Ref tempering a knife

Keith asked a question in comments a while back on this. I think I covered this at some point, but I'm too lazy to check so here's a quickie.

For 5160 spring steel, I vary the tempering color by use and size. A big blade for heavy cutting I temper to a medium/dark bronze color; smaller general-use blade a dark yellow to light bronze. I take the hardened blade, clean off the quenching oil and use a fine grinding belt or buffing wheel, no coarser than 220 grit, to shine up the surface, then stick it in the oven at about 375-400F; if you're not certain of the accuracy of your thermostat start low. Leave it in for at least 45 minutes, long enough for the whole piece to get up to heat, then take it out and look at it. If not dark enough, let it cool, shine it back up, turn up the heat about 25F and do it again. When the color is what you think it should be, give it another heat at that temperature, preferably two heats, and after the last turn off the oven and let it cool off inside.

One of the best tests to see if you've tempered it correctly is a real simple one that I first read about in an article by Wayne Goddard. After tempering, sharpen the blade as you would for use. Get a piece of brass rod(I keep a piece of 1/4" brazing rod for this) and clamp it lengthwise in a vise so about half is above the vise jaws. You need good light for this test, as you lay the blade so the edge itself is resting on the brass; not the bevel, and not like you're chopping into it, as though you were going to slice a layer off lengthwise, and press straight down, watching the edge closely. If properly tempered, the edge will flex, and return to straight when you release the pressure, kind of amazing to watch. If it's too hard, it will usually chip slightly; back to the oven and add another 25F. If it bends, as in flexes when you press but stays bent when you release, it MAY be too soft.

I say MAY, because it's subjective. In a big blade for heavy chopping, a lot of people prefer one that's easier to sharpen when it does dull, and unless you've really overheated it this may work out perfectly for you. This is where experimenting comes in, especially with steel you're unsure of. You can temper it once, check the color, and try it on the brass; if it chips, back to the oven. If it flexes, give it the two more heats and then use it to see how it works. If it's a touch too hard, you can go back to the oven for a heat at higher temp and try it again.

I can't give a hard 'use this color' because I don't know what steel you might use it with; an old file has to be tempered down a bit more, but with the wear resistance added by the high carbon content will work quite well. Spring has lower carbon than file, so it might have to be tempered a bit harder to do what you want.

To see the colors mentioned, best thing I know of it to take a piece of steel, thin and long with as shiny and clean a surface as possible(not stainless), and stick one end into a torch flame or something else hot. As it heats you can watch the colors move out away from the heat source, paler colors meaning less heat, darker as the temp in the steel rises.