I've read the tables showing what kinds of tools get tempered to what colors. And I've tempered some knives and other tools that seemed to come out okay. But I didn't feel like I really knew what temper was best for different tools. So I decided to experiment. I tried to standardize everything as much as possible, but this is a "quick and dirty" test, not a rigorous one. Still, I thought the results might be interesting.
I took some 1/4" rod from a coil spring (high carbon steel) and made some small tanged points. Roughly 1" of point and 1" of tang. I annealed them, and ground/filed/sanded just enough to get a fairly smooth, flat surface. Heated to red, quenched in oil, sanded lightly, and tempered. I tried to get a range of temper colors.
1 - quenched, not tempered at all
2 - tempered until it just barely starts to darken
3 - tempered to light yellow/straw
4 - dark yellow/straw (tip started turning purple)
5 - purple
6 - blue
I drilled holes in small pieces of scrap wood to make basic handles for all of these.
My initial idea was to stab each blade into a piece of wood and then push to one side to see if the tip broke or bent. Using a scrap piece of maple, it was hard to stab them very far in by hand, and they just pried slivers out of the wood. So I switched to using a mallet to drive the blades deeper into the wood and then pushing to one side.
With the blades driven deeper into the wood, #3 and #4 bent slightly, the rest were unaffected. I tried again, driving in a bit farther, and using a hardwood board that was harder than the maple. With this test, #1 and #2 snapped off, #4 bend a little and then snapped, and #6 bent (#5 was a bit thicker than the others). (I lost the broken tips from #2 and #4)
Results: There was a trend to being softer with higher temper temperature, but blade thickness also made a big difference. Also, all the blades were pretty sturdy and needed quite a bit of force to break or bend (even the one that was quenched and not tempered). Any of #3-#6 would have made a decent blade as long as the tip wasn't made too thin. Even a blade from #2 would have been fine for light use, as long as it wasn't used with much prying force.
I would like to do a more rigorous and precise test, and also look into the effect of different quench mediums, but for now I feel like I got a good idea of the effects of different amounts of tempering.
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