Heat treating temperature vary depending upon the alloy of steel being tempered, varing between 300-1200 degrees F (although the extremes are usually reserved for the super-alloys.)
The steel used in files can vary quite a bit so it isn't a big help. I will also point out that you can't trust a home oven to even be in the slightest bit accurate; double check the temp with a digital oven thermometer.
I know the files are a crap shoot, but I generally just make small dress seax's that don't get tough use. Hopefully one day a can set up a small forge, but until then I have to work with what I have.
I'll get the thermometer as well. I already make killer brownie though.
a rought color chart is in a number of good books.
Edge of the Anvil
The art of blacksmithing
The compleat modern blacksmith
Pattern-Welded Blade: Artistry in Iron
Wayne Goddard's $50 Knife Shop
Or if you don't mind reading books in a digital media
Sounds like you were running a bit hot; but for "ornamental blades" a good spring temper is not a bad chouce---keeps them from breaking if dropped and they won't be expected to have to keep an edge through rough usage.
Or in other words---test the edge first to see if it's hard enough before re-doing the heat treat (and yes you need to start over with -re-hardening before drawing the temper again to a cooler value.)
Thomas
Last edited by Thomas Powers on Mon Jul 26, 2010 12:15 pm, edited 1 time in total.
Tempering only works one direction. If you start with straw, you can go further to purple or blue. But if you go to blue, you can't come back to straw without rehardening first.
Memento, homo, quod cinis es! Et in cenerem reverentis!
Guessing from the color, you have that tempered to about where you would want a sword, not a small knife. It's not a big deal (assuming the file wasn't case hardened which would mean it's just mild steel except the outside skin) a bit softer than optimal but still usably hard (also assuming that you didn't get it even hotter than the temper when you were grinding the blade).
If you make another one or redo that one, here's my method. Get a magnet (a hard one, not a bendy fridge magnet) and a heat source. Heat the blade until it's glowing just a bit and see if the magnet sticks, if not put it back in until it's a bit hotter and check again, if so put it back in anyways just to get it a hair hotter (blacksmiths call the optimal color for hardening simple steels "cherry red" though I've never seen a cherry that color) then take it out and let it cool to "room" temperature. Do this two more times, this is called "normalizing" (at least in smithing circles, industry defines terms a little differently) and it relieves strain in the blade and makes the grain smaller. Heat it the same way one more time, and then QUICKLY quench it in oil. Test the edge with a file, it shouldn't bite at all, it should just slide over the surface almost like glass. If it does bite, repeat the heat and quench but use water instead of oil, if the file still bites after using water then you probably have a case hardened file, useless for your purpose.
To temper it, put it in the oven (I like the toaster oven if the blade is small enough to fit) in a way that it sits evenly (notice how your blade is different colors on each side? that's uneven heating) and start lower than needed, maybe 325-350F, and leave it in for an hour and then take it out to cool. For a small blade of simple steel (which most good files are made from) you want the color to be a yellowish bronze color. If it didn't turn yellowy bronze then turn the oven up 25 degrees and do it again, repeat raising the temp until you get the right color, then do it one more time at the right temp. Don't worry about the extra tempering sessions you did at lower temps, it won't hurt anything.
