Steel Hardness

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KennyB

Joined
Jan 19, 2010
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So I don't really have any knives that are of particularly high HRC values. I think the hardest one I have is my BM Kulgera, which I believe is at 58-60 HRC.

That being the case I wonder how different levels of hardness would perform. I mean, just so that there's not a lot of confusion, let's imagine that I need to cut a piece of copper wire. This is what I assume might happen at different levels of hardness

56-58 - Cuts the wire, but leaves a really big dent.
58-60 - Cuts the wire and just just leaves a small roll.
60-62 - Cuts the wire and does not take any deformation, just wear
62-64 - Cuts the wire, but has chipped out a little bit

What are your guys's real world experiences with how different levels of hardness will perform? I understand that as steel gets harder, it has a tendency to chip, and as it gets softer it tends to deform more easily. However I'm looking for some kind of idea of how much more a blade that's 58-60 HRC will deform compared to one that is 60-62 given various tasks. Maybe just list some things that you would use a knife with a relatively high HRC value for that you wouldn't use one with a low value of and vise versa.
 
I'd say your chart is good, but the highest hardness wouln't chip if it is properly hardened and shaped. Also, it depends on how the cut was made. If the blades are pressed straight down, there shouldn't be any chipping unless the edge was so thin it buckled.
 
It also depends on the steel, edge angle, cutting method like me2 said.
I've cut copper wire, aluminum discs and steel wire with M2 64HRC and ZDP-189 67HRC. Neither suffered a single chip. Details and misrographs in superhard vs. soft edges article.
Back then I've had BM 154CM chipping at 60-61HRC on copper wire, later on Phil Wilson rehardened 154CM Nimravus to 62HRC and with thinner edge (30 inclusive vs. 40 inclusive before) rehardened and harder 154CM didn't chip.
So, it's not as simple.
 
I think overall HT is a larger factor than a blades final hardness, as a example get yourself a kershaw in S30V to compare to your BM. IMO kershaw has a superior HT, just in sharpening alone it feels like a different steel. The wear resistance is higher, it has much less edge rolling, and deformations from hard contact is less. Its also harder to sharpen yet most likely spec'ed at a similar final hardness to most every other makers S30V.

A steels final hardness means something but not nearly as much as the quality of the HT itself.
 
knifenut1013 said:
I think overall HT is a larger factor than a blades final hardness, as a example get yourself a kershaw in S30V to compare to your BM. IMO kershaw has a superior HT, just in sharpening alone it feels like a different steel. The wear resistance is higher, it has much less edge rolling, and deformations from hard contact is less. Its also harder to sharpen yet most likely spec'ed at a similar final hardness to most every other makers S30V.

A steels final hardness means something but not nearly as much as the quality of the HT itself.
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HT is important, but I feel it's a difficult task to tell who has the "better" HT. At the very least, my ZT 0301 had the edge roll from some corn cutting, and I feel the final hardness is responsible for that. Though to be fair, I would think you would want the final hardness to be on the softer side for a "hard use" knife, as a rolled edge is preferable to a chipped one. And cutting corn is like cutting through half an inch of wet wood, definitely no cake.

I find that edge rolling pretty much stops at the Rc 60-62 range. To me, that's sort of the optimum hardness I'm looking for in a knife. Modern steels made with Powder Metallurgy process have proven to be tougher in each generation(i.e. Elmax tougher than S30V). So I would think it only natural to raise the hardness to take advantage of that fact.
 
A higher hardness does not mean the edge will not roll, your getting things a bit confused. Hardness is only one of many factors in edge performance.

It will also be extremely hard for you if not impossible to tell HT differences because of your sharpening method.
 
I think the first part plays the edge angle.

A thin edge angle would deform sooner, than a thick one. Same knife would perform different with a different edge angle.

Than, if HT was done correct (not special, just correct) hardness, for it means the ability to cut somthing softer.

Than HT as a finer difference. But, how can you tell that difference, if you can´t be sure, you compare blades with same hardness?
 
knifenut1013 said:
A higher hardness does not mean the edge will not roll, your getting things a bit confused. Hardness is only one of many factors in edge performance.

It will also be extremely hard for you if not impossible to tell HT differences because of your sharpening method.
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Pardon my impudence, but while I didn't exactly get an A in Metallurgy(only a C), a few concepts got through crystal clear to me(no pun intended).

One in particular was when the instructor snapped a steel bar in half after quenching, but without tempering(which made it very hard but brittle). Another was with the steel bar without hardening, which bent very easily. To me, it seemed pretty clear that the most important property of the steel that made it resist plastic deformation was the hardness.

A soft piece of steel bends easily and will most likely deform rather than break(like H1 steel). Whereas an extremely hard material will break rather than deform(ZDP-189, ceramic blades, glass). So it seems odd to say that the ability to resist plastic deformation(which is what edge rolling is) is NOT directly proportional to hardness.

I was under the impression that the temper of a steel is what most people botch up, and that the purpose of the temper is to relieve stress in the matrix of the steel caused by warping from the quench, which would make the steel softer and less brittle. I simply assumed that if both steels are at the same hardness with one heat treated properly while the other is not, then one would be more prone to chipping/breaking than the other.
 
In the basic line of thought that is correct but it does not explain real world events. You also must remember HT can get very complex for very complex steels and what we see on a factory level is much different from the custom market. Edge rolling can be a factor of many things but having a higher hardness will not solve the issue, it will still happen. The steel itself, the HT, and the edge grind will all play into the performance of a knife.

Saying a Rc above 60-62 pretty much stops edge rolling raises a red flag in my memory banks because I've had M4 and ZDP roll where S90V (58-59) didn't. The M4 and S90V both being mule teams with near exact edge angles and finishes cutting the same media. So with edge formation and hardness taken from the equation what caused this to happen?

Even the steel itself can have differences from batch to batch so naming one variable as a reason in a pile of variable's only scratches the surface of the issue.
 
Just remember how thin an edge really is when properly sharpened for knife use (see knifenut's avatar). Anything that thin will bend/flex because the stress just isn't enough to break it. Also, consider that even high speed steel wood turning scrapers have enough ductility to allow a hook to be burnished onto them, though tungsten carbide is generally used for the burnishing.

You are correct in that higher hardness reduces the amount of plastic deformation possible. Edge rolling is still possible, but it's just very small before chipping happens.

PS: I tried my M2 knife at around 64-66 HRc on some bread tie wire (small diameter steel) and it didn't chip. I've also whittled on soft tie wire with nearly every knife I have and only the hardest will come out with minimal rolling, and only the best heat treated will come out with minimal or no rolling and no chipping. This tie wire is the kind used to tie rebar together before placing concrete around it; very soft, very ductile, and about 1mm in diameter.

I'd say people can commonly botch pretty much every step heat treating a knife. I came to this conclusion when I saw someone on youtube heating M2 HSS to nonmagnetic and quenching in oil.:eek::mad:. If its all you have, sometimes you have to go with what you've got, but that's taking it a bit far.
 
Gator97 said:
It also depends on the steel, edge angle, cutting method like me2 said.
I've cut copper wire, aluminum discs and steel wire with M2 64HRC and ZDP-189 67HRC. Neither suffered a single chip. Details and misrographs in superhard vs. soft edges article.
Back then I've had BM 154CM chipping at 60-61HRC on copper wire, later on Phil Wilson rehardened 154CM Nimravus to 62HRC and with thinner edge (30 inclusive vs. 40 inclusive before) rehardened and harder 154CM didn't chip.
So, it's not as simple.
Click to expand...

What knife was the 67hrc one?
 
How the cut was made can make a huge difference on how an edge reacts to cutting a certain material IMHO. I agree that the OP's chart is pretty fair but so many people push cut rather than slice and that plays a huge role on how the edge responds to cutting some materials. Cutting flexible cord or cable usually puts much more force on the tang end of the blade when you wrap the material around the blade and start to draw the blade through the material. At first you tend to pull straight through the cable before the knife angles more and begins to slice. I believe that friction point before the blade angle lessens and begins slicing can have a dramtic effect on any edge damage or other effects on the blade than if the slice was even throughout the stroke from the beginning.
 
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