Higher RC = better edge holding?

[Seb]

Does a high RC always mean better edge holding
a in general
b if its the same steel
?

Seb

 

[Paracelsus]

a) No, RC does not allow comparison of edge holding between different steels

b) Maybe, RC does correlate to some extent with edge holding when comparing hardness of the same steel. However, if the steel is too hard (eg RC63), it may be very brittle and prone to chip out the edge. An RC of 59-60 will hold an edge better than an RC of 50 when comparing the same steel.

When your throw in non-iron based blade materials like talonite, all bets are off. Talonite normally has an RC in the low 40's, but holds an edge better than many much harder blade materials.

There is always a compromise of functional properties in the composition and making of blade steels. RC is only one measure of performance. A blade of 52100 at RC58 will not be identical in edge holding to a blade made of 440A at RC58.

RC measures resistance to Penetration. There are many other physical properties of steels that contribute not only to edge holding, but to other aspects of blade function as well (e.g. toughness and flexibility). RC is easy to measure, so its a commonly cited number. But its utility is limited and often overstated. There is a tendency of many production companies to push the hardness envelop too far. I personally think many blade steels make better knives at RC values slightly lower than sometimes produced. They sharpen more easily, and are tougher.

Size matters too. Small blades can tolerate greater hardness compared to large blade which needs to be tough for chopping. Differential heat treatment is probably the ideal way to make a blade because to edge can be produced with a very high RC, but the back of the blade and the tang can be made much softer, giving the blade both toughness, and great edge performance.

Paracelsus

[This message has been edited by Paracelsus (edited 11-09-2000).]

 

[dePaul]

Bravo Paracelsus, really slick

I was about to say some good words, but you´ve said it all

*joke*

 

[Joe Talmadge]

Paracelsus gave a good reply, but let me emphasize. You can't compare RCs between different steels and expect meangingful data. Within the same steel, higher RC always means more wear resistance, but wear resistance does not always mean better edge retention. If the edge is too hard, it could chip out during use. For example, if you had two big camp blades in ATS-34, one at 63 Rc (don't ask me how you got it that hard!) and one at 58 Rc, you might very well micro- (or macro-) chip out the 63 Rc version, so the softer 58 Rc version would hold an edge better.

Joe

 

[russ]

finally saw a topic i wanted to get in on...but para said it so well.right on target.i recently got an atkinson folder in 154cm. i speculate around rc60. it really holds an edge well.[i.e. resistance to penetration].i know i did'nt say much, para said it so well, just had to get in on this thread.

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russ aka bladezealot.{that's blade zealot! not bladeze alot!


a cat almost always blinks when hit in the head with a ball-peen hammer.

 

[Cliff Stamp]

Joe :

Within the same steel, higher RC always means more wear resistance

In general yes, but there are exceptions. It may be possible that there are differences in grain refinement and carbide formation which could give high alloy steels greater wear resistance at lower RC. To make matters worse, steel composition and quality can vary significantly, which can effect performance.

For example I have used A2 from Mission at their spec'ed 56 RC and A2 from Reeves which is spec's at 55-57 RC. They performed very differently. I saw a large improvement in terms of edge strength and retention from the Reeves blade. I would suspect that the heat treatment is more involved and possibly a better source for the steel is used.

-Cliff

 

[tom mayo]

although the resistance to penetration statement is true....The Rc on knives is a measurement of HARDNESS.....most knife steels can be made VERY hard....and come out of the oven after initial heat treat as such. After the cryo cycle they are then tempered down to the appropriate hardness, where the toughness comes up and the hardness goes down....the balance making the knives a much better tool. You can harden ATS and most of the tool steels far above 60Rc....but they remain brittle and unsuitable for a functionable knife blade. Stellite is not really as soft as some make it out to be....there are hard cobalt/tungsten/chromium molecules in a softer matrix and that is what is measured by the rockwell tester....It can NOT be drilled thru with cobalt drill bits (actually I did it once....it took me about 1/2 hour....constantly resharpening the bit)

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http://www.mayoknives.com

 

[dogboye]

Originally posted by Paracelsus:
<center> SNIP </center>
When your throw in non-iron based blade materials like talonite, all bets are off. Talonite normally has an RC in the low 40's, but holds an edge better than many much harder blade materials.
<center> SNIP </center>

Yes, BUT the edge on my Talon will roll much easier, given any lateral stress. While it passed the "cutting through the bottom of a plastic bottle" (across the center, though the hard stuff) test without tearing the edge, AND remained shaving sharp, the edge rolled SIGNIFICANTLY when I accidentally nicked the edge of a faucet while cutting up veggies. Straightened out easily, and back to shaving sharp without permanent deformation. So, while Talonite (or cobalt alloy, at least) is my favorite small knife blade material, it does have some limitations that people should be aware of. Wear resistance isn't everything with respect to edge holding (probably most of us reading this thread know that... but I state it anyway, because it took me a long time to accept it), and I'm almost coming to the conclusion that edge holding isn't everything WRT to a good knife.

Sorry to ramble a bit there.


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iktomi

 

[Cliff Stamp]

Tom :

Stellite is not really as soft as some make it out to be

It is as soft as the RC indicates. Saying it is not because it is different is like saying a pound of feathers is lighter than a pound of lead.

there are hard cobalt/tungsten/chromium molecules in a softer matrix and that is what is measured by the rockwell tester

The same is true for steels, but you don't see people arguing that low 50 RC steels with 75+ RC Vanadium Carbides are not soft because of the really hard carbides. The matrix is what indents and bends and such not the carbides.

-Cliff

 

[Walt Welch]

Tom Mayo said:

...there are hard cobalt/tungsten/chromium molecules in a softer matrix and that is what is measured by the rockwell tester.

I believe that Tom meant to say that there are hard tungsten/chromium (as well as Fe and Mo) CARBIDE molecules in a softer matrix (Co), and the softer matrix is what is measured by the Rc test.

Jerry Hossom put it much more elequently: http://www.bladeforums.com/ubb/Forum3/HTML/001292.html

One point I'd like to note, however. I think it might be worth considering that heterogenous structures like dendritic, damascus and wootz steels behave much differently than homogenous materials like normal steel and talonite. While the matrix in those structures may be relatively soft, they gain a great deal of strength and wear resistance from the much harder inclusions or layers in the composite. The soft matrix can erode a lot before the harder material is influenced.
Isolated parameters like the Rockwell hardness of a part of that structure may poorly define its overall performance when compared to homogenous materials we are more familiar with.

As Cliff Stamp pointed out further down in the same thread, expounding on Jerry's post:

Thanks for pointing that out Jerry, I have noticed that transition point with steels and just was not making the connection here. It was probably because of the significant difference in matrix composition which could quite easily make the transition points happen at two very different times.

There are basically two stages of blunting, the matrix deformation and the carbide wear. The second is much longer than the first, but with a hard steel the first one is still very long and what most people refer to as blunting in general. Since the cobalt alloys have a soft matrix the very edge may deform and go out of crisp alignment very quickly. Thus the optimal performance takes a sharp hit quickly. However after that the wear resistance becomes the dominant factor and they compare well to the best steels in that area.

Thus it seems fairly well shown that Rc per se often, but not always, is an indicator of more wear resistance whether considering the same steel or different alloys.

As for the latest word on what happens when the Talonite (r) meets the chopping board, here is the latest word from Nick Blinoff, who was the lucky recepient of the prototype Talonite (r) chef's knife. Nick is a professional chef who owns a busy restaurant, and uses his knives more than most people.

In regards to the Talonite monster... It is still going strong! I still
am using it as my number one chopper everyday, and I am still an
enthusiastic Talonite supporter.... Boy does this stuff hold an edge or
what?

The only thing that is taking some getting used to is the technique of
sharpening the blade. I think I'm up to four times on the Edge pro ( not
bad for over a year! ). The first three times I got a scary sharp edge
fairly quickly, but the last time it took me nearly an hour to regain the
sharpness. Is this what is known as the slippery edge? Also I've found that
the Talonite seems to respond to my fine stone rather than a coarser stone.

It would seem that in real life use, the Haynes alloys, such as Stellite (r) and Talonite (r) are notable exceptions to the general rule that Rc is proportional to edge retention.

Hope this helps, Walt

 

[Roger Blake]

Just my opinion, and somewhat off topic, but RC seems to be over emphasized and the ability of the steel to take a keen edge is under emphasized.

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Roger Blake

 

[Cliff Stamp]

Walt :

Thus it seems fairly well shown that Rc per se often, but not always, is an indicator of more wear resistance whether considering the same steel or different alloys.

Increasing the RC of a steel can increase its wear resistance, and it usually does, depending on several factors and exactly what kind of wear you are talking about. So there is a relationship there and all else considered equal you might even be able to draw a performance ratio on two different RC's, however, and this is a big however, very rarely is all else equal when it comes to knives.

As Paracelsus noted, there are lots of other ways to increase wear resistance, such as alloy content, carbide structure, grain refinement etc. . A deep cryo treat can change wear resistance significantly and have almost no measurable effect on RC. And besides this there are several other factors like strength, ductility and impact resistance that contribute to edge holding.

RC is a useful specification to have as it indicates hardness which is resistance to indentation, which is an important property, making generalizations about the other material properties from RC is not nearly as easy and you need far more information such as alloy content, and heat treat info.

-Cliff

 

[Keith Montgomery]

If it is the same steel a higher RC will mean better edge holding. That being said, there is an RC range for steels that takes into account edge holding, ease of sharpening, strength and durability. It does not make any sense to make a blade so hard that it can not be sharpened, or that it breaks or chips easily.

 

[Ironhorse]

Everybody said it so well, I stayed out of this one, besides I'm a 2 fingered typer!! HA! HA!
Ironhorse