What steel takes the keenest edge?

[james terrio]

No need to over-complicate this. Let's discuss one topic at a time... attainable keen-ness. Not factory edges, how much of a pain a given knife is to sharpen, or even geometry.

The keenest edge requires the finest "grain" structure. That's not "theoretical" or "subjective" at all, it's just dependent on chemistry and HT. Generally speaking, the simpler the steel, the easier it is for the heat-treater to keep the grain very fine. A touch of vanadium (but not enough to form a lot of carbides) helps with that... otherwise, all you really want to achieve the finest, crispest edge possible is iron and .6-.9% carbon.

For instance, D2 has a reputation for "taking a lousy edge, but holding it forever" because it's chock-full of big clumps of carbides. 10xx steels are beloved for the way they get scary sharp, because they have little to no carbides and very fine grain when HT'ed properly.

 

[singularity35]

Not steel but if we talk sharp, obsidian gets very sharp.

 

[J W Bensinger]

No need to over-complicate this. Let's discuss one topic at a time... attainable keen-ness. Not factory edges, how much of a pain a given knife is to sharpen, or even geometry.

The keenest edge requires the finest "grain" structure. That's not "theoretical" or "subjective" at all, it's just dependent on chemistry and HT. Generally speaking, the simpler the steel, the easier it is for the heat-treater to keep the grain very fine. A touch of vanadium (but not enough to form a lot of carbides) helps with that... otherwise, all you really want to achieve the finest, crispest edge possible is iron and .6-.9% carbon.

For instance, D2 has a reputation for "taking a lousy edge, but holding it forever" because it's chock-full of big clumps of carbides. 10xx steels are beloved for the way they get scary sharp, because they have little to no carbides and very fine grain when HT'ed properly.

^this

 

[EChoil]

~~~ misplaced post ~~~ Apologies....

 

[Twindog]

I'm struggling to understand this one. At a practical level, any steel capable of holding an apex should be able to get as sharp as any other, assuming proper sharpening techniques and super-fine stones. We can sharpen right through grain boundaries. We can sharpen through carbides. The differences will be in how hard it is to achieve that keen apex and how long it will last.

 

[DeadboxHero]

No need to over-complicate this. Let's discuss one topic at a time... attainable keen-ness. Not factory edges, how much of a pain a given knife is to sharpen, or even geometry.

The keenest edge requires the finest "grain" structure. That's not "theoretical" or "subjective" at all, it's just dependent on chemistry and HT. Generally speaking, the simpler the steel, the easier it is for the heat-treater to keep the grain very fine. A touch of vanadium (but not enough to form a lot of carbides) helps with that... otherwise, all you really want to achieve the finest, crispest edge possible is iron and .6-.9% carbon.

For instance, D2 has a reputation for "taking a lousy edge, but holding it forever" because it's chock-full of big clumps of carbides. 10xx steels are beloved for the way they get scary sharp, because they have little to no carbides and very fine grain when HT'ed properly.

Wow, great post, definitely opened a door for more learning.
Thanks.

I'm struggling to understand this one. At a practical level, any steel capable of holding an apex should be able to get as sharp as any other, assuming proper sharpening techniques and super-fine stones. We can sharpen right through grain boundaries. We can sharpen through carbides. The differences will be in how hard it is to achieve that keen apex and how long it will last.

I disagree, in reality, some steels just get sharper then others. YMMV.

 

[Fire_Brand]

No need to over-complicate this. Let's discuss one topic at a time... attainable keen-ness. Not factory edges, how much of a pain a given knife is to sharpen, or even geometry.

The keenest edge requires the finest "grain" structure. That's not "theoretical" or "subjective" at all, it's just dependent on chemistry and HT. Generally speaking, the simpler the steel, the easier it is for the heat-treater to keep the grain very fine. A touch of vanadium (but not enough to form a lot of carbides) helps with that... otherwise, all you really want to achieve the finest, crispest edge possible is iron and .6-.9% carbon.

For instance, D2 has a reputation for "taking a lousy edge, but holding it forever" because it's chock-full of big clumps of carbides. 10xx steels are beloved for the way they get scary sharp, because they have little to no carbides and very fine grain when HT'ed properly.

This post kind of hits toward some of my thoughts. I definitely am not the most qualified to answer your initial question but I'll add to it because this actually has been a question I've wondered about myself as well.

Rather than absolute keenest initial edge I'm interested in what can take an incredibly keen edge and keeps a super high sharpness longer. Like mentioned in this post D2 is one of the steels where if you see cutting pressure graphed out it loses its initial razor like sharpness very quickly but then starts to plane out and then keeps a working sharpness for a long, long time.

I don't process piles of boxes everyday and knives are more a hobby for me so keeping a working sharpness for ages really isn't that interesting to me because it isn't an issue to sharpen my knives regularly. What would be great is a knife that I can get scary sharp and it stays this way for a reasonable amount of time even if after that it dulls relatively quickly (if this makes sense).

 

[james terrio]

Not steel but if we talk sharp, obsidian gets very sharp.

That's true... if you look up some microphotographs of edges, obsidian, flint and even glass make our favorite steels look downright jagged in comparison. They don't hold up very well to any kind of impact or lateral stress, though.

We can sharpen right through grain boundaries. We can sharpen through carbides.

In theory, maybe. In practice, it's not nearly that simple. This is obviously a silly exaggeration, but it's helpful to think of "plain" steel as Jello, and carbides as chunks of peanut mixed into it. Push your spoon through a bowl of that, and what's going to happen right where the spoon passes? Those chunks of peanut are going to either be pushed aside or tear out of the Jello, they won't be cut cleanly like the Jello itself is. Sharpening, and dulling, a carbide-rich steel is sort of like that.

Grain boundaries and the matrix between carbides wear away pretty much instantly even while you're sharpening, leaving little pokey carbides sticking out. That's not necessarily a bad thing (lots of people love "toothy" edges for various tasks, including me), but it doesn't lend itself to the very finest edge. Now if you took a blade made entirely of tungsten or vanadium carbide, or certain ceramics, that wouldn't be an issue... but then we're right back to serious brittleness problems, sort of like glass or obsidian.

Rather than absolute keenest initial edge I'm interested in what can take an incredibly keen edge and keeps a super high sharpness longer.

And that's what's so fun and interesting about tool steels (it's basically why O1 was invented - to retain the toughness of 10xx alloys but achieve more thorough hardening on quench and add a splash of carbides...) and especially powder metallurgy steels; trying to find that balance.

For instance 154CM vs. CPM-154, or D2 vs. CPM-D2. The chemical formulas for the two versions of each are almost identical. But the particle manufacturing process prevents big clumps of carbides, helping achieve a finer structure, closer to that of the simpler steels. Those carbides are still there, they're just smaller and more evenly distributed, retaining that boost in greater wear-resistance, without nearly as much problems from uneven wear and tear-out. (You might say, the peanuts in the jello are finely diced, instead of whole and clumped together.)

What would be great is a knife that I can get scary sharp and it stays this way for a reasonable amount of time even if after that it dulls relatively quickly (if this makes sense).

One way to achieve that is to embrace the simpler steels like 1095 and O1, but HT them much harder for more wear-resistance. It's really a shame that most makers and manu's run all their "carbon" steels at 56 or maybe 58Rc max, hoping to maximize toughness and keep them easy to sharpen. Those very same alloys at 60 or 62Rc (some people report even higher hardness levels) get just as crazy sharp because they still have that really fine structure, but they keep that edge a lot longer. The Japanese embraced that concept quite a while ago, especially in kitchen cutlery. It's steadily catching on in America for high-end kitchen stuff, too.

Of course, you don't want to be chopping wood or cleaving bone with a screaming hard, paper-thin edge. So there's that... but for pure cutting and slicing, even with abrasive materials like rope or cardboard or a dirty old wild hog's hide, higher hardness is your friend. :thumbup:

 

[Twindog]

My own sense is that when people say that some steels get sharper than others they are referring to their own experience and their own equipment, both of which are usually the key limiting factors.

Sandvik uses 13C26 for its razor blade steel. It has 13 percent Cr for corrosion resistance. Through a proper heat treat, the large, primary carbides are avoided, along with nonmetallic inclusions, leaving a clean steel with finely dispersed carbides in the matrix. Nothing wrong with the sharpness of a fresh razor, even with the 13 percent load of carbides.

Powder steels -- usually called super steels here -- can hold much larger amounts of carbides because of both the heat treat and steel processing techniques. These steels have a fine grain structure and small, evenly dispersed carbides and few inclusions. Steels like D2 will have much larger carbides and be more susceptible to carbide shedding at the edge, but D2 is still considered one of our better knife steels.

With fine abrasives, whether as stones or pastes, at diamond or near-diamond hardness, there is really no practical limit to the sharpness that can be obtained by any of these steels, provided the right equipment and proper technique.

But without proper technique or equipment, most people will probably be able to get the simple carbon steels sharper then more complex steels.

 

[AntDog]

For me in my own experience it's A2, VG-10, 52100, SR-101 (which is modified 52100), CPM-154, AEBL, and INFI.

 

[james terrio]

Sandvik uses 13C26 for its razor blade steel. It has 13 percent Cr for corrosion resistance. Through a proper heat treat, the large, primary carbides are avoided, along with nonmetallic inclusions, leaving a clean steel with finely dispersed carbides in the matrix. Nothing wrong with the sharpness of a fresh razor, even with the 13 percent load of carbides.

You're at least half-right... 13C26 is indeed very clean and has 13% chromium. I'm not sure where you're getting "13% load of carbides" from... Chrome (or any other alloy) content does not necessarily equate carbide volume. It has to have sufficient carbon that's not already tied up with iron, to form carbides... that what carbide means.

13C26 (and its predecessor AEB-L) have almost no carbides at all, because there's purposely not enough carbon in them to ever form carbides with the chrome (just under .7%... just enough to help the steel get good and hard, no more). All the chrome is free, specifically for corrosion resistance, and specifically not for carbides. That's the exact reason they were designed. To have a very fine-grained simple carbon steel with, essentially, a lot of free chrome mixed throughout to shield it from rust. I like to refer to that whole class of alloys as "the stain-resistant steels for people who don't like stainless steel."

On the other hand you'll see tool and "stainless" steels approaching 2% carbon in some cases; that doesn't help the martensite matrix itself get any harder. The extra carbon in those instances is there specifically to form carbides with large amounts of chrome, vanadium, molybdenum, etc.

In practical terms, how much difference does all this really make? Well, that depends partly on whom you ask. For Joe Sixpack or Suzy Homemaker, not much. For technical geeks and knife knuts, a lot. *shrug*

 

[Twindog]

Sandvik says that almost all of the carbon and chromium in 13C26 are bound in carbides. This is from Sandvik's data sheet on 13C26:

The ferritic matrix has low contents of carbon and chromium because these elements are bound in carbides.


After hardening and tempering, Sandvik 13C26 consists of a martensitic matrix with undissolved carbides and some retained austenite


http://www.smt.sandvik.com/en/mater.../strip-steel/sandvik-13c26-razor-blade-steel/

 

[DeadboxHero]

This post kind of hits toward some of my thoughts. I definitely am not the most qualified to answer your initial question but I'll add to it because this actually has been a question I've wondered about myself as well.

Rather than absolute keenest initial edge I'm interested in what can take an incredibly keen edge and keeps a super high sharpness longer. Like mentioned in this post D2 is one of the steels where if you see cutting pressure graphed out it loses its initial razor like sharpness very quickly but then starts to plane out and then keeps a working sharpness for a long, long time.

I don't process piles of boxes everyday and knives are more a hobby for me so keeping a working sharpness for ages really isn't that interesting to me because it isn't an issue to sharpen my knives regularly. What would be great is a knife that I can get scary sharp and it stays this way for a reasonable amount of time even if after that it dulls relatively quickly (if this makes sense).

We are both on the same sheet of paper. Great share

My own sense is that when people say that some steels get sharper than others they are referring to their own experience and their own equipment, both of which are usually the key limiting factors.

Sandvik uses 13C26 for its razor blade steel. It has 13 percent Cr for corrosion resistance. Through a proper heat treat, the large, primary carbides are avoided, along with nonmetallic inclusions, leaving a clean steel with finely dispersed carbides in the matrix. Nothing wrong with the sharpness of a fresh razor, even with the 13 percent load of carbides.

Powder steels -- usually called super steels here -- can hold much larger amounts of carbides because of both the heat treat and steel processing techniques. These steels have a fine grain structure and small, evenly dispersed carbides and few inclusions. Steels like D2 will have much larger carbides and be more susceptible to carbide shedding at the edge, but D2 is still considered one of our better knife steels.

With fine abrasives, whether as stones or pastes, at diamond or near-diamond hardness, there is really no practical limit to the sharpness that can be obtained by any of these steels, provided the right equipment and proper technique.

But without proper technique or equipment, most people will probably be able to get the simple carbon steels sharper then more complex steels.

Glad that works for ya.

That's true... if you look up some microphotographs of edges, obsidian, flint and even glass make our favorite steels look downright jagged in comparison. They don't hold up very well to any kind of impact or lateral stress, though.



In theory, maybe. In practice, it's not nearly that simple. This is obviously a silly exaggeration, but it's helpful to think of "plain" steel as Jello, and carbides as chunks of peanut mixed into it. Push your spoon through a bowl of that, and what's going to happen right where the spoon passes? Those chunks of peanut are going to either be pushed aside or tear out of the Jello, they won't be cut cleanly like the Jello itself is. Sharpening, and dulling, a carbide-rich steel is sort of like that.

Grain boundaries and the matrix between carbides wear away pretty much instantly even while you're sharpening, leaving little pokey carbides sticking out. That's not necessarily a bad thing (lots of people love "toothy" edges for various tasks, including me), but it doesn't lend itself to the very finest edge. Now if you took a blade made entirely of tungsten or vanadium carbide, or certain ceramics, that wouldn't be an issue... but then we're right back to serious brittleness problems, sort of like glass or obsidian.



And that's what's so fun and interesting about tool steels (it's basically why O1 was invented - to retain the toughness of 10xx alloys but achieve more thorough hardening on quench and add a splash of carbides...) and especially powder metallurgy steels; trying to find that balance.

For instance 154CM vs. CPM-154, or D2 vs. CPM-D2. The chemical formulas for the two versions of each are almost identical. But the particle manufacturing process prevents big clumps of carbides, helping achieve a finer structure, closer to that of the simpler steels. Those carbides are still there, they're just smaller and more evenly distributed, retaining that boost in greater wear-resistance, without nearly as much problems from uneven wear and tear-out. (You might say, the peanuts in the jello are finely diced, instead of whole and clumped together.)



One way to achieve that is to embrace the simpler steels like 1095 and O1, but HT them much harder for more wear-resistance. It's really a shame that most makers and manu's run all their "carbon" steels at 56 or maybe 58Rc max, hoping to maximize toughness and keep them easy to sharpen. Those very same alloys at 60 or 62Rc (some people report even higher hardness levels) get just as crazy sharp because they still have that really fine structure, but they keep that edge a lot longer. The Japanese embraced that concept quite a while ago, especially in kitchen cutlery. It's steadily catching on in America for high-end kitchen stuff, too.

Of course, you don't want to be chopping wood or cleaving bone with a screaming hard, paper-thin edge. So there's that... but for pure cutting and slicing, even with abrasive materials like rope or cardboard or a dirty old wild hog's hide, higher hardness is your friend. :thumbup:

Mind blown...
Outstanding post, packed with knowledge thanks man we all appreciate it.

 

[Alberta Ed]

High carbon steels with very fine grain take the keenest edges in my experience, although edge geometry plays a part. 52100 is very fine grained and extremely tough, judging by my Marbles Campcraft and Fieldcraft (from Mike Stewart's era at Marble's) for superb edge holding. Both have convex edges that stand up to hard use. My sharpest knife overall is a Scandi by Ivan Campos in humble (but very tough) 1070 -- with no secondary bevel it cuts like a banshee.

 

[Insipid Moniker]

Not steel but if we talk sharp, obsidian gets very sharp.

Yeah, silicates can be waaaaay sharper than it's physically possible to make steel.

 

[chiral.grolim]

13C26 (and its predecessor AEB-L) have almost no carbides at all, because there's purposely not enough carbon in them to ever form carbides with the chrome (just under .7%... just enough to help the steel get good and hard, no more). All the chrome is free, specifically for corrosion resistance, and specifically not for carbides. That's the exact reason they were designed. To have a very fine-grained simple carbon steel with, essentially, a lot of free chrome mixed throughout to shield it from rust. I like to refer to that whole class of alloys as "the stain-resistant steels for people who don't like stainless steel."

On the other hand you'll see tool and "stainless" steels approaching 2% carbon in some cases; that doesn't help the martensite matrix itself get any harder. The extra carbon in those instances is there specifically to form carbides with large amounts of chrome, vanadium, molybdenum, etc.

13C26 has ~0.7% Carbon so that it DOES form carbides to increase wear-resistance but they remain sufficiently small and dispersed to avoid loss of toughness or ease of sharpening, and the level is low enough that not much chromium is pulled into carbides so that corrosion-resistance remains relatively high. Add much more carbon and the chromium gets pulled from corrosion-resistance to carbide-formation, and chromium carbides have a tendency to aggregate, they are also larger and softer than vanadium and tungsten carbides.

But add LESS carbon and you gain MORE corrosion-resistance and toughness and ease of sharpening at the cost of carbide-formation and wear-resistance. 12C27 and 12C27M (or 420HC) have decreasing levels of carbon (0.6 and 0.5) for exactly that reason - tougher, more stainless, but not as strong or wear-resistant when optimized for hardness.

S-7 shock steel hardened/tempered to 56-58 Rc is among the toughest steels used in knives... it is 0.5% carbon and is characterized by ~2% total carbide volume without the presence of so much chromium (a potent carbide former).
A2 Tool Steel has ~1% carbon and at ~60 Rc is characterized by ~5% total carbide volume.
M2 Tool Steel has ~1% carbon but also has more tungsten and vanadium that help it achieve closer to 10% total carbide volume at 60-62 Rc.
CPM-10V has ~2.5% carbon and ~10% vanadium to achieve ~15% total carbide volume.
WC-Co hard-metal diatome blades are among the finest-edged I've ever used, can be ~100X sharper than ANY steel knife, and they are >95% tungsten carbide.


Carbide is many times harder than the steel matrix, it can take a MUCH finer edge than the matrix, so the finest/keenest edges come from the highest carbide blades... but the edge is not automatic, it must be achieved through a process, i.e. sharpening = forming that ultra-fine apex. WC-Co blades are sharpened on sharp diamond and CBN to achieve their incredible geometry. Using those same tools on steel will never achieve the same level, but the more carbide present the more of the apex can be made to that level keenness. So more carbide = keener achievable edge... in theory.

In practice most users neither need nor are able to achieve the level of sharp that those high-carbide materials can provide. Heck, using a butcher's rod can align a softer, weaker steel to face-shaving keenness with a few swipes, a technique that will NOT work on materials with much higher carbide loads. Try sharpening a thin, hard carbide-loaded apex with a dull or overly soft abrasive and you'll only knock (chip) the carbides out of their binder and ruin the fine apex, but the same abrasive might easily plough through the matrix of a low-carbide blade to produce a finer edge. That finer edge will not be as fine as the high-carbide edge could be made IF the user had the proper tools and technique, but it's usually sufficient for their needs and that ultra-fine edge would likely be lost on the first cuts through tough material anyway. *shrug* So in practice, low-carbon / low-carbide steels like 420HC and 1095 etc. will more easily take a keen edge than high-carbide steels. But Twindog is right, with better equipment and technique those high-carbide steels can get just as keen if not keener.

 

[bpeezer]

With fine abrasives, whether as stones or pastes, at diamond or near-diamond hardness, there is really no practical limit to the sharpness that can be obtained by any of these steels, provided the right equipment and proper technique.

Most obsessive sharpeners aren't "practical" Keenest measured edge I've seen (courtesy of ToddS SEM measurements) is ~30 nanometers. I'd really like to see a steel apex measured in picometers

Edit to add: Atomic radius of iron is around 156 pm while carbon is around 67 pm. It's been a while since my materials class, but regardless of BCC or FCC cubic structure I think it'd be impossible to get into the picometer range using abrasives.

 

[Airborne 1]

Now that the people that know what their talking about have posted...I'll give you my take on this (like you all give a damn)I myself can get 1095,01,A2,D2 the sharpest .Then comes Au8 VG10,35v. Thats all I own. Please correct Me if I'm wrong, the keenness of a blade has nothing to do with how long it will keep that edge.


2 Panther

 

[Sunyata]

So far M4 and M390 for me

 

[me2]

Sandvik says that almost all of the carbon and chromium in 13C26 are bound in carbides. This is from Sandvik's data sheet on 13C26:

The ferritic matrix has low contents of carbon and chromium because these elements are bound in carbides.


After hardening and tempering, Sandvik 13C26 consists of a martensitic matrix with undissolved carbides and some retained austenite


http://www.smt.sandvik.com/en/mater.../strip-steel/sandvik-13c26-razor-blade-steel/

Sandvik is being a little unclear. That first line in your quote is referring to the steel in its annealed condition. Its not discussed much, but annealed steel has more carbide than hardened steel. After heat treatment, there is a small amount of chromium carbide, 3%-5%, depending on the exact procedure.

The sharpest I've been able to gat steel is the White #2 in my Tojiro petty. It was noticably sharper, and measured sharper than anything I've tested before, requiring 11-12 grams to push cut thread on a scale. Now, I might be able to get that in other steels with different procedures or more work, but that's the best I've seen so far.