Edge Fatigue -- Real or B.S.?

[knarfeng]

We really need Bear to explain what he means. I did watch his video. He says the edge fatigue occurs with stones softer than diamonds and CBN. I think the idea is that softer stones are unable to cut the large, hard carbides in the high-carbide steels; so they plow up the softer steel matrix, leaving the carbides standing proud on the edge. In that condition, they can be knocked out of the apex, leaving a fracture point that damages the edge and reduces cutting performance.

With diamonds, the carbides are sharpened along with the matrix, so you don't get that carbide pullout.

I think this effect would be most likely in steels like 440C, D2 and ATS 34, where the carbides are large and often clumped together.

DeadBoxHero and I have been having a discussion offline.
What he describes is a crappy, brittle, edge in high hardness, high carbide volume alloys when they are sharpened with aluminum oxide stones.

My opinion is that this is due more to what you are describing than to actual "fatigue". The abrasion media needs to be harder than what you are trying to abrade; else you end up with shattered carbide chunks and carbides torn out of the steel matrix. Such tears would be expected to weaken the steel.

 

[Sharperthansticks]

This is the first discussion I've seen in a long time on the Internet that's gone from "he's full of s--t" to "We have been talking and may have come to an agreement," which is exceptionally impressive; I'm glad I started this little kerfuffle. When I saw the video, I remember thinking, "Huh?" It's nice to actually have some explanation.

Next thing you know, we might see a Slysz Bowie go for less than a month of rent in downtown Manhattan.

 

[Sharperthansticks]

As for the question about steel not being made of molecules, just about everything on Earth is made of atoms, ions, or molecules. Metals atoms share electrons, but they're still molecular. Salt in water breaks up into sodium and chloride ions, so that's an example of ionic matter. And the noble gases (helium, neon, argon, and so forth) don't like to react because their outer energy levels are full, so you find them as solitary atoms unless you really f--k with them.

If you want to experience stuff that isn't an atom, molecule, or ion, you either need a particle accelerator or a place somewhere else in the universe where temperature and/or pressure allows (forces, actually) matter to exist in a different form -- say, in a neutron star. (I wouldn't even consider a still functioning star as a great candidate for this thought experiment, since nuclear fusion results in an output of a an atom: helium.) Atoms and molecules do not exist in a neutron star because the pressure is high enough to overcome the internal electron degeneracy pressure in the atom -- the pressure that, in our world, prevents an atom from collapsing in on itself no matter how hard we push on it.

 

[REK Knives]

Personality I totally see the validity of this argument (that fatigue can happen). Clay Allison has documented this as well as having his idea reinforced by cliff stamp's testing and @ToddS observations.

https://knife.wickededgeusa.com/forums/topic/steel-fractures-on-utility-blade-toughness-vs-hardness/

 

[hardheart]

The abrasives are able to remove the iron and carbon in the steel, but it may not leave the edge with a surface finish that can deal with certain stresses because of the other features in the steel.

Edges are very thin (or generally should be) and can fail at very low forces since it is very little material involved. I have no trouble tearing down spider webs, yet it is about the same strength as steel in diameter (about five times the strength by mass) while about ten times the size of the radius of a sharp edge. I have dented the edge of razor blades push cutting sewing thread, visible under a loupe. And of course knives are made with steels that vary so widely in toughness that no blanket statement can be made about their capability.

 

[allenC]

I've never heard of such a thing.

 

[Twindog]

Personality I totally see the validity of this argument (that fatigue can happen). Clay Allison has documented this as well as having his idea reinforced by cliff stamp's testing and @ToddS observations.

https://knife.wickededgeusa.com/forums/topic/steel-fractures-on-utility-blade-toughness-vs-hardness/

I'm not seeing the damage in that photo as edge fatigue. Fatigue is something where the steel is taken past its yield strength and takes a permanent bend. The borders of that bend will have weakened metal. Bend it back and it will be weaker yet. Bend it again, and it will break. I still struggle to see edge fatigue in normal sharpening, unless someone is using a lot of pressure.

The damage in the photo was caused not sharpening, but by an extremely high PSI blow. As Clay said, "I took my EDC and set the edge on the edge of the utility blade I was working on and gave the EDC a little tap on the spine with a diamond stone block. It definitely made a mark:"

So that's damage from an apex-on-apex blow, nothing like you'd see under sharpening.

I can see that coarse abrasives can leave deep scratches in the metal along the edge to create stress risers that will be more prone to damage. Diamonds can cause that weakness as well as softer abrasives. And that's not really edge fatigue.

 

[REK Knives]

I'm not seeing the damage in that photo as edge fatigue. Fatigue is something where the steel is taken past its yield strength and takes a permanent bend. The borders of that bend will have weakened metal. Bend it back and it will be weaker yet. Bend it again, and it will break. I still struggle to see edge fatigue in normal sharpening, unless someone is using a lot of pressure.

The damage in the photo was caused not sharpening, but by an extremely high PSI blow. As Clay said, "I took my EDC and set the edge on the edge of the utility blade I was working on and gave the EDC a little tap on the spine with a diamond stone block. It definitely made a mark:"

So that's damage from an apex-on-apex blow, nothing like you'd see under sharpening.

I can see that coarse abrasives can leave deep scratches in the metal along the edge to create stress risers that will be more prone to damage. Diamonds can cause that weakness as well as softer abrasives. And that's not really edge fatigue.

True, I agree and thank you for pointing that out. I guess it just demonstrated time to me that damage can occur below the apex itself. But you may be right... I am not so sure edge fatigue can be an issue except for maybe under a very specific set of circumstances.

In some articles by @ToddS he demonstrates some important things and also makes some claims which I assume he's seen but not posted pics of yet.

For example, in this article he states "It is important to remove this large burr with edge-leading strokes as the steel near the base off the burr will be damaged from the burr flipping side to side."

In the comments he responded to my question stating that yes it can happen with a foil burr the fatigued metal can be removed with Stropping (micro convexing)

" These can be several microns thick and can torque the apex when if they flip. The foil-burrs that I show in this article are usually 1 tenth of a micron thick. By micro-convexing the apex, we remove that (potentially) damaged steel anyway."

In this article he demonstrates that even a smooth steel does not actually bend the apex back and forth but will actually abrade the damaged steel.

" worn" apex

Smooth steeled apex

 

[Danke42]

Is Metal Fatigue real? Yes.
Can you sharpen a knife to such a fine edge it's fragile? Yes.
Can you affect the heat treatment of the blade with your sharpening method? Yes.

Can you combine a few things to make your result worse? For sure.

 

[Sharperthansticks]

DeadBoxHero and I have been having a discussion offline.
What he describes is a crappy, brittle, edge in high hardness, high carbide volume alloys when they are sharpened with aluminum oxide stones.

My opinion is that this is due more to what you are describing than to actual "fatigue". The abrasion media needs to be harder than what you are trying to abrade; else you end up with shattered carbide chunks and carbides torn out of the steel matrix. Such tears would be expected to weaken the steel.

Some time ago I sent in an M4 Manix 2 to Spyderco, complaining that it was chipping and/rolling after not much abuse at all. The technicians called me, and we discussed the matter; they said that, in their experience, hard steels such as M4 should not be sharpened past approximately 1000 - 1500 grit. I think this is consistent with knarfeng's hypothesis.

In a previous thread months back, I likened this phenomenon to periodiontal disease, where the teeth themselves (the analogs of our carbides) don't wear away, but the supporting jaw bone structure does, so the teeth wobble and eventually fall out.

True, I agree and thank you for pointing that out. I guess it just demonstrated time to me that damage can occur below the apex itself. But you may be right... I am not so sure edge fatigue can be an issue except for maybe under a very specific set of circumstances.

In some articles by @ToddS he demonstrates some important things and also makes some claims which I assume he's seen but not posted pics of yet.

For example, in this article he states "It is important to remove this large burr with edge-leading strokes as the steel near the base off the burr will be damaged from the burr flipping side to side."

In the comments he responded to my question stating that yes it can happen with a foil burr the fatigued metal can be removed with Stropping (micro convexing)

" These can be several microns thick and can torque the apex when if they flip. The foil-burrs that I show in this article are usually 1 tenth of a micron thick. By micro-convexing the apex, we remove that (potentially) damaged steel anyway."

In this article he demonstrates that even a smooth steel does not actually bend the apex back and forth but will actually abrade the damaged steel.

" worn" apex

Smooth steeled apex

I read Josh's last response a number of times and am not sure what to make of it. (I am being literal and intend no disrespect.)

It seems to me that the logic and evidence presented in this thread support the assertion that an abrasive that isn't abrasive enough may not evenly abrade the steel, leaving unsupported carbides, but this does not to me seem akin to what we normally refer to as "fatigue," so while I still have an open mind, I am not at this point convinced that the concept really has much applicability to knife sharpening. With regard to metal in general, yes, fatigue can be an issue, as is easily demonstrated by beer-canning a soda can back and forth until the metal breaks, but no one in this discussion has demonstrated this phenomenon, yet, with regard to knife edges, unless you consider the burr itself, which we all agree needs to be removed -- no matter what abrasive you choose.

 

[Twindog]

Some time ago I sent in an M4 Manix 2 to Spyderco, complaining that it was chipping and/rolling after not much abuse at all. The technicians called me, and we discussed the matter; they said that, in their experience, hard steels such as M4 should not be sharpened past approximately 1000 - 1500 grit. I think this is consistent with knarfeng's hypothesis.

I wouldn't agree. If your M4 edge is chipping and rolling with light use, something went wrong with the heat treat. That's really how you know you got a bad heat treat.

The idea that you should use just coarse grits is poor advice, too. If you want to avoid carbide tear-out and maintain a strong edge, then you should use higher grit diamonds and light strokes, so you abrade the carbides and steel matrix evenly.

 

[REK Knives]

Some time ago I sent in an M4 Manix 2 to Spyderco, complaining that it was chipping and/rolling after not much abuse at all. The technicians called me, and we discussed the matter; they said that, in their experience, hard steels such as M4 should not be sharpened past approximately 1000 - 1500 grit. I think this is consistent with knarfeng's hypothesis.

In a previous thread months back, I likened this phenomenon to periodiontal disease, where the teeth themselves (the analogs of our carbides) don't wear away, but the supporting jaw bone structure does, so the teeth wobble and eventually fall out.



I read Josh's last response a number of times and am not sure what to make of it. (I am being literal and intend no disrespect.)

It seems to me that the logic and evidence presented in this thread support the assertion that an abrasive that isn't abrasive enough may not evenly abrade the steel, leaving unsupported carbides, but this does not to me seem akin to what we normally refer to as "fatigue," so while I still have an open mind, I am not at this point convinced that the concept really has much applicability to knife sharpening. With regard to metal in general, yes, fatigue can be an issue, as is easily demonstrated by beer-canning a soda can back and forth until the metal breaks, but no one in this discussion has demonstrated this phenomenon, yet, with regard to knife edges, unless you consider the burr itself, which we all agree needs to be removed -- no matter what abrasive you choose.

I'm basically saying that it doesn't appear to be a "real thing" except for in very limited circumstances (foil/wire edge which would be stropped off anyway).