Toughness as a function of edge retention (?)

[Cliff Stamp]

Recently I have been doing some edge retention comparisons to look at the edge retention as a function of micro-bevel angle using the Wicked Edge 1000 grit diamond plates with a Spyderco Lum : VG-10 and a Paramilitary : S30V. I am about half way through the data now (http://www.spyderco.com/forumII/viewtop ... =2&t=65212" target="_blank) however at this stage even with the numbers being fairly rough there is an interesting question asked, or maybe point made about the relationship between toughness and edge retention. To start :



That is the edge retention as a function of micro-bevel angle. Now again this is only three runs so the conclusions are not that strong (precise) but still the general trend is all that is important to this discussion here and specifically that the edge retention decreases as the micro-bevel angle increases. Now if this makes you pause then just consider the following very crude image :



The edge on the left is a very low micro-bevel, the edge on the right has a very large micro-bevel angle. Both red lines are the same thickness of the apex after dulling. Which apex do you think will reach that red line faster, the high angle or the low one?

Now some people might step in here and say "But wait, if you reduce the apex angle too much at some point surely it will break off and not slowly dull?". Yes, exactly right so we have to be clear that we are talking about micro-bevel angles here which are stable .

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Here is where it gets interesting. Lets say that when you compare VG-10 vs S30V you find that if the VG-10 blade has a ~2 dps lower micro-bevel it has the same edge retention then as S30V (again, I don't have enough data to give clear numbers yet so these are just approximates at this stage). How then would you describe those two knives?

They have the same initial sharpness, the edge retention is the same, the cutting ability is the same (nearly identical both would for example slice 1/2" hemp with 4-5 lbs on a 2" draw). Well what happens is that the S30V micro-bevel is slightly more resistant to damage if you do accidentally over load it as that extra 2 dps makes it stronger/tougher.

What this means interestingly enough is that while you can just sharpen all the steels at the same micro-bevel angles, if you take advantage of the fact that in some cases S30V will have better edge retention than VG-10 this means you can run the micro-bevel a little higher and sacrifice some of that edge retention ability to make the very apex a little tougher/stronger.

In this way while you could look at S30V as being having a slight advantage in edge retention over VG-10 in cutting soft materials like hemp and cardboard (which is how it is commonly done/promoted) you could also look at it as having the same edge retention on those materials but by adjusting the micro-bevel angle make the apex stronger/tougher - i.e. the steel is more durable.

Here is a funny part (to me anyway) this means that the vanadium carbides, in essence (though indirectly), make the steel stronger/tougher at the apex if you regrind the knife so as to take advantage of it.

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Now this isn't a VG-10 vs S30V issue, it holds across all steels and it is just a way to look at steels in a new way possibly. However it isn't always as easy to make this kind of comparison. For example if you try it with AEB-L vs S30V it is more complicated because AEB-L is actually stronger/tougher at the apex than S30V in thinner sections this means that if you compare them it is possible that AEB-L could have higher edge retention and apex durability at the same time. If you want to get something really interesting just try it with 420HC vs 10V and watch the numbers come out and what it says about edge retention, durability, etc. .

[sharpnessis]

Hi cliff stamp. I tried to send a pm but not sure if it went through. Can you post some pics of your edges with these ultra low angles?

[gijoe945]

I think you could write a Thesis on what you know about and experiments with metallurgy. Then we could call you Doc!

[me2]

You already can. Cliff has been a PhD for several years now.

[Cliff Stamp]

... Can you post some pics of your edges with these ultra low angles?

The knives with very low edge angles such as 6.5 dps have been sharpened flat to the primary grind, they look like this :



+



There is no distinct edge bevel, just one bevel from the spine to the edge. The reason they are slightly higher than the primary grind angle is because the stones are usually water stones which due to wear/slurry will add a little angle, plus the force is often applied unevenly and the little extra force near the edge will again add to the angle. The bevel thus curves a little near the apex and so the edge, or bevel under about <0.015" will start to thicken and get up to ~6.5 dps.

This is intentional as for awhile I was using knives with true flat bevels right down to 1.5 dps but at that angle, the micro-bevel was not stable unless you were cutting very soft materials. I have found that in the 6-8 dps range most steels will be strong enough so that a micro-bevel can easily undergo slow wear. Thus unless the blade has a very heavy primary grind I add a little curvature to the knife when sharpening so as to get it in that range.

This is just for the knives which do light cutting, for the knives which do harder work (bones, metals, shingles, etc.) the edge angle has to be higher 10-15 dps to prevent significant deformation/fracture, similar if you are chopping.

[sharpnessis]

Thank you, sir. That answered my question perfectly.

[Cliff Stamp]

Note that the finish of the knife doesn't need to be that rough or uneven. If you take a look at traditional Japanese sharpening for example they tend to sharpen in this manner and frequently work on the primary grind, however the finish is very critical to many and they even have speciality stones to bring out certain finishes. Knives like the Paramilitary however get used for rough work often though so the primary grind gets scratched up anyway as the edge dulls so I don't put much time into raising the finish, unless I am experimenting with a new stone.

[sal]

Interesting concept Cliff. I have also thought about corrosion resistant alloys and how it might effect edge retention.

The edge is a woundrous thing. Ideally we would have a .00001 thick wire that would cut through anything, never break, never corrode and be adjustable for length. Unfortunately we have to "back up the thin wire" so we get caught up in "what will do that?"

Knife afi's are pretty far out, steel junky's more so, but "edge junky's" are just nuts. :p

sal

[paladin]

The edge is a woundrous thing. Ideally we would have a .00001 thick wire that would cut through anything, never break, never corrode and be adjustable for length. Unfortunately we have to "back up the thin wire" so we get caught up in "what will do that?"
sal

I never thought of the apex of an edge in this fashion...

The wire, of course, needs to be able to generate a force equal to & opposite of the force the target material is providing...hence the "what will do that?"

[Cliff Stamp]

Ideally we would have a .00001 thick wire that would cut through anything, never break, never corrode and be adjustable for length. Unfortunately we have to "back up the thin wire" so we get caught up in "what will do that?"

I believe it was Wayne Goddard I first read talk about the knife being just support for the edge which in many cases is pretty close to true. That idea of a super thin wire has been popular in science fiction for a long time, I believe Gibson used it first.

[ManixFan]

Interesting concept Cliff. I have also thought about corrosion resistant alloys and how it might effect edge retention.

The edge is a woundrous thing. Ideally we would have a .00001 thick wire that would cut through anything, never break, never corrode and be adjustable for length. Unfortunately we have to "back up the thin wire" so we get caught up in "what will do that?"

Knife afi's are pretty far out, steel junky's more so, but "edge junky's" are just nuts. :p

sal

I love the pun......I've been "bitten" enough times to truly appreciate the statement.

[Cliff Stamp]

Summary article : http://www.cliffstamp.com/knives/articl ... bevel.html" target="_blank .

[JD Spydo]

Cliff you may disagree with me and that's cool because that's how I learn a lot of things I didn't know previously. But in my own using and my own experimenting here in the past 2 months I've truly come to the conclusion that steels with more of a toughness property primarily do seem to make better Spyderedged blades. I've seen more endurance out of 440V, ATS-55, AUS-8, ATS-34 and even the ancient GIN-1 than some of the other steels known for their plain edge performance.

I will admit that when Spyderco said by their own tests that H-1 proved to be their best performing Spyderedged steel I was even sort of shocked to hear it. Then I got to testing some of my older Spyders with older blade steels and I was impressed at how well they maintained.

But for plain edges I've had my better luck with ZDP-189 which I know if not know for toughness, But with XHP I've been pleasantly surprised and I much want to try it in Spyderedge if I ever get them to make one. VG-10 my go to steel which most of my hard users are seems to do fair in both edges.

[Cliff Stamp]

... steels with more of a toughness property primarily do seem to make better Spyderedged blades

That would be expected as the edge is a series of points and low toughness tends to be caused by high carbide volume which can cause the points to fracture easily. Hence why ZDP-189 tends to be problematic in serrated knives. Now if you only cut soft material this isn't an issue, but often people use serrated knives for very rugged/demanding work, often because they will cut even when very dull so making them out of brittle steels can be problematic.

[The Mastiff]

I've thought on this subject for a few years now. When I really get into it I begin to think how not just different steels but each steel with different heat treats and final hardness's will give different results. Cliff you have used the example of 154cm and the high and ....not as high heats (relative) before. The use of cryo to help reduce the retained Austenite and increase wear resistance but lose toughness. There are so many variables that it would take quite a while to get through testing one steel with so many variables. Even plain carbon steels can be pretty complicated. Getting into some of the highly alloyed steels makes my tires spin when I begin thinking about it.

I've heard 3V described as the perfect steel for knives. I can't say I agree with that but it is a good example to use when talking about toughness and edge retention. The toughness can vary a pretty large amount with just a few hardness points.

Anyway, it's beyond me pretty much but it is an interesting subject. It effects wear resistance a great deal when you can keep the steel together at very thin edges. Comparing a Phil Wilson blade of Cruwear the way he does it compared to a ( excellent as far as production knives go) production blade.


Joe

[arty]

I think we are talking about toughness as a function of edge geometry, hence the effect on edge retention. A thinner blade edge will last a long time, all else being equal. If I need to do some rough chopping with a chisel, I want softer and tougher steel, with a more obtuse edge. I wouldn't expect to see a mortise chisel with the same edge geometry as a paring chisel, since it wouldn't hold up to chopping. With two identical steels, one harder than the other, one would expect a higher RC to give more edge retention and lower toughness.this assumes the same edge geometry.
I have two Tojiro knives in the kitchen, both VG10. One is a thin blade Gyoto, while the other is a heavy Western Deba with a convex grind. I can't Compare edge retention, since both are excellent. They are used for different things. I can cut through lobster tails and cut up a whole chicken with the Western Deba. It has no problem chopping through bones. If I tried that with the thin blade of the Gyoto, I would probably be looking for a new knife. My guess is that the core steel is harder in the Gyoto, but that is a guess.

[Cliff Stamp]

There are so many variables that it would take quite a while to get through testing one steel with so many variables.

The way a steel is processed does significantly influence its properties, however there are ranges. For example there is very little influence on the corrosion resistance of 1095 by how it is processed, it is simply uniformly very low. This is also a heavily researched aspect of materials so there is a lot of materials data available. It is very well known for example on the properties of ATS-34 with a low vs high tempering. But at the same time there are some questions which are interesting (I find them anyway) but the questions are not so well known because there are many influences and some of them are contradictory.

For example on the surface it would appear that heat resistance in a knife (used by people) isn't of any use and that HSS and similar steels are just wasteful as there is alloy which doesn't produce a productive outcome and this is money not being spent on useful function. However try to look into the data and find an answer to the question of how do hot work steels compare to cold work steels at the same wear resistance/hardness in terms of toughness. After I could not find the answer to that I asked a bunch of metallurgists like Landes, Verhoeven and even guys like Krauss - the answer simply isn't known .

The point is well made though, and it is something that many makers like Dozier and Fowler have long argued - that in essence, flittering around from one steel to the next as they become popular maybe not be ideal compared to working with one steel to fully try to explore what it can do. However, at a basic level, the steel has to have the alloy to generate the properties (hardness, corrosion resistance, wear resistance, etc.) . The interesting thing happens when you have steels which have enough alloy to generate a wide range of end results - figuring out what balance gets you to optimal is a non-trivial question.

[Cliff Stamp]

I think we are talking about toughness as a function of edge geometry, hence the effect on edge retention. A thinner blade edge will last a long time, all else being equal.

Yes, the point I was trying to make in the above, which maybe didn't come across so well, is that for example if you compare two steels and one of them has higher edge retention due to its composition (hardness, abrasion resistance, etc.) then you could increase its angle, and so lower its edge retention but it would still have the same edge retention as the other one, but now the edge would be tougher/stronger.

To make it a little less abstract, when S30V and VG-10 were compared in the above slicing a semi-abrasive material then S30V was found to have about a 30% advantage - this isn't surprising or unexpected. However if you look at the main result, the influence of angle on edge retention, then you could make VG-10 have the same edge retention by simply reducing the apex angle of the VG-10 blade.

At this point the S30V blade and VG-10 blade now have the same edge retention, but the S30V blade is ~2 degrees higher at the apex and thus it will be tougher/stronger. The point I tried to make is that you can thus use the inherent increased edge retention and trade it away as it were to make the edge tougher/stronger.

This raises an interesting question because for example how would you normally answer this question - which steel is tougher/stronger and has better edge retention in slicing hemp/cardboard :

-AEB-L vs 420J2

The obvious answer would seem to be that AEB-L is stronger and would have an edge retention advantage but 420J2 is tougher. But, if you use the above information and adjust the geometry of the AEB-L blade it might be possible for it to be superior (or at least equal) in all of the aspects at the same time.

[Mallus]

Hi Cliff,

Thanks for the summaries, this and the others (http://www.cliffstamp.com/knives/articles/index.html" target="_blank). Nice to have the results of your work collected in one place - much more convenient to check things, should the mood strike. :)

[Cliff Stamp]

Thanks for the summaries ...

NP, i recently criticized someone for siting a reference incompletely which made it difficult to check and a friend pointed out that I recently referred to something I did and point to a 20+ page thread with a one line conclusion which would be a tangle for anyone to verify. I did it so it is easy for me to remember it, but for anyone else, trying to sort through pages of updates is a little difficult. I have started to write summary articles to make this at least possible, a big target is the pass arounds as they are very tangly but contain a lot of interesting information, especially the one on the XM-18 by Hinder compared to the Spyderco Delica.