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What does Rockwell hardness really tell you about your knife?

Oct 11, 1998
Each time we read about knives, the term RC hardness pops up. Ok, now we all know (sort of) how it's been determined and that higher RC means harder steel. But what does it mean when it comes to performance? The general idea is, more RC = better edgeholding. Well, maybe not.

My opinion is, you have to see the whole picture. That means, RC and the type of steel used. For example: D2 or CPM4X0M with an RC of >60 makes it fairly brittle and chipping the edge on a bone for example is not what you could call "edgeholding". In addition there is the term abbrasion resistance (AR). Now I'm not a metalurgist, but I don't think there is a relationship between RC and abbrasion resistance. Titanium for example has a fairly low RC but high AR. Ceramics like MirageX has a very high RC and also a high AR. Then there is Stellite/Talonite, which you could call a sort of metal matrix, meaning harder particles embedded in a softer base. The overall RC is low but AR very good. It is probably safe to conclude that RC doesn't tell you anything about edgeholding at all.

As a result I would say that abbrasion resistance is more important when it comes to edgeholding than RC hardness. The only thing RC can give you is to determine how brittle the specific steel is. In other words, what to expect of your blade in terms of toughness.
RC = Rockwell Scale C
depth of penetration of a 150 kg mass on a diamond cone, angle of incidence 120 deg.
Measures hardness only. Edge holding can be indirectly predicted based on the properties of the material but no direct relation.
What does Rockwell hardness really tell you about your knife?

RC to me is only applicable to the same steel type. What do I mean? ATS-34 at RC 57, BG-42 at RC 57 and A-2 at RC 57 will give you vastly differing performance characteristics.

In A-2 I want an RC of 57 nominally because that is where that steel performs best. A higher RC will make it generally brittle when used in a psuedo-axe application as A-2 seems find itself so often. Do you split firewood with your knife? If so, an A-2 knife is what I would choose.

ATS-34? I want it hard like Benchmade does it. Why? Edge holding for that combination is very good for the tasks it is meant to perform. No, you won't get an "axe" level performance.

Each steel has an application and an RC value that makes it perform. If you find that knife steel with an RC that is not in the accepted range, it is a warning that the knife is either for a special use in a specific application and you need to evaluate it closely or, somebody misread the heat treat specs, yielding a knife that is apt to perform poorly.

To call the RC value a quality control indicator is not accurate but, on the other hand in practical usuage for a given steel, it comes pretty close.


[This message has been edited by Sid Post (edited 07 April 1999).]
as you have seen above...the rockwell reading is a hardness factor...but the main thing it tells you is whether or not you ran your heat treat profile properly....if you follow the receipe and your knife comes out 54 (say its 440c) then something is wrong.. when paul bos does mine and this happens he anneals the steel and does it again...if it happens twice he calls me up and yells at me that its not 440c (or whatever) one time i made a knife out of 416 and sent it to him.. it is imperative that you keep your steels seperated and marked (something i have failed to do a number of times) as you cannot run ats the same way as 440c or any other steel....d2 is totally different and all of these steels have great variations in temps at hardness, stress relief, and temper temps.....one thing also....you may get sent the wrong steel by the distributor...and you will never know until the telltale rockwell tester says its 416....not 440c.....it happened to me.....p.s. just remembered..on a lot of the high chromium exotic types the Rc must be high or the corrosion resistance goes out the window. (thats what paul bos told me)so you cant lower the hardness to get toughness with losing a lot of rust resistance.....dont know why...

[This message has been edited by tom mayo (edited 07 April 1999).]