What's the "RC" value of knife steel mean?

D

DGG

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May 3, 2005
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Okay, I'll admit it. In spite of using knives all my life I'm a big dummy when it come to specifics.

I understand the RC value relates to the hardness but what or where did it come from? Is it also related to strength or toughness?

Is 60 RC 10% harder than 55RC or there a log scale being used.

What is the optimal RC value for a fixed knife, folder, etc. or is it all the same?

Can all steels types be hardened to the same RC number or is dependent on the type of steel, ATS34 vrs CS60v for instance?

Are ceramic blades like those used in chef knives given RC numbers?

I'll shutup now. I said at the beginning I was a big dummy.
 
RC is an acronym for Rockwell hardness scale C. Rockwell (the corporation) made hardness testers and depending on the material different scales were used. Scale C is used for steels.

The number is based on a measurement made of how deep of dent the tester makes in the material. I do not think it is logarithmic but I am not sure of that.

People will debate all days about optimum hardness. Harder means it is usually more work to sharpen. It all depends on the heat treatment of the steel. Now I am getting out of my depth so I will shut up
 
Optimum hardness depends on what you are using the knife for. Also, depends on what steel you are using because each steel has it's own maximum hardness. But generally speaking, most knife steels are hardened between 58 and 62. Don't know about ceramic blades. I would think that because they can't be sharpened, they are made much harder than 62. That way they stay usable longer.
 
Hardness depends on the steel. The basic rule of thumb is that the harder the steel, the better the edge retention...to a point. Each steel alloy has a certain point where it becomes too hard and is excessively brittle. A brittle blade will chip and damage easily.

On the other side of the coin, a soft blade will wear quicker, and is more susceptible to rolling (where the tip of the V of the edge turns to one side).

For every steel, there's a balance point between too hard and too soft. For most modern premium steels (ATS-34, 154CM, S30V, VG-10), the sweet spot happens to be about 58-60Rc.

However, not all steels will be that hard. Largely, attainable hardness while retaining maximum performance depends on the amount of Carbon (but the rest of the elements in the alloy play their parts too). 420J for example has pretty low carbon, and high chromium. You won't see it beyond 55-56Rc, because trying to make it harder would make it too brittle. Optimal performance in a steel in terms of edge retention generally come with higher carbon, and less chromium (at the sacrifice of stain resistance).

But this is a very general rule of thumb. Every steel has its own quirks, using it's particular alloy composition to get the job done. And not every steel follows the rule of thumb. For example, CMP-440V (aka S60V) is an extremely wear resistant steel, but can become brittle and nearly impossible to machine/sharpen. Because of this, it's usually only hardened to about 56-57Rc. However, when compared to other steels of this hardness (AUS-6, 440A), it'll outlast them by far in terms of edge retention. Easilly several times longer. The disadvantage is that it still has more potential to roll than a harder steel.

I honestly have no idea how Rc is measured, or what the numbers actually mean. I just know how some steels work at certain hardnesses.

I hope this helps, and I hope I didn't confuse you too much.
 
If you aim for HRC 55 to 59, i guess any steel with a carbon content over .5% can be tempered to that. Not to be specific but Roman Landes and some other custom knife makers were hardening and tempering a .5% carbon 14% chromium steel to HRC 59 and the blades work great.

Initial hardness (right after quenching) is the reaction of the steel to a certain temperature treatment. See, that various steel grades are hardened at same temperatures but behave different depending on what they are made of. Most important: The higher the carbon content, the higher the hardness. Other alloys can raise the hardness too.

If i got it right, the difference between two points of HRC is the depth of 0.02mm. So HRC 60 is not 10% harder than HRC 55. The diamond tooth was into the steel .1mm deeper.

Optimum is related to intended use.
 
RC measures hardness. A file is hard, but shatters if you drop it on a cement floor. Toughness is the ability to avoid breaking when something like bending stress is applied to the material. One way that is measured is with a Charpy test. This sort of hammers a sample in a bending direction. High charpy numbers implies a high limit where the material neither breaks nor takes a permanent bend when hammered. To increase the Charpy of a file you might bake it in your oven at maximum temperature for a couple hours. Then you could grind it into a knife blade that would not break.
 
DGG said:
I understand the RC value relates to the hardness but what or where did it come from?
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A steel is hit with a harder material of a specific shape and the depth of dent indicates the hardness. Take a hammer and whack a piece of wood, same thing, HRC testors just do this with in a very precise way.

Is it also related to strength or toughness?
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Yes, the strength relationship is fairly simple, almost linear, but changes slighly from steel to steel. The toughness one is non-linear, complicated and changes massively from steel to steel. One steel can be very brittle at a given hardness and the other one quite tough.

Is 60 RC 10% harder than 55RC or there a log scale being used.
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The HRC scale is linear, it is just C-d, where C is a constant and d is the depth of the dent. Almost no property however is linear on it.

What is the optimal RC value for a fixed knife, folder, etc. or is it all the same?
Click to expand...
It depends on the steel and what you want the knife to do.

Can all steels types be hardened to the same RC number or is dependent on the type of steel, ATS34 vrs CS60v for instance?
Click to expand...
Those two steels are similar in that they both can reach ~62 HRC, steels range from ~22 HRC to 72 HRC in terms of maximum hardness. Most cutlery steels fall between 40-60 HRC, with the vast majority being between 55-60.

Are ceramic blades like those used in chef knives given RC numbers?
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No they are too hard for that scale, which is usually limited to ~65/66 HRC, though it is often extrapolated higher.

-Cliff
 
Larry S. said:
This may help....

http://www.gordonengland.co.uk/hardness/rockwell.htm
Click to expand...

Larry

That's a great link and helps to explain the Rockwell testing procedure. It also references many other good links related to hardness/toughness. I added it to my "favorites" .

Thanks to all who posted a response. It appears that the RC value "Depends". No one rule fits all situations.

Are less expensive knives made from 420, etc. of the same hardness as the better ones? I have never seen an RC number referenced like you do with the more expensive knives (probably for a good reason).
 
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