What makes a steel 'aggressive'?

[Bolster]

Good discussion, thanks. Kennbr did a good job of describing what I mean by aggressive, where he says:

"...a kind of sticky and biting feeling when pressed to the skin, where it seems to grab at the epidermis with barely any pressure at all…"

and BBB:

...it slices into the material with great ease and depth...

I noticed the difference with the three M4s despite sharpening to the same angles as other knives (12-13 dps back bevel, 15 dps micro bevel) and to a high polish. I am surprised by the aggression even with mirror polish on the blades. This is a subjective perception, I have no objective measurements of it. Yes, the sharpening was done with Venev diamonds. Finished with a 1200 Venev Orion. Part of what has me confused is that I would not expect this "grabby" edge at such a high polish. At a lower grit edge, not as surprising.

[elena86]

Is that question what aggression is, or how aggression is achieved?

I'm no expert, but aggression in an edge to me means the edge has a kind of "sticky" and biting feeling when pressed to the skin, where it seems to grab at the epidermis with barely any pressure at all. Sometimes if you cut material that's under a tensile load like rope, fabric, sheets of plastic etc, it's almost as if it invades the material at a microscopic level and then causes it to pull apart under the tensile load without it needing to actually push or slice, but simply touching it. For example, if I open up a bag of cat food or feed, I like to pinch the corner and pull it so the material is being pulled on, and an "aggressive" knife will just touch it and cause it to separate along a naturally formed seam without any real exertion of pressure. Though, this definition of 'aggression' for me may also be conceived by how it is accomplished...

Which is just making the edge toothy, 'micro-serrated' or however else you want to phrase it. This is accomplished in a lot of different ways depending on the steel and abrasive used. I think simple, low-alloy, and softer steels tend to get their toothiness imparted by the grit level. Other steels like carbide-rich variety seem to have it naturally by merit of the carbides in the steel matrix. So that is one reason a lot of people will say "aggressive steel" because it tends to be harder to polish them into a smoother apex, and they generally remain aggressive even at higher grit levels. Ultimately though, however they're formed, they basically act like little needles that can penetrate material with extreme readiness because they're at such a small scale.

One empirical test I have done to support this in the past is with a BESS tester. Typically, people will see this as a test of a blade's push-cutting ability, and generally people will say that a highly polished edge is better at push-cutting than a very toothy/aggressive one. However, I have found in the past that I have been able to achieve higher BESS scores by using a very toothy edge, and loading the thread under high tension, so when the microscopic teeth just barely penetrate the test media, it immediately pulls apart and counts as being severed. I've gamed that system in such a way to get absurdly low BESS scores that weren't very representative of how sharp the knife would be if testing in good faith, but showcase how the micro-serrations definitely penetrate and cause taught material to pull apart.
……………..

^^^ That.

[Vamais]

To me, aggression is something that I see when opening boxes. A non-aggressive steel will politely just cut the tape, and stay in the gap between the cardboard flaps. An aggressive steel, however, will wander into the cardboard, and instead of being repulsed back into the tape region, will happily bite into the cardboard and cut right through it with ease. It will cut where I point it, instead of finding the path of least resistance. I have noticed this difference particularly between my M4 GB1 and S35VN Native.

[Fireman]

Testosterone

[Flash]

Large carbides - For example Cast 440C has huge carbides formed in its microstructure and as a result cuts like a very finely serrated blade without all the inconvenience of sharpening one.

Custom Maker Stuart Ackermen uses cast 440c and I believe Spyderco has used it in collaboration with him.

Good stuff for cutting, just so don’t go battoning through logs with it!

[Bolster]

Large carbides...

...coarser edges...

I still have questions.

How do we explain particle metallurgy steels with relatively small carbides (like CPM M4) being so aggressive?

And how do we explain this aggressiveness, even at high-grit, high-polish?

Just thinking out loud here, but is it possible that high-carbide steels that are sharpened on a lower-hardness stone would expose more carbides (compared to diamonds, which may actually shape the carbides)? Although I've been using Venev diamond stones up to mirror polish and I'm still finding notable aggression in the M4 knives.

And yes, come to think of it, the ZDP-189 knives. I don't have many XHP, but they are aggressive too. So far, M4 is still the most aggressive I've experienced, but that's a subjective statement without legitimate control groups.

[Erich]

Large carbides...

...coarser edges...

I still have questions.

How do we explain particle metallurgy steels with relatively small carbides (like CPM M4) being so aggressive?

And how do we explain this aggressiveness, even at high-grit, high-polish?

Easy: it does not exist. You bought into the "super dooper" steel marketing hype. It makes you feel cool. It's all psychological.

M4 is no more "aggressive" than s30v or vg-10, because "aggressiveness" is not an inherent property in steel. You can not detect wear resistance by touching a steel with your finger. You cannot feel the carbides. The behavior of an edge is determined by abrasives. High carbide steels do wear more slowly and blunt via fracture relative to their carbide amount but that all happens within the confines of the abrasives used when sharpening. It is all in your imagination. 420 hc is not going to cut a paper or whatever any less "aggressively" than maxamet with equally sharpened edges. The only thing you will see is 420hc dull much more quickly over time.

[Bolster]

Easy: it does not exist...

Yeah, I read your "it's your imagination" post earlier. Your explanation is that I have psychological problems, that I "need to feel cool," and that I am a dupe of marketing hype. You are calling me a fool for asking the question. No steel is more aggressive than VG-10 or 420HC. OK, I read your posts, thanks for your input Erich.

[skeeg11]

Easy: it does not exist...

Yeah, I read your "it's your imagination" post earlier. Now your explanation is that I have psychological problems, that I "need to feel cool," and that I am a dupe of marketing hype. No steel is more aggressive than VG-10 or 420HC. OK, I read your post, thanks for your input Erich.

IMHO it's not your imagination. Years ago when David Boye first came out with dendritic 440C, it was kind of magical. Never had a chef's knife go through silver skin so easily. Carbides really are a thing. How you sharpen them and the matrix plays a role for sure, tho.

[Bolster]

...Carbides really are a thing. How you sharpen them and the matrix plays a role for sure, tho.

For sure, re: carbides and sharpening. Can you explain more about how the matrix affects the result?

[skeeg11]

Clean stones so that the supporting matrix is not unnecessarily abraded away.

[Naperville]

Large carbides...

...coarser edges...

I still have questions.

How do we explain particle metallurgy steels with relatively small carbides (like CPM M4) being so aggressive?

And how do we explain this aggressiveness, even at high-grit, high-polish?

Easy: it does not exist. You bought into the "super dooper" steel marketing hype. It makes you feel cool. It's all psychological.

M4 is no more "aggressive" than s30v or vg-10, because "aggressiveness" is not an inherent property in steel. You can not detect wear resistance by touching a steel with your finger. You cannot feel the carbides. The behavior of an edge is determined by abrasives. High carbide steels do wear more slowly and blunt via fracture relative to their carbide amount but that all happens within the confines of the abrasives used when sharpening. It is all in your imagination. 420 hc is not going to cut a paper or whatever any less "aggressively" than maxamet with equally sharpened edges. The only thing you will see is 420hc dull much more quickly over time.

I've been alive for 63+ years. Millions of products and processes have undergone continuous refinement during that time. To think that those continuous refinements did not result in better outcomes would be like saying that nothing in the USA has improved since the arrival of Leif Erikson or other settlers who followed. Of course things improve. You are here and alive to see and experience it.

[Bill1170]

As Sal Glesser has said, “The edge is a ghost.” There’s no dishonor in admitting that we don’t fully understand the dynamics of matter separation. We know a lot that wasn’t known a century ago. Isn’t it possible that we know a lot less than will be known a century from now?

[Deadboxhero]

How do we explain particle metallurgy steels with relatively small carbides (like CPM M4) being so aggressive?

Small compared to what? 1095, AEB-L and 51200 have finer carbides. Don't forget about carbide volume, also overall hardness HRC and hardness of carbides.

There isn't enough testing to put numbers to the exact numbers of sizes and volume to experience aggression or no aggression currently.

Can't just make things up.

But there is something there to explore with controlled testing.

And how do we explain this aggressiveness, even at high-grit, high-polish?

The carbides and matrix are at different hardnesses. Some people sharpen with polishing abrasives that are softer than the carbides which can produce an exposed carbide edge since the softer abrasive will cut away the surrounding matrix if the steel is at sufficient overall hardness. This effect at higher grit will make an edge seem a little more toothy than it would be of all the phases were cut flush to the apex with an abrasive that's harder than all the phases in the microstructure.

[Erich]

Easy: it does not exist...

Yeah, I read your "it's your imagination" post earlier. Your explanation is that I have psychological problems, that I "need to feel cool," and that I am a dupe of marketing hype. You are calling me a fool for asking the question. No steel is more aggressive than VG-10 or 420HC. OK, I read your posts, thanks for your input Erich.

Glad I could convince you that you are seeing things that don't actually exist. Keep feeling special because you have a "m4" stamped steel in your pocket.

[Bolster]

...There isn't enough testing to put numbers to the exact numbers of sizes and volume to experience aggression or no aggression currently. Can't just make things up....

Agreed, that's the fly in the ointment, no accepted method to quantify it yet. Aggression is often mentioned as an attribute but poorly defined. You proposed a method to test for aggression earlier in this thread that sounded pretty good: "We would need to slice with a fixed load, fixed speed and fixed length on a known material with the same edge angle/finish/sharpness using the same sharpening stone on a range of samples and measure the depth cut." I would propose maybe just the same finishing stone, guessing that's the one that would be important. As to your other insights re carbide size, matrix hardness (had not thought much about that, good point) and relative abrasive softness/hardness, thanks for those.

Glad I could convince you that you are seeing things that don't actually exist. Keep feeling special because you have a "m4" stamped steel in your pocket.

I have many dozens of Spydercos in 22 different steels, and they are all special in one way or another. They are not equally aggressive cutters, even though my sharpening procedure is reasonably consistent with a Hapstone R2 and Venevs. I don't think owning a knife has ever affected my self-esteem in one way or the other, but thanks just the same for your concern about my mental health. If Erich has convinced anybody of anything, please post up.

[Vamais]

Erich,
Bolster has reported an observed difference in how different steels cut, which property he has termed "aggressiveness". True, there is currently no standardized test or measurement of steel "aggressiveness", but that doesn't mean we should immediately discount his observations. Two possibilities exist: either that he is noticing something that is there, or that he is perceiving something that is not actually there. It is very possible that what he is noticing is a combination of measurable properties that lead to his observed results. It is also very possible that he is experiencing a placebo effect. The only sure way to know is to perform a controlled study, which would require much time, money, effort, and vigor. I believe that Bolster asked his question to see if anyone else on this forum had experienced something similar, or to see if there had been studies done (scientific or not) that validated or disproved his observations.

A quick answer (either in support of or disagreeing with the stated observations) without evidence kind of defeats the purpose of this forum.

[Traditional.Sharpening]

I am no steel or sharpening expert. I read what Larrin produces and make good use of the charts. I just know what I like and stick to buying carbide rich steels for knives under 6 inches if at all possible. Carbide rich steels are chosen for a reason, they cut better than the older non carbide steels, not just that they are newer.

As to BESS testers, they ONLY test what is under less than one millimeter of the knife blade and the remainder of the blade is ignored. I think Larrin has a better steel tester that slices cardboard test pieces, and that would be a better tester overall of how a knife edge is going to perform. I just don't know, will a BESS tester show that a high carbide steel cuts better? I do not think that is what it is measuring.

I could be wrong about Larrin's steel testing rig too, don't take my word for it.

What testing rig tests the quality of a cut, employing the length of a knife blade?

What do you mean that a carbide rich steel 'cuts better'? This is rather vague and I don't know if you mean cuts longer, gets sharper, etc.

[Bolster]

Edited ... changed my mind about asking a particular forumite to respond to Shawn's recent experiment (after proposing that edge aggressiveness is merely a psychological phenomenon) ... However, if the threads get separated and someone is reading this thread in the future, don't miss the related experiment that demonstrates edge aggression:

viewtopic.php?f=2&t=95803

[zhyla]

We really need a better term than “aggressive”. We’re really talking about micro serration size here, right?