# Hardness and edge geometry



## milkbaby (May 5, 2017)

Inspired by one of the threads in the sharpening sub, I was wondering how people decide on what hardness to temper to and how much you adjust that to accommodate edge geometry and sharpening angles.

I don't have a hardness tester, but I'm working with simple eutectoid (or almost) carbon steels 1084 and 15N20, and I do my own heat treat in a small two brick forge with canola oil quench. I'm guessing that I'm probably getting close to listed "as quenched" hardness for these two steels and use internet info (like Kevin Cashen's site and Alpha Knife Supply) to choose tempering temps. I will probably try some 1095 in the future too as in the thin cross sections used for kitchen knives, it seems people have decent results quenching in medium fast quenchant like canola oil. 

Will much more acute sharpening angles always be more chippy even at lower hardness? 15N20 is supposed to be very tough like L6, so can it be left at higher hardness than 1084 or 1095 and be less chippy? Also, since these steels are relatively simple carbon steels without a lot of alloying elements, they should be less wear resistant yet suffer less microchipping from carbide tear out, right?


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## RDalman (May 5, 2017)

They should perform great if you have decent temp control. I would temper for high hardness as they're so tough even at upward 63-64. Go as thin as you can on edge, shouldn't be any problems. Maybe play with waterquenching if you're going to try 1095. It's difficult getting longer knives to come through quench straight if ground thin before, and for canola you sort of need to be pretty thin to get the edge to full hardness.


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## jessf (May 6, 2017)

small side note, in the absence of any HRC testing equipment I've taken to using the edge of two knives who's HRC i trust or feel is credible and is between 61 and 58. It's a crude method and they have to be close in cross sectional geometry to the blade your testing. I'm testing just the edge in several places. Helps me understand a ballpark area of hardness, not unlike using a magnet to test a ballpark area of temperature.


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## PolishAvenger (May 7, 2017)

I would do a test blade with "extreme hardness" as your goal. Finish the blade as finely as you like, then put it through a battery of tests looking for undesirable traits along the way. If you run into unacceptable chipping, throw it back in the tempering oven and add 50 degrees. Rinse and repeat until you get perfection. I'd suggest the brass rod test....but thin, hard blades are what you're going for and they'd probably always fail just as a matter of course.

Lower hardness will not be as chippy, and yes, the fewer carbides formed, the less tear-out you'll have.

On a side note...unless you're going to be able to achieve a proper soak at temp to get 1095's alloys into solution, your resulting product will be basically equal in performance to 1084. If, however, you wanna see hamon.....

-Mark


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## scott.livesey (May 8, 2017)

all blades will eventually either bend or chip. softer edges bend, harder edges chip. part of our job as knife makers is to find the hardness spot for a particular steel and geometry where it takes a long time for this to happen. if you can find hardness vs. charpy testing, it will give you target area to start from. an example https://www.crucible.com/eselector/prodbyapp/tooldie/ketos.html. tempering at 350F to 400F would be the 'Sweet Spot' for O-1. 
also look at what the blade will be used for. a very thin, say 0.03" 1/2" above the edge, hard, Rc62-64, blade is not the knife to use for deboning, but will excel at slicing boneless proteins, most fruit and veg.
scott


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