your blade. They burn the coal until it converts to coke, and use that. Something else that works well is charcol. It needs to burn in a small covered container(like some of the small charcol grills out there) preferably with some sort of air blowing into it to increase the charcol's temperature. A small fire box with coal in it also needs that airflow to complete the coal to coke transformation. A length of pipe with that is fitted into the bottom with a blowdryer duct taped to the other end works. If the blade is small and thin enough, it can be heated with an oxyacytelne, oxy-gas, or propane torch. I've a small oxy-gas torch that I use to "soften" the backs and tangs back when I was making only file knives and that worked. You'll also need a quenching medium which can be either oil or brine. I started out using used motor oil as a quench for when I tempered the blade, then switched to brine(you heat water and add enough non-iodized salt until an egg floats). Most files are either W-1 or W-2, but some(nichols) may be 1095 or something else. I used Sandvik(swedish)files which I could get for 50 cents, and had no idea what the steel was, but it acted like one of the ones I've mentioned.
The quenching medium should be heated to around 140F(oil) or 170F(brine). For the steels mentioned, brine is the better of the two quenching mediums. The basic process is: heat the steel until it starts to turn dark red, then start checking it with the magnet until it won't stick. It's important to catch the steel just as it becomes non-magnetic, you don't want to exceed that critical temperature, so check it often. Once the blade is at critical temperature,plunge it straight down(point first)into the heated quenching medium. Don't "stir" the blade in the quench. After it stops vibrating/sizzling, it can be drawn out or left in until it's cool enough to be handled with bare hands. It's normally advisable to start tempering while the blade is still warm. There are other way's to do this process, but they involve items/materials that aren't commonly available, and quite frankly I've never tried so have no expertise in using them. 'Course if you have access to a heattreat/ceramics oven, then the whole process is a lot simpler. Hope this helps.

 

[Jeff Vicars]

Thanks Joel, I'll let you know how it turns out.

 

[Joel]

Good luck. One really important safety factor I should have mentioned. If you use oil as your quenching medium, DON'T do it inside. Also you need a non-flammable cover that can be fitted over the quenching container, because I can almost quarantee you the oil will catch on fire when you stick that red hot blade in there. You need the cover to put over the container after it ignites. Never had it happen when I was tempering, but then I was only heating the steel to 400F, but I still kept a cover handy. Also, if you try heating the blade with a torch, make sure you run the flame on both sides of the blade. If you heat from just one side(with a torch) the blade will warp away from whatever sides being heated. I should never try and answer posts while still on my first mug of tea. Hope you read this before you try that route.

 

[Jeff Vicars]

Well I heated my file knife till a magnet wouldn't stick, quiched it in oil. Repeated this(why I don't know, guess I wanted to make sure I got it right) heated the back to a "straw" color and put it in a welding rod oven for 35 minutes. A file will cut the blade, but not easily. I think it's about the right hardness. I sharpened the blade today, the stone on the blade had a good "feel". Put a good edge on. Just got some more polishing to do. Thanks for your help. I'll try to post a picture.

 

[Joel]

Glad things are working for you Jeff, but I'm a little confused. Did you just heat the back to straw and leave the front(cutting edge) as is? What did the time in the welding rod oven do as far as color on the blade goes. Normal tempering sequence for your steel is to heat the whole blade to "straw" color in the oven at 400F and quench, then heat the back and tang to "blue" with a torch, being sure I didn't run the blue too close to the tip. Way I kept the cutting edge at straw while heating the back, was to place the cutting edge in about 1/4" of water and holding it in place with Vise grips back on the tang while heating the back. Straw color gives you a Rockwell of around 62-64. If you didn't temper the whole blade, that untempered cutting edge will probably be way too hard and brittle, and that 62-63 Rockwell back won't help matters much. 'Course, that doesn't mean it won't work, but wouldn't put a lot of side stress or hammer on it much(in case it's a stick tang and you're going to put a guard on it). Broke several files cause I didn't heat the tang to "blue"; plus there was no way I could drill holes with regular steel drills. When I tried to hammer the guard down the tang(for a tight fit), the steel snapped right at the juncture of the tang/blade. Still, if it works, it works. Good luck with the rest of it.

 

[R. Tillery]

I've used old motor oil, chainsaw bar oil...even olive oil for hardening blades.

AFTER you harden it, DO NOT forget to 'draw' it in an oven. I place mine on a baker's cooling rack, in a 400* oven until the polished blade turns a 'straw' color.

I'll never forget the blade I forged from a file, and forgot to anneal prior to hardening it. Prettiest crack you ever did see right in the belly of the blade.