Chasing Ghosts: The effect of edge aggression between steels.

[Deadboxhero]

The Edge is A Ghost…A ghost is something that we can never be sure of, know it all or even agree on if there is such a thing and the controversy is endless.”
-Sal

The concept of edge aggression is nothing new and as others have pointed out it is a subject made popular by David Boyd and his knives in Cast 440C and Dendritic Cobalt
These knives were advertised to have increased slicing aggression at the cost of edge stability and toughness due to the large branching carbide networks left in the steel and metal.


Picture: Credit Nemo Knife Reviews

Stuart Ackerman, a popular and well known knife maker also had a collaboration knife with Spyderco called the Serrata which took use of the cast 440C and was highlighted as a feature for increasing the slicing aggression at the edge.


Figure 1: Metallography of cast 440C by George Vandervoort, large carbide networks observed on left, same steel with further magnification on right.

Testing will be done to see if larger carbides at higher volume produce a measurable effect on the edge aggression at the same edge angle, behind the edge thickness, edge finish and sharpness.

Edge aggression is not a measured, quantified feature currently however it is known amongst experienced knife users and sharpeners. Edge aggression is usually experienced by slicing or drawing the edge across a material with extremely light touch yet still have the edge "grab" and cut with the least amount of force. A very aggressive edge could even use less than the weight of the knife to slice. Conversely, a very keen, less aggressive edge can still be very sharp especially in push cutting but may show edge "slipping" when drawing the edge lightly across a material as if strangely there is no edge there at all.

It is well known that edge finish has a huge effect on edge aggression with lower grit edges having deep scratches all the way to the apex that almost act like a micro serration by reducing contact area with the material being cut. However, it would be interesting to further explore the idea that the microstructure of the steel itself that could be able to synergize this edge aggression effect.

Experiment

Two steels with drastic differences in microstructure were chosen for testing

52100 at 65.0rc
440C at 59.6rc

Micrographs show a significant difference in carbide sizes and volume which can also be seen in the figures below.

Figure 2a: 440C with large M7C3 primary carbides in excess of 10um with smaller M23C6 secondary carbides under 5um. Total carbide volume at 12%


Figure 2b: 52100 with significantly finer M3C iron carbides. The majority of the carbides are sub micron in size with a total volume of 6%. Micrograph by Dr Larrin Thomas


The edges were sharpened on a Edge Pro Professional model to 15 degrees per side (DPS) using a 400 grit (40um) cBN metallic bonded sharpening stone.
Utilizing a fixed angle system will help remove any hidden convexity near the apex that can happen with freehand sharpening.

The initial BESS in grams sharpness can be seen in the table


Table 1: Sharpness before testing, 400grit edge, no strop.

The edge angle was measured with a laser goniometer and both edge are confirmed at 15 dps as seen in the figures. A laser goniometer is one of the most accurate and convenient ways to measure the actual angles at the edge.

Figure 3a: laser goinometer measurement of 52100


Figure 3b: laser goinometer measurement of 440C


Table 2: Geometry of test blades.

The edges will be tested using a Edge On Up ELab A which has a slicing feature with blade fixture.

Test media will be a cherry tomato which as most knife users know has rubbery, elastic like skin that dull edges or even over polished edges can slip off of or not pierce before crushing and squashing the delicate tomato flesh inside making it a deceptively challenging material to cut.


Figure 4: Edge Lab Slicing test fixture slicing tomato and measuring force in grams to slice the tomato.

The edges will be tested followed by rope cut testing and testing again to see if the slicing aggression follows the initial testing.

Results

Testing shows despite the lower BESS sharpness and lower HRC hardness the 440C at the same edge angle, edge finish, blade curve/profile and behind the edge thickness showed less force in grams needed to initiate the slice in the tomato skin.


Figure 5: Test results show lower cut force in grams with 440C before and after rope cutting.

After cut testing in rope the same effect was still observed with 440C showing less force is required to initiate the cut into the tomato.


Conclusion

Edge Aggression is not just a feature of edge finish, testing shows the microstructure of the steel itself particularly the carbide size and volume can play a huge role in the edges ability to slice with the least amount of force possible.

Further testing is required to see what ranges of sizes, volumes and perhaps even hardness have the biggest and least effect on edge aggression.


-Shawn

[Wandering_About]

Thanks for the great info, Shawn!

[WilliamMunny]

Great article, there is a difference between push cutting and slicing.

Cool BBB sticker on the scale, any way to order one?

My gut feels like there is a way to put hard carbides next to softer steal so that as it breaks down with use, material is removed exposing new carbides. Pretty much the way bonded diamond stones work. But I could also be totally off on that one.

BTW how is the baby doing, almost a year old by now?

[Enactive]

Thanks, Shawn! Good stuff here and looks like great experimental design and execution.

[Erich]

Ackerman's project revolved around how 440c was *cast* not 440c itself. He also constantly emphasized the need for this knife to be sharpened with very coarse grits as finer grits would nullify the "dendritic" teeth produced by the cast.

This is not a measure of carbide but rather how the knife is forged.

[Deadboxhero]

Thanks for the great info, Shawn!

Glad you enjoyed the read.

Thank you.

[Deadboxhero]

Great article, there is a difference between push cutting and slicing.

Most definitely, it is certainly something to explore for those that may not know.

Cool BBB sticker on the scale, any way to order one?

Thank you,
Soon

My gut feels like there is a way to put hard carbides next to softer steal so that as it breaks down with use, material is removed exposing new carbides. Pretty much the way bonded diamond stones work. But I could also be totally off on that one.

They're materials like that, stellite 6k and talonite comes to mind and they would be interesting to test but I don't feel like they would be as interesting in real world use.

BTW how is the baby doing, almost a year old by now?

Dude is a tank. I'm a very happy Dad.

[Deadboxhero]

Thanks, Shawn! Good stuff here and looks like great experimental design and execution.

Thanks for reading, I appreciate the compliments.

[dsvirsky]

Thanks Shawn. I know it's just a start, but it's good to see objective, measurable, evidence showing that one steel can, in fact, have greater edge aggression than another, under otherwise identical conditions.

[zhyla]

As I read this I was impressed with the writeup and methodology… until you got to the tomato. how was this tomato calibrated? I know we all note tomatoes as being challenging to start a cut in but there must be some better material for this purpose?

I’m also confused what the data point after the rope cutting is meant to show. At that point the two steels have dulled to different degrees. What conclusions should we draw from the 2nd data point?

Last question: isn’t the effect of the carbides/edge “aggression” likely be more relevant in a slicing cut than a push cut?

[Deadboxhero]

Thanks Shawn. I know it's just a start, but it's good to see objective, measurable, evidence showing that one steel can, in fact, have greater edge aggression than another, under otherwise identical conditions.

Yeah looks curious enough to build a bigger set of data with more conditions to see if we can learn something interesting.

Thank you.

[Josh Crutchley]

Thanks for taking the time to test and share with us here. Very interesting to see the difference in Bess vs ELab numbers. Has anyone every run this type of test with 440c vs cast 440c to see if there really is a difference in aggression?

[Deadboxhero]

Thanks for taking the time to test and share with us here. Very interesting to see the difference in Bess vs ELab numbers. Has anyone every run this type of test with 440c vs cast 440c to see if there really is a difference in aggression?

Thank you,

Yes, that would be interesting to hear if there was testing done, and what the testing was in the past.

[ZrowsN1s]

And this is why I was jealous when you got an E-Lab.

Even when you have Larrins wonderful steel comparison charts with toughness, edge retention, and corrosion resistance; A laser goniometer and guided sharpener to take the guess work out of things; A BESS Tester to quantify 'sharp'...... There are still many more variables to account for to find your perfect steel. And I say 'your' because it's different for everyone and their use case.

"The world of edges has a small doorway in, but opens into a cavern that is both wide and deep." -sal

[p_atrick]

Conclusion

Edge Aggression is not just a feature of edge finish, testing shows the microstructure of the steel itself particularly the carbide size and volume can play a huge role in the edges ability to slice with the least amount of force possible.

Further testing is required to see what ranges of sizes, volumes and perhaps even hardness have the biggest and least effect on edge aggression.


-Shawn

Out of curiosity, what would some of these further tests look like? Would you want to compare D2 and CPM D2 to check for carbide size and its effect on aggression? Are there two steels that have carbides of similar size, but one steel has substantially more of them?

Edited to say thank you for this and the other contributions you make to the community.

[Deadboxhero]

As I read this I was impressed with the writeup and methodology… until you got to the tomato.

Thank you for the backhanded compliment

how was this tomato calibrated? I know we all note tomatoes as being challenging to start a cut in but there must be some better material for this purpose?

Well, it seems you can never win. I use something too obscure and people then accuse me of it not translating to something more real world and that the effect can only be seen in a lab like setting.

Use something more familiar and people complain that it's not calibrated in a clean room on Mars.

When it comes to testing, the objective is to see differences between test samples. If the behavior is big enough and present there will be separation between the conditions that are being controlled for.

What people need to understand is that there is always scatter in any measurement, even when using a ruler. We just need the scatter to be low enough for us to see distinctions between the test samples.

Given that geometry, edge finish and starting sharpness are the biggest variables It was interesting to see such a clear separation between the two samples even after cutting the same amount of rope.

I’m also confused what the data point after the rope cutting is meant to show. At that point the two steels have dulled to different degrees. What conclusions should we draw from the 2nd data point?

The effect was still present after cutting the same amount of rope, It wasn't just a fluke with the initial sharpness that we saw the behavior.

Last question: isn’t the effect of the carbides/edge “aggression” likely be more relevant in a slicing cut than a push cut?

I never said it didn't.

[vivi]

Cool stuff shawn, thanks for sharing.

Cherry tomatoes were one of the things that got me more into sharpening as a matter of fact.

I was working as a prep cook years and years ago at my first fine dining establishment. The house knives couldn't slice a cherry tomato, and I needed to halve 8 quarts of them each morning for prep. It got frustrating real quick.

I tried bringing my sharpmaker in and it helped, but the knives still required much more frequent sharpening than I wanted. Then I remembered Cliff Stamp talking about coarser edge finishes, so I went from the ultrafines down to fines, then eventually medium and got much better results.

At this job I bought myself my first culinary work knife. 10" Victorinox chef knife (unknown to me at the time I ordered the slim version). I'd generally sharpen it on the fine sharpmaker rods whenever it started getting dull, but I also kept a DMT coarse / fine diafold so I could apply a coarse microbevel as needed.

Not too long after I got more serious about experimenting with different edge finishes to increase edge holding in PE H1. I tried every possible finish my equipment at the time allowed on a PE Aqua Salt and tested it cutting cardboard up for recycling. UF, F and medium rods on the sharpmaker, as well as fine, coarse and extra coarse DMT stones. I found the lower I went the better edge holding got on slicing cardboard with PE H1.

To be honest since getting into coarse edges I haven't noticed a drastic difference in slicing aggression between steels themselves. PE H1, K390, M4, AUS8, 1095.....they all get sharp and slice aggressively and demonstrate edge holding abilities in line with what I'd expect.

So this topic has me thinking, maybe I'm not focusing on the steels themselves enough?

Currently I find I prefer high hardness steels for polished edges. As a real world example, I keep my ~65rc aogami super 10" gyuto at 5,000 grit and it holds it well, while my old Victorinox 10" chef knife that's closer to 55rc I give it a coarse edge.

In general I find the softer steels deform too rapidly to hold a polished edge well, while higher hardness steels with superior edge stability work well for that role.

Now, I haven't found any steels that I think work poorly with coarse edges. It extends the edge holding of every steel I've tried when it comes to general purpose knives. However, now I'm wondering, which steels are ideal for this role?

Out of Spydercos current line-up, any suggestions for the best steels for running at 200 grit at the apex?

From my casual, 100% unscientific testing, low carbide steels seem to show the biggest leap in edge retention comparing 2,000 grit edges to 200 grit edges, while K390 seems to take the most aggressive edge out of my current collection.

One last thing - how long were you citting tomatoes for? Asking out of curiosity whether corrosion resistance became a variable?

[Deadboxhero]

Conclusion

Edge Aggression is not just a feature of edge finish, testing shows the microstructure of the steel itself particularly the carbide size and volume can play a huge role in the edges ability to slice with the least amount of force possible.

Further testing is required to see what ranges of sizes, volumes and perhaps even hardness have the biggest and least effect on edge aggression.


-Shawn

Out of curiosity, what would some of these further tests look like? Would you want to compare D2 and CPM D2 to check for carbide size and its effect on aggression? Are there two steels that have carbides of similar size, but one steel has substantially more of them?

Edited to say thank you for this and the other contributions you make to the community.

Thank you.


I believe about 6 years ago. I was curious about the subject. There was talk about the difference between conventional ingot, particle metallurgy and spray form. Some folks felt all of them creating different carbide sizes led to people feeling as though they had a real world effect on the cutting dynamics along with how the edge itself felt. Which I thought was something interesting but didn't know how it could be tested to see if it was actually there or just in people's heads.


There is a spray form version of CPM 10V called PSF59. (Not to be confused with PSF27) There is also a conventional steel called A7 that has massive carbides at high volume so it would be very fascinating to test the differences between those three.

The PSF59 was notable since it won a rope cutting competition in Russia even beating steels like Rex121.
Unknown how, perhaps just scatter from the variability of the force needed to cut rope "real world" or just nuances in heat treatment and processing bringing out small flaws in the microstructure at the edge.

It would be fun if we had a competition like that in the US.

[Deadboxhero]

Cool stuff shawn, thanks for sharing.

Cherry tomatoes were one of the things that got me more into sharpening as a matter of fact.

Same, I wanted to follow a recipe and my knife was so dull it just squished the tomatoes miserably. It made me so angry that I was dead set on learning how to sharpen and taught myself out of pure stubbornness.

I was working as a prep cook years and years ago at my first fine dining establishment. The house knives couldn't slice a cherry tomato, and I needed to halve 8 quarts of them each morning for prep. It got frustrating real quick.

I tried bringing my sharpmaker in and it helped, but the knives still required much more frequent sharpening than I wanted. Then I remembered Cliff Stamp talking about coarser edge finishes, so I went from the ultrafines down to fines, then eventually medium and got much better results.

At this job I bought myself my first culinary work knife. 10" Victorinox chef knife (unknown to me at the time I ordered the slim version). I'd generally sharpen it on the fine sharpmaker rods whenever it started getting dull, but I also kept a DMT coarse / fine diafold so I could apply a coarse microbevel as needed.

Not too long after I got more serious about experimenting with different edge finishes to increase edge holding in PE H1. I tried every possible finish my equipment at the time allowed on a PE Aqua Salt and tested it cutting cardboard up for recycling. UF, F and medium rods on the sharpmaker, as well as fine, coarse and extra coarse DMT stones. I found the lower I went the better edge holding got on slicing cardboard with PE H1.

That is a fantastic revelation, thank you for sharing

To be honest since getting into coarse edges I haven't noticed a drastic difference in slicing aggression between steels themselves. PE H1, K390, M4, AUS8, 1095.....they all get sharp and slice aggressively and demonstrate edge holding abilities in line with what I'd expect.

So this topic has me thinking, maybe I'm not focusing on the steels themselves enough?

Currently I find I prefer high hardness steels for polished edges. As a real world example, I keep my ~65rc aogami super 10" gyuto at 5,000 grit and it holds it well, while my old Victorinox 10" chef knife that's closer to 55rc I give it a coarse edge.

In general I find the softer steels deform too rapidly to hold a polished edge well, while higher hardness steels with superior edge stability work well for that role.

Yeah, maybe when the surface roughness of the edge is coarse enough there's little distinction but we would need testing to be sure. However, perhaps there is a specific surface finish range where we start to see the steel itself show it's aggressive qualities and perhaps if we go too high we just smooth everything out and there is little to no difference?


That could be an active area of more testing to see if there's a numbers to it.

Now, I haven't found any steels that I think work poorly with coarse edges. It extends the edge holding of every steel I've tried when it comes to general purpose knives. However, now I'm wondering, which steels are ideal for this role?

Out of Spydercos current line-up, any suggestions for the best steels for running at 200 grit at the apex?

From my casual, 100% unscientific testing, low carbide steels seem to show the biggest leap in edge retention comparing 2,000 grit edges to 200 grit edges, while K390 seems to take the most aggressive edge out of my current collection.

Those are fantastic questions that I would hope more testing would shine more light on by giving us more objective information.

One last thing - how long were you citting tomatoes for? Asking out of curiosity whether corrosion resistance became a variable?

There wasn't enough tomato cutting to cause problems with corrosion and the blades were wiped off after cutting the tomatoes, Yes, 51200 can be quite reactive especially in real world in a kitchen environment like your experience has shown.

[Deadboxhero]

And this is why I was jealous when you got an E-Lab.

Even when you have Larrins wonderful steel comparison charts with toughness, edge retention, and corrosion resistance; A laser goniometer and guided sharpener to take the guess work out of things; A BESS Tester to quantify 'sharp'...... There are still many more variables to account for to find your perfect steel. And I say 'your' because it's different for everyone and their use case.

"The world of edges has a small doorway in, but opens into a cavern that is both wide and deep." -sal

**** yeah man, edge nerds FOREVER!