How does Rockwell hardness affect edge holding?

Cosmic Superchunk

Cosmic Superchunk

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Jan 28, 2001
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I've been reading several articles regarding Rockwell hardness, and it seems people have different opinions on what is acceptable for high quality cutlery. How does a high Rockwell hardness affect the overall edge holding ability of a knife? Also, if a knife is Rockwell tested at 58-60, and the steel is say, 420HC, does this mean that it will perform as well as a blade made of AUS-8 or 440C at the same Rockwell hardness?
 
not at all.

every steel has it's own optimum hardness, on which it perfoms best.
There is just too much involved in a good cutting edge, and it certainly isn't only hardness. Other stuff like geometry, grainsize, structure, yielding strength and toughness are factors too.

420 HC (what is 420 HC? I can read metalurgy, but I can search up at least 4 different formulas for 420 with higher carbon) will perform well at 58 HRC, given tempering..cryothreath and all that.
440C can be taken higher (HRC 59-60), if given the matching high quality HT. I also think it is a better steel in the first place.

It's all in the heat-treath folks.. even a widely know and used steel like 5160 can still be improved by better heat-treaths, and it certainly hasn't given up all it's secrets. Given that 5160 has been used for sooooo long, how much do you think the HT of say 440C or 420HC can be improved? Maybe it will perform better if we find a way of dissolving more carbides and less free chrome...maybe a smaller grainsize will do better..maybe even a bigger?
I dunno..I have no equipement to test it out.

But, to get back to your question.. Yes, if all factors are equal and if the steels recieve their heatreath with equal care and expertise, the HRC may be an indicator of better vs worse edgeholding, but also worse VS better rustresistance and brittle vs tough steel.

I once tried to quench 5160 in brine. The blade was sooo hard, I had trouble getting an edge on in the first place (if you let me guess, something like HRC 65). Then I tought, it would cut fabulously, altough it may be too brittle. I tried cutting meat..excellent..the I cut paper..excellent..then I tried cord... hmm..half the edge lost tiny chips... then with the rest I tried to cut a point to a wood stake.. I lost about 22 chips of more then 1/10 inch before my edge was more like a saw then a knife and totally unusable to cut anything.

Just an example that high hardnesses mean nothing. On the wrong steel, 1 HRC point can differ between excellent edgeholding and bad edgeholding.

Anyone ever complained about edgeholding of opinels? they are about HRC 55, same as douk-douks, while other makers get their 1075 up to 60.

Hope I helped ya on this,

greetz, bart.
 
I had to reply to this because its something Ive been thinking about for a while. Basically, the way I understand things, is if the edge is too hard, it will chip too easily, leaving a broken and dull edge behind. If the steel is too soft, then it will just roll over, like trying to cut something with the edge of a piece of aluminum foil. A medium point must be found and it will differ from one steel type to another, and even one manufacturer to another for the same steel. Some companies add extra elemements into the mix. A list of D2, for example, had over 10 variations from different manufacturers. The alloy makeup for O1 in my metallurgy book differs from that mentioned in some of my knife making books. I tend to ramble, but the point is that there is an optimum hardness for different steels, and that point may differ depending on manufacturer, steel type, added elements, etc.
 
Thanks, for the info Bart. I had the impression that it was all in the heat treating as well. You mentioned how 440C can be treated up to a Rockwell hardness of 59-60 given the right heat treatment, so what advantage is there in using ATS-34 or 154CM at the same 59-60 Rockwell hardness, if the 440C blade can perform just as well?
 
440C tend to be more corrosion resistant then ATS-34 or 154-CM, because it has more chrome, which also leads to more carbides and bigger grain structure. Basically this means that if particles get lost from the 440C cutting edge, they will leave bigger holes, thus creating a duller edge then an 154-CM cutting edge which loses particles too. basically 154-cm will get duller too, but still be more usefull then 440C.
This is also the great benefit of 52100... it's grain size is sooo small (about 50 times smaller the 154-cm) that if it loses a particle, it doesn't really damage the cutting edge.

Dragon2 : you said it better then I could have as non-english speaking guy...actually what you said should be framed in and hung in every shop where a knife is being made. :D

moral: know the excact composition of your steel along with the excact way to heat-treath it for cutlery use (most manufacterers give HT direction for average use which is NOT cutlery use).

greetz, Bart.

P.S. wouldn't this be better in the shoptalk forum?
 
It is safest to say that there is no direct correlation between Rockwell hardness and edge holding, but in general, on the same steel, with the same edge shape, higher hardness will mean longer edge holding up to the point where the edge is so hard that it is too brittle for the work being done with it and the edge chips rather than wears.

Putting carbon in a steel makes it more hardneable and adding alloys such as chrome and vanadium make the steel more stain resistant and more wear resistant, because these are very hard substances. But, if you put too much alloying material into steel, it grabs up a lot of the carbon and forms big crystals in a weakened matrix, resulting in a steel that, though it is wear resistant, will chip easily. So 420HC will not be as resilient at high hardness as 440C, potentially, because 440C has more carbon, and less alloys mucking up the grain structure. I do not know if a 420HC blade would be much good at 60 RcH, if it can be hardened that high, but 440C should be much more durable than 420HC at 58.
 
You can not say that because two blades of different steels have the same hardness they will perform the same. Some steels will work fine at an RC value of 60 while others will be brittle and tend to chip. So many other things go into how well a blade will perform other than it's hardness. The chemical makeup of the steel and the grain structure are just a couple of examples.

High RC values are very important for edge holding, but are detrimental for other aspects of blade performance. The biggest problems being chipping and breaking. The harder a blade gets the harder it will be to sharpen as well.

Steels used to make a blade are picked for many different reasons. Resistance to corrosion and rusting, toughness, grain structure, low impurities, ease of sharpening and edge holding are some the reasons. The ability to put a certain finish on the blade is another reason to choose a certain steel. Some will not take a mirror polish. Some makers like a steel because they have used it a lot and become familiar with how to get the most out of it.

Sorry if this has wandered away from the original question a little. I do tend to go on and on.
 
Dont worry. I had to go bttt and check the question again myself.
Another thing to take into consideration is how the edge will fail. Did it fail because the edge rolled over or because the edge was actually abraded away, or did the edge just chip off bit by bit like the 5160 that bart student tested after brine quenching? Harder steels tend to roll less, but may chip easier. Actual loss due to abrasion doesnt seem to be as common as I once thought. IMO this type of dulling has more to do with the alloy content than hardness. However, since alloy affects hardness, they are related.
I cant take credit for coming up with the abrasion vs rolling vs chipping part. That came from Mr. Stamp back in the archives somewhere.
 
I think the first thing to understand is that wear resistance and edge holding are not necessarily the same thing. Let's start by looking at two blades of the same steel, then we'll talk about comparing hardnesses of blades made from different steels. Given two blades made of steel X, one heat treated to 56 Rc and one heat treated to 61 Rc, it is reasonable to assume the 61 Rc version will be more wear resistant. That does not necessarily translate to more edge holding. It will translate to more edge holding if the cause of edge degradation is that it is wearing down. But what if these knives are being used as heavy choppers? In that case, the 61 Rc version will likely be much more brittle, and might chip out badly. In that case, the tougher less-wear-resistant 56 Rc blade might hold its edge longer than the harder, more brittle, more wear-resistant 61 Rc blade. Cliff really brought this out and I'm convinced he's right.

So, since edge holding is a function of many factors, including wear resistance, toughness, strength, the particular job the knife is being used for, etc., it will be more correct to use the term "wear resistance" instead of "edge holding" for the rest of the discussion.

Now, given two knives made from different steels, one from steel X at 56 Rc and one from steel Y at 60 Rc, will the harder steel Y blade have more wear resistance than the softer steel X blade? The answer is absolutely not (note that you've seen a different answer in this string; I believe that answer is wrong). It is easy enough to demonstrate that 440V at 56 Rc will have more wear resistance than ATS-34 at 60 Rc. Talonite, with an Rc measured in the 40s as I recall, will have more wear resistance still. Wear resistance is to a large extent a function of the types, amounts, and distribution of carbides in a steel. Given two different types of steel, it is not just possible, but not that uncommon in cutlery steels, that a steel with a lower Rc might have more wear resistance than a steel with a higher Rc. While it is more often true that a higher-Rc blade of one steel will have more wear resistance than a lower-Rc blade of a different steel, there are enough important exceptions that it's easy to make mistakes if you use that as a rule-of-thumb.

So, in summary, what do we think we know about wear resistance and edge holding?

Steel X at 60 Rc is probably more wear resistant than Steel X at 56 Rc.

We have no way to knowing offhand if Steel X at 60 Rc is more wear resistant than steel Y at 56 Rc.

Wear resistance is not the only determiner of edge holding.

Joe
 
This is one of those good questions that comes up every so often. Unfortunately, there is no simple answer. Joe and others have made the important point, you can not compare edge holding between different steels based on an RC value.

Many other factors will play a role in wear resistance (e.g. alloy composition and heat-treatment). Edge holding is affected by even more factors such as the blade size and edge geometry. Wear resistance between different steels at the same RC may be quite different (for example, S90V will be several times more wear resistant than 440C even if both are RC58). Depending on the heat treatment, edge holding may be very different in different blades with the Same steel alloy at the Same RC value.

This old thread from the archives has some great posts on this topic: High RC = Better Edge Holding?

Para
 
Congratulations to all of you.
The light at the end of the tunnel is beginning to show.
Happy sharpening :)
 
hehehehe..
All the answers are here.. simply too many factors determining that correlation, and we're summing them up. Alloy composition, HT, structure, grain size.... And all those factors correlate amongst themselves to make a bit more complicated. I better get a factor analysis and a variantion analysis :D

greetz, Bart .
 
Since I forge the carbon steels, I have always liked my knives to have between a 55 Rc and 57 Rc. For a while I didn't think they were that low, according to the tables of heat treat, until I had them tested on a diamond tester. At that time I thought that there was something I was doing wrong because the softer blades were not supposed to perform that well. I have come to the conclusion that it all depends on the use of the blade as to what Rc is best. The best Rc for the blade will be determined by the type of steel, heat treat, and end use.
Due to the small variations in the heat treat of factory knives, I would buy one and use it for a while, if it didn't "hold an edge" like I expected, I would trade it off and buy another. The good ones I would carry for several years.
Bladeforums has helped to eliminate the possibility of buying a POS for a lot of people.
 
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