Spyderco Nilakka : edge retention, durability, strength vs tough

[Cliff Stamp]

Video :

http://www.youtube.com/watch?v=oZxl7SSwa3A

This is a long video and is about the following topics :

-edge retention of the Nilakka/S30V on cardboard
-strength vs toughness
-edge stability and carbide volume

In short, after repeated sharpening, the Nilakka can :

-whittle pine and similar woods including cross grain

-whittle basswood and similar woods including cross grain

-will take damage on OSB/plywood with heavy force, damage is limited to about 0.001"

To increase the durability :

-0.005" at the edge is moderate use (plywood, etc.)

-0.010" is heavy use, knots and similar

-0.015" is extremely heavy use (impact splitting on difficult woods)

On some of the discussions :

-strength is a property of steels which deals with slow application of force

-toughness is a property of steels which deals with a fast application of force

-a high carbide volume does not strengthen the edge it weakens it

-higher edge angles are needed in higher carbide steels

Reference :

http://www.cliffstamp.com/knives/reviews/cardboard.html

[tvenuto]

Thoroughly enjoyed, thanks for taking the time.

[Cliff Stamp]

Thanks, there were a few other things I wanted to mention such as contacts off of staples and such, but the video was getting long and I decided to trim it back a little.

[kbuzbee]

Nice Cliff. I love the penny example. Really brings the concept home.

The one concept kinda missing was "what does that mean for tasks it is suited to?" I mean, you say you like the cutting ability and you show a whole carbide won't fit near the edge, but you mention you will sharpen (or Spyderco has ground) carbides all the way to the edge, so in your model, you'll have various sizes of cut pennies with their flat side along the grind....

I get the whole maximal edge retention from wider angles and coarser grinds but given a polished edge at this 5° per side how does it behave compared to your reference AEBL?

Ken

[Cliff Stamp]

The one concept kinda missing was "what does that mean for tasks it is suited to?"

Yes, I did have some discussion on that regarding edge angle as that is the critical part so essentially what happens is :

-higher carbide volume -> thicker edge angles
-thicker edge angles -> lower cutting ability

In general then what you have is that as the carbide volume increases you move away from a precision cutting tool and more towards something which is designed for lower sharpness/cutting ability and more towards scraping and rougher cutting and non-knife like work.

However care needs to be taken because if damage does happen it is far harder to remove so that is a counter balance which needs to be taken into account on larger and heavier use knives.

For example if you were making say a medium size knife which was not going to be used for precision/light use and you had a 0.030"/20 dps edge you might be inclined to think that ATS-34 class steels would be optimal (the angle is high and the edge is thick so the carbides will be stable) but if that edge is over loaded then the low grindability will make the user spend a lot more time.

This tend tends to be really, and optimally suited for people who don't maintain their own tools and who are going to have them just sent in for sharpening/repair and are going to use knives until they are extremely dull, almost smooth at the edge.

(assuming of course there are no gross impacts which would remove high carbide steels as a sensible choice)

These are just highlights of course, it isn't a trivial question and I didn't want to skim over it in the video as that would be more likely to confuse than anything productive.

[kbuzbee]

Yes, I did have some discussion on that regarding edge angle as that is the critical part so essentially what happens is :

-higher carbide volume -> thicker edge angles
-thicker edge angles -> lower cutting ability

Right, but here you have higher carbide volume -> thinner edge ange -> superior (though lighter use) cutting ability. So if it were AEBL rather than S30V and you still used it carefully... The S30V would hold it's edge longer than the AEBL? Or not?

Ken

[Cliff Stamp]

The steel with the higher carbide volume will remain stable in a thinner cross section and not take damage, so the edge retention would be much higher. If the edge is thickened to the point the carbides are stable then the edge retention advantage would be to the high carbide steel. Note the edge retention advantage will only become significant at low levels of sharpness regardless, < 5% or so.

[chuck_roxas45]

The steel with the higher carbide volume will remain stable in a thinner cross section and not take damage, so the edge retention would be much higher. If the edge is thickened to the point the carbides are stable then the edge retention advantage would be to the high carbide steel. Note the edge retention advantage will only become significant at low levels of sharpness regardless, < 5% or so.

Did you mean to type that?

[kbuzbee]

Did you mean to type that?

Thank you Chuck. Sometimes I get SO confused. I think I got it, and then..... I ain't got it!

Ken

[chuck_roxas45]

Thank you Chuck. Sometimes I get SO confused. I think I got it, and then..... I ain't got it!

Ken

You and me both Ken. :)

[Cliff Stamp]

Thanks, no that is incorrect, it should be :

"The steel with the lower carbide volume will remain stable in a thinner cross section and not take damage, so the edge retention would be much higher. If the edge is thickened to the point the carbides are stable then the edge retention advantage would be to the high carbide steel. Note the edge retention advantage will only become significant at low levels of sharpness regardless, < 5% or so."

There are also a number of assumptions here, mainly that the steels are similar in other respects and that the main effects on the edge are the ones being discussed. If for example the low carbide steel had poor thermal processing, is full of impurities, etc. then it could have lower edge stability than a high carbide steel.

However in such discussions there is always the assumption "all things being equal but ..." as otherwise you can't actually have a discussion. For example in general I can't win a 100 m race against an Olympic athlete, however if he is 80 years retired then that would not be true.

[chuck_roxas45]

So how thin of an angle is stable for a high vanadium steel like 20CP?

[kbuzbee]

So how thin of an angle is stable for a high vanadium steel like 20CP?

It's a good question. As I'm coming to understand there are two factors at work here.

The size of the largest carbides

The total volume of carbides

Increasing either reduces edge stability as your edge gets thinner and more refined. Increasing both even more so.

I would love to find a carbide size chart to help some of this analysis.

And I have "weird" thoughts, like iirc Niobium carbides are quite small and very hard, so what would happen if you made a partical steel with only iron, carbon and Niobium, would that work? What kind of edge would that have?

Ken

[chuck_roxas45]

Aren't tungsten carbides pretty fine too? I also wonder if say, 30 inclusive is too fine for 20CP or M390?

[captnvegtble]

I'm also wondering about the South Fork. When it first came out, I know people (like Cliff) were a bit disappointed it wasn't a thinner grind like Phil Wilson's customs... but this discussion begs the question - since S90V has such a high vanadium carbide content, maybe it's better not to have a thinner grind?

[Cliff Stamp]

It's a good question. As I'm coming to understand there are two factors at work here.

The size of the largest carbides

The total volume of carbides

Increasing either reduces edge stability as your edge gets thinner and more refined. Increasing both even more so.

Yes, what happens is that as the volume increases the carbides simply form so close to each other they aggregate into super-carbide structures which are far larger than the individual carbides themselves. This is no different for example than increasing the population of people in a court yard for example. At some point the population will be so large that you will see groups of people form which are far bigger than the individual people. If you keep increasing it the groups will merge and at some point, even though the people themselves never get any bigger, there will be that many people there is almost no free space (i.e., 121 REX).

I would love to find a carbide size chart to help some of this analysis.

The carbide sizes are effected by heat treatment as well as volume, so it isn't trivial. For example there is nothing stopping you from actually dissolving almost all of the carbide in S30V into the steel if you wanted, you would just have to soak the steel hot enough. However that would be a rather odd thing to do as what did you put it in there in the first place for then?

In short, if you want to see what it looks like carbide wise, you need to look at a properly prepared micrograph - but again, realize (and this is critical), the thermal processing effects this greatly. You can easily process AEB-L and make it very coarse both in carbide and aus-grain, though again I don't know why you would do that, but you could.

And I have "weird" thoughts, like iirc Niobium carbides are quite small and very hard, so what would happen if you made a partical steel with only iron, carbon and Niobium, would that work? What kind of edge would that have?

All of the steels made to cut things and retain a sharp edge are made with that perspective in mind. As you no longer focus on the sharp part you simply ramp up the carbide volume and as well usually the cobalt and moly to dramatically increase the hot hardness as usually such tools are ran at extreme speeds as they are roughing cuts.

Niobium, and nitrogen are becoming more used now for a number of reasons, one of which is that they form very hard and very small carbides, but how and when they form is also critical. But thermal processing as well is also advancing by changing how steels are rolled/normalized to effect the size of the martensite lathe packets inside the aus-grain.

This has a dramatic effect to increase all of :

-strength
-toughness
-wear resistance (at the edge, not grossly)

Note it increases strength and toughness at the same time. The only downside of course is cost so it is mainly done now in parts of extreme service demands (aerospace is one such market).

... maybe it's better not to have a thinner grind?

It isn't the thickness but the angle which is critical because even what we would consider to be very thin edges are very thick compared to carbides. To be specific, 0.001" is 25 microns, so even a very thin edge of 0.005" is > 100 microns. The only steel which has aggregates that size is D2 (assuming normal steels, there are others but they are rarely if ever used in knives). The cobalt alloys also have very coarse carbide aggregates as well, similar to D2.

[Ankerson]

I'm also wondering about the South Fork. When it first came out, I know people (like Cliff) were a bit disappointed it wasn't a thinner grind like Phil Wilson's customs... but this discussion begs the question - since S90V has such a high vanadium carbide content, maybe it's better not to have a thinner grind?

The Spyderco South Fork was designed to be more of a general use knife, the reason for the more standard thickness behind the edge opposed to the Customs that Phil grinds much thinner and are more purpose made blades.

This is a Custom South Fork that is .006" behind the edge at around 10 DPS.

[Blerv]

Thanks Cliff, great stuff. :D

If you are talking about low-edge stability how do the low/no carbide steels stack up compared to say CPM-M4? I'm assuming it should work better at steeper angles with the higher carbide content than low ones; high and low being relative describers. Additionally the steel relative to other relatively stainless recipes like CPM-S30v or CPM-S90v, can be treated harder and still retain a high level of shock-toughness.

I guess I'm just curious if something like M4 at very low edge angles would be still tougher, on average, than something like Superblue or a very high RC 1095.

Note: I have a pretty thin reground ZDP-189 Stretch that now is starting to make me rethink things. It hasn't chipped so I'll just cross my fingers and hope it is optimal, lol.

[Ankerson]

Thanks Cliff, great stuff. :D

If you are talking about low-edge stability how do the low/no carbide steels stack up compared to say CPM-M4? Based on what you are saying (seemingly) at more steep working edge angles with the higher carbide content it should (in theory) work better than low angles. Additionally the steel relative to other relatively stainless recipes like CPM-S30v or CPM-S90v, can be treated harder and still retain a high level of shock-toughness.

I guess I'm just curious if something like M4 at very low edge angles would be still tougher, on average, than something like Superblue or a very high RC 1095.

Note: I have a pretty thin reground ZDP-189 Stretch that now is starting to make me rethink things. It hasn't chipped so I'll just cross my fingers and hope it is optimal, lol.

A lot will depend on the following.....

The knife in general.

The Steel and how it's heat treated, tempered etc.

Blade and edge geometry.

What the media is that is being cut.

What the testing process used is ment to show exactly.

There are a lot of variables to take into consideration here so there really isn't just one answer or outcome overall.

So it depends is the real answer here.....

So in general just saying that a low alloy steel will be more stable and have higher edge retention than a high alloy steel is misleading at best....

Edge retention follows carbide volume as in the higher the carbide volume the better the edge retention will be all things being equal and that is a known proven fact that has been documented more than a few times over the years.

[kbuzbee]

FIGHT! FIGHT!

Edge retention follows carbide volume as in the higher the carbide volume the better the edge retention will be all things being equal and that is a known proven fact that has been documented more than a few times over the years.

So..... You're saying you disagree with Cliff that two blades ground at 10° inclusive full to the edge, the S30V edge will outlast the AEBL edge?

Bring it!

Ken