Hardness, Edge Retention and S30V

[Confederate]

One of the reasons hard steels do well is that they retain their edges better than softer steels. But as steels become harder, they also become more subject, generally speaking, to cracking and breaking. Thus, good steels are a balance between hardness (edge retention) and softer (strength). If the heat treat is good, an optimum balance is struck and tough, hard steels are produced, albeit at significantly higher costs.

But what's with S30V and hardness? If the above is correct, edge retention should be better if the steel is harder. But Kershaw states regarding its Bump line with S30V: "The four-inch blade is flat ground for strength and built of S30V stainless steel. S30V is made through a metallurgical process that results in extremely uniform molecules; so much so that inclusions or impurities are eliminated or insignificant. The process creates steel that is strong, workable, and has excellent edge-retention properties. It rates a durable 55-57 on the Rockwell Hardness Scale."

This raises questions of how S30V can be so edge retentive, yet so soft when compared to inferior steels. The upper RC of AUS8 is said to be 58, but as low as 56. I assume based on this that it's a hit or miss thing...is this right? If so, it would be technically possible for an AUS6 to have a harder RC than an AUS8, correct?

I'm a little confused on how Rockwell relates to edge retention. Which is more important? Steel compositon or Rockwell hardness? Yes, I know heat treat is a vital element, but I'm more interested in tolerances.

Finally, what equipment would someone need to test RC? Is it expensive and where would it be available, if it is affordable?

Thanks.

 

[knarfeng]

Edge retension is a combination of hardness and wear resistance.
S30V has a high Vanadium Carbide content, which provides excellent wear resistance.

You would need a Rockwell hardness tester to test hardness. I've seen used ones for about $900 on the bay place.

You could also send your knives to a metallurgical lab. The tes takes about 5 minutes to run and leaves a pinprick in the blade.

 

[hardheart]

rockwell test equipment is expensive. Heat treatment is critical, because that's how you get the rockwell test results you want. Steel composition seems more important to what you are asking about. You can get 1095 to 66 Rc, you can get ZDP-189 at least that high. Nobody sells a mass produced 1095 blade at that hardness, and for good reason.

The two point spread is fairly standard, from what I had read before. Really it comes down to the fact that these blades are being batch heat treated, and there are acceptable levels of variance in the test results. I have no idea why someone would heat treat S30V to only 55-57. I also have no idea why molecules were mentioned, steel does not have molecules.

 

[salty141]

I also have no idea why molecules were mentioned, steel does not have molecules.

Of course steel has molecules!
- Kinetic Molecular theory-
-Matter is composed of small particles (molecules)
-Matter-is commonly defined as the substance of which physical objects are composed

They are refering to the cpm process, and what is does to the basic composition of the steel when it says uniform molecules.


The process of producing CPM (Crucible Particle Metallurgy) steels involves gas atomization of pre-alloyed molten steel to form powder. This powder is then screened and then isostatically compressed into 100% dense compacts. The CPM process produces steels withe no alloy segregation and extremely uniform carbide distribution characterized by superior dimensional stability, grindability, and toughness compared to steels produced by conventional processes.

check here for an example of cpm steel in comparison to conventional steel (a bit down on the right)
http://www.crucibleservice.com/datash/dsS30Vv4b.pdf

 

[dantzk8]

One of the reasons hard steels do well is that they retain their edges better than softer steels.

It depends of the task you use the blade to. For instance a soft steel chopper will keep a better edge than the same tool made of a hard steel which will chip or even crack.

Thus, good steels are a balance between hardness (edge retention) and softer (strength).

You confuse there strength and toughness. Hardness increases strength, sotness increases toughness.

I'm a little confused on how Rockwell relates to edge retention.

Because edge retention is not only function of hardness. It's function of a balance between strength, toughness, wear resistance. That balance can be good for a task, bad for an other one. To such a point that stain resistance can have some importance as well.

dantzk.

 

[Joshua J.]

If I recall correctly, when Kershaw stated their Rc numbers they were quoting some theoretical number, or at least those numbers don't apply any more.
Kershaw reps have stated several times that they heat treat S30V to around Rc 59 like most people.

CPM S60V is another story.
For some reason the combination of alloys in S60V makes it more brittle than average and is usually hardened to Rc 55-57. This is very soft for a high quality blade steel, however it still holds an edge quite well due to the extremely high alloy content combined with the particle metallurgy process.
This is one example of a low hardness metal with good edge retention.

That is, good edge retention for certain tasks.

Part of the problem is that there is no scientific way to measure edge retention.
There is the CARTA machine, but that only cuts a specific material in a specific way. Real world usage is extremely dynamic.
A knife made to whittle hardwood should have very different steel compared with a knife made to cut dirty rugs.
Your edge will dull differently depending on how it is used, sometimes higher hardness is more important than alloy content.

Not that it's impossible to make steel that does both, it's just on the fringe of what current technology is capable of producing.
Friction Forged D-2 is one example.
http://www.diamondbladeknives.com/

Crucible has also made CPM S125V. Which is so hard and wear resistant that knife manufacturers have yet to come up with a way to efficiently make knives out of it.
And there are only a very few custom knife makers who will touch the stuff.

 

[mete]

Metal have metallic bonds and therefore have crystals not molecules .Molecules are found in plastic which has molecular bonding....The benefit of the CPM process is that , in addition to low amounts of inclusions , the carbides are smaller and more evenly distributed. This makes it tougher along with other benefits....The working hardness of most S30V blades as I understand is a bit higher,59-60 HRc....Edge retention is due in large part to carbides and in this case V carbides are very wear resistant !..Often the 'measure ' of a blade for many is hardness but that doen't tell the whole story ,you have to look at all the properties.For example Talonite blade matrix is quite soft but the wear resistance is very high due to the carbides !

 

[HoB]

Well, about the manufacturers blurp, I think it is not sensible to read and take it apart with the eyes of a classically trained scientist, not even with those of a BF trained knifenut. Historically, all small particles were were called molekels or molecules without referring to a certain chemical bond (in fact, some old german chemistry Professors might still do it) It is not uncommon to read it in pre-WWII scientific literature. Now, the term "molecules" refers to atoms bonded in small predetermined units by a covalent bond (in a nutshell). If I understand the blurp correctly, it refers to the small particles that are produced in the CPM process, they should be larger than the crystal size, so I don't think it would be correct to replace "molecules" with "crystals" (not to mention that crystals by definition should be pretty uniform), not to mention the effect the blurp would have on the general knife buyer, if it was talking about crystals: "Hey, check out my new knife, the blade contains CRYSTALS! Must be a form of diamond dust.....!". No, I think for the intended purpose is perfectly acceptably phrased. Don't be so uptight, guys .

As to the Rc range: could it be simply a misprint?

 

[Frank Hillary]

Hi,

I like to knw to, but you got to ask a matelurgist. Any left on the forum? Cliff was one but he left.

Christmas Cheers
Frank

 

[hardheart]

a metallurgist already answered in post #7 . Cliff wasn't a metallurgist, I think he was a particle physicist, can't exactly remember. Technically, he didn't leave.

Cold STEAL!
Sebenzas are overpriced!
Strider knives are low quality!
Benchmade vs Spyderco!

Hopefully this will prevent thread drift, as the flaming would be spread too thin to survive

 

[Frank Hillary]

hardheart

a metallurgist already answered in post #7 .

Sorrrrrryyyyyyy. I didnt rlize that mete is a metalurgist. It be nice if he could explain this a bit more. I dont quite undetrstand what he measn by inclusions. And if talonite works at a low hardness why does S30V have to be higher thatn 55-57? Anyway one hardness test is not enough and cant do one on the edge where it counts, only the spine. I reckon many knives are much softer on the edge cause of overheting during grinding.

Christmas Cheers
Frank

 

[magneto]

Mete rocks!!! It is so cool to have someone of his caliber here for us Bozo's.

 

[rprocter]

i need a little help here. quote Mete "metal have metallic bonds and therefore have crystals not molecules". but the crystals are made up of molecules (made up of atoms, etc.), and what is a "metallic bond" ? thanks, i would like to understand this. roland

 

[Confederate]

You confuse there strength and toughness. Hardness increases strength, softness increases toughness.

Thanks. Can you elaborate on the difference between toughness and strength? If one needs to abuse a knife in an emergency by prying or taking external impacts, would it be strength or toughness that would be required?

Sometimes I think too much of an emphasis is placed on edge retention, especially because I've noted that even relatively dull blades will still cut, except in the most exacting of circumstances. Some have said that Damascus steels also tend to maintain a degree of sharpness because of the many layers, and that as they're worn away, succeeding layers keep the blades from being entirely dull.

If I recall correctly, when Kershaw stated their Rc numbers they were quoting some theoretical number, or at least those numbers don't apply any more. Kershaw reps have stated several times that they heat treat S30V to around Rc 59 like most people.

Well, I fer one would certainly recommend to Kershaw that they update their specifications if the above is true.

 

[hardheart]

strength is resistance to deformation, toughness is resistance to failure from impact. plate glass won't bend and take a set, but it shatters when struck; bubblegum is soft and malleable, but it won't shatter if you hit it.

 

[StretchNM]

Far be it from me to argue with the wealth of experience found in varying degrees in this Forum, LET ALONE argue with a metalurgist.....BUT....

I'm pretty sure rproctor (and others) has a point: what is there on earth that has no molecules? As proctor suggests and I ask, wouldn;t the crystals be made of molecules? And if S30V has no molecules, is it the only thing on earth not comprised of them?

If my S30V blades are moleculess, that is without moleculii, they must be evil and should be banished! (( ))

 

[hardheart]

I sure hope mete gives the proper answer, but I'm gonna go ahead and botch it first

steel isn't made of molecules because the iron and carbon aren't bonded. AFAIK, the reason we can harden steel is because it is not a molecular structure. When you change the temperature of the solid solution, the alloying elements can move about a bit. Hard or soft, it's still called steel. You can have a chamber full of hydrogen and oxygen gases, or a chamber full of water. There isn't a 'water' equivalent in steel, the cementite, ferrite, martensite, bainite, etc. are just ways the iron and carbon intermingle. As water, steam, or ice, it's still H2O, you lose that specific bond, it isn't the same thing.

 

[dantzk8]

Can you elaborate on the difference between toughness and strength?

Toughness: ability to take an impact without damage. Important point for chopping tasks.

Strength: ability to take a load without permanent deformation. Important point for hard stuff cutting and when a lateral load is applied to the edge ( like wood cutting).

If one needs to abuse a knife in an emergency by prying or taking external impacts, would it be strength or toughness that would be required?

A tough steel will forgive somes excess. A strong steel will be uncompromising.

Sometimes I think too much of an emphasis is placed on edge retention, especially because I've noted that even relatively dull blades will still cut, except in the most exacting of circumstances.

More important, for me, than the edge retention is the cutting ability. i can sharpen an edge, i can't change a geometry.

dantzk.

 

[Joshua J.]

Well, I fer one would certainly recommend to Kershaw that they update their specifications if the above is true.

The information you quoted is old. The Ti Bumps were made back around 2004 and that was one of their first models to use S30V, so they were still learning the ropes.
Kershaw's new website doesn't quote stats like steel hardness, but If you ask they will usually tell you what hardness they are treating their knives to.
When they had their steel chart up I made a similar complaint, which is one of the reasons they don't publicly display that kind of information anymore.

 

[neeman]

Good information here
Thanks to all