Chasing a burr on REX 121

[hardheart]

I don't actually use my Rex 121 mule and 15V fixed blades from Farid, though I've had them for years. I was searching for something else when I saw a post about SiC not working on steels above 4% vanadium, which didn't seem to jive with my memory. So I took out my Norton combo Stone from Home Depot and gave it a shot on the mule. I had a weak polish on the edge bevel back when I last played with the knife, though it wasn't sharper than paper slicing.

The SiC scratched the edge bevel well enough, and then when I checked the edge, I was surprised by a meaty burr. I had to work at low pressure on both sides several times to remove it. Then I went to a vintage SiC/carborundum razor hone. That put a hazy polish on the bevel... and raised a small burr again. I chased that one down quickly, then stropped with 1 micron diamond on leather for no good reason because I hadn't really refined the edge.

Has anyone else experienced such burring from Rex 121?

I went ahead and did the 15V on waterstones after, finishing with 13k sigma power. Really minor burr formed at 1k that was easy to remove.

 

[DeadboxHero]

Please use diamond/CBN on those steels in particular. You're leaving a lot of performance on the table using SiC and Alumina

If you don't like the rough cut from a coated stone you can use a bonded stone.

These gentleman have some options for those steels if you're interested.
@Gritomatic
@Diemaker

I don't actually use my Rex 121 mule and 15V fixed blades from Farid, though I've had them for years. I was searching for something else when I saw a post about SiC not working on steels above 4% vanadium, which didn't seem to jive with my memory. So I took out my Norton combo Stone from Home Depot and gave it a shot on the mule. I had a weak polish on the edge bevel back when I last played with the knife, though it wasn't sharper than paper slicing.

The SiC scratched the edge bevel well enough, and then when I checked the edge, I was surprised by a meaty burr. I had to work at low pressure on both sides several times to remove it. Then I went to a vintage SiC/carborundum razor hone. That put a hazy polish on the bevel... and raised a small burr again. I chased that one down quickly, then stropped with 1 micron diamond on leather for no good reason because I hadn't really refined the edge.

Has anyone else experienced such burring from Rex 121?

I went ahead and did the 15V on waterstones after, finishing with 13k sigma power. Really minor burr formed at 1k that was easy to remove.

 

[hardheart]

I've got a full set of dmt plates (which I actually don't like too much), was just using the SiC to make sure it worked. I read a few posts where people thought it couldn't cut high vanadium steel alloys.

 

[DeadboxHero]

The biggest problem is its not so black and white.

So when folks get an ANY effect or result from softer abrasive there is a "A-HA! Myth busted!" Moment.

We need to remember, just something has an effect doesn't mean it's the best tool for the job.

There are different types of SiC and different ranges of hardnesses from measurement differences etc. Lots of details to get lost in and argue.

SiC does have a similar hardness to Vanadium Carbides but it doesn't have the overmatch in hardness for superior cutting power. I've noticed when the Vanadium carbide volume in the matrix blasts towards and past ~15% and you have a high matrix hardeness moving toward the mid 60s HRC you'll notice a drop in the effectiveness of SiC. The edges don't come up as crisp as they do off of diamond/CBN and it's not just surface roughness, some folks do confuse the more toothy, rougher edge off coated diamond plates as being crisper, I'm using a variety of bonded stones that can produce similar and superior surface quality to SiC.

These steels need harder abrasive to shape the apex better,the otherwise experience and performance will not be as good as it could be.

This is the biggest reason folks aren't seeing the performance these steels have to offer since it requires more understanding to maximize their potential.

15v and Rex 121 at the furthest end of the spectrum and need abrasive overmatch for the best performance.


This has been directly from my experience working with both of these steels for my custom knives.

I've got a full set of dmt plates (which I actually don't like too much), was just using the SiC to make sure it worked. I read a few posts where people thought it couldn't cut high vanadium steel alloys.

 

[hardheart]

I'll keep an eye out on that gritomatic site for when they get more full size bench stones in. I haven't had bonded diamond, just used the coated plates along with pastes and powders on a variety of strops. Couldn't find a website for diemaker.

Just wanted to check the sharpening for my own curiosity. I figured there was no way to not get an edge when the abrasive is harder than two thirds of the blade material in Rex 121, but I wouldn't expect a great edge from the process. But suboptimal sharpening is the most fun for me to play around with. With a few dozen knives on hand, it typically isn't about needing to get it done in the most efficient way possible to have a sharp cutting edge available. I go back to my better stones and materials when I actually need to sharpen properly, but that's maintenance, not play time.

I was just surprised at just how easily it burred. If anything, I was expecting much slower material removal. Got my curiosity up even more.

 

[Twindog]

I can raise a small burr on my Rex 121 from @bluntcut . I use only diamonds, and the hardness is 70 Rc, so it may not behave exactly like yours.

The knife came from Bluntcut sharpened on diamonds. The edge is very thin -- somewhere around 0.007 inches.

The burr seems stiffer and smaller than other steels, but not difficult to remove with short, edge-leading strokes on fine diamonds.

I use the knife regularly on cardboard, but it needs sharpening so infrequently that I don't have a lot of experience chasing the burr.

 

[Vicv]

I've never done those two steels but 10v at 64hrc isn't a problem for me with Norton crystolon. I'm not sure about the whole crisp edge thing but 3 finger sticky across the whole edge seems good to me (shrugs). I don't like diamonds as you pay a lot of money for these plates and there is only one surface on it. Once those diamonds are dull that's it...... When my stone loses bite (which is extremely rare) I lap it with loose sic on glass. Then it's good again. I can do that 10000 times. Seems a no brainer.

 

[DeadboxHero]

Well, 10v at 64rc is not as difficult for SiC to shape.
If hardness is increased to 67rc though it will be noticed.


10v is ~17% Vanadium carbide volume.



15v and Rex 121 are at a different level.

15v is ~22% volume Vanadium carbide volume.

Rex 121 is special, it makes ~24% with an vanadium volume 6% M6C Carbides for a


Unrelated to the carbides is the hardness the matrix can be heat treated to.

Rex 121 gets the hardest of available steels, unrelated to carbides.

So Rex 121 at 70rc and 15v at 67rc had less performance on SiC than bonded CBN/diamond.

The edges were not shaped as crisp and more burnished.

Folks haven't seemed to catch up to the fact that "diamond stone" no longer means coated plates that wear out and make rough finishes for the given grit size with deep scratches.

Bonded stones last much longer than any SiC stone. Less prone to dishing. Cleaner cut and better surface finish all with less burnishing and better apex shaping for steels packed with hard~ 3-5um Carbide clusters at ~85-90hrc at high volume with +65hrc matrixes


I'd recommend you try for yourself and see the difference.

I use bonded CBN/diamond for any steels with +3% vanadium carbide volume and +60rc

I've never done those two steels but 10v at 64hrc isn't a problem for me with Norton crystolon. I'm not sure about the whole crisp edge thing but 3 finger sticky across the whole edge seems good to me (shrugs). I don't like diamonds as you pay a lot of money for these plates and there is only one surface on it. Once those diamonds are dull that's it...... When my stone loses bite (which is extremely rare) I lap it with loose sic on glass. Then it's good again. I can do that 10000 times. Seems a no brainer.

 

[Vicv]

The only stones I've seen that are made this way are prohibitively expensive. And in a finer grit than I need. JKI sells them. I guess I'm just happy with what I have. I prefer fine grained steels anyway and alox cuts these fine.
That is an interesting perspective though. I'm under the impression that carbides don't sharpen anyway. They are too brittle. So they either break apart or pull out of the steel matrix while sharpening. And being that these steels seem at their best at ~300 grit levels anyway the abrasive is bigger than the carbides so who cares. The reason aluminum oxide is bad for grinding these steels isn't because they can't cut the carbides. It's because the carbides dull the abrasive grains and it can no longer cut the steel matrix well. Sic doesn't have this problem so it still abraded the matrix. Even if the matrix is at 70hrc even aluminum oxide is still much harder than that so the hardness of the steel should not really be a factor. Or not enough to dull the abrasive and cause burnishing

 

[Vicv]

That being said I have not sharpened these steels so I am going by theory not experience. And even though I disagree with her on this point; my wife does tell me I am occasionally incorrect about certain facts

 

[tomhosang]

I can raise a small burr on my Rex 121 from @bluntcut . I use only diamonds, and the hardness is 70 Rc, so it may not behave exactly like yours.

The knife came from Bluntcut sharpened on diamonds. The edge is very thin -- somewhere around 0.007 inches.

The burr seems stiffer and smaller than other steels, but not difficult to remove with short, edge-leading strokes on fine diamonds.

I use the knife regularly on cardboard, but it needs sharpening so infrequently that I don't have a lot of experience chasing the burr.

That's exactly my experience with Maxamet. It forms a very small but brittle burr. You can actually push it around with your finger and it'll break off. I think that might just be a characteristic of these higher hardness steels.

 

[tomhosang]

The only stones I've seen that are made this way are prohibitively expensive. And in a finer grit than I need. JKI sells them. I guess I'm just happy with what I have. I prefer fine grained steels anyway and alox cuts these fine.
That is an interesting perspective though. I'm under the impression that carbides don't sharpen anyway. They are too brittle. So they either break apart or pull out of the steel matrix while sharpening. And being that these steels seem at their best at ~300 grit levels anyway the abrasive is bigger than the carbides so who cares. The reason aluminum oxide is bad for grinding these steels isn't because they can't cut the carbides. It's because the carbides dull the abrasive grains and it can no longer cut the steel matrix well. Sic doesn't have this problem so it still abraded the matrix. Even if the matrix is at 70hrc even aluminum oxide is still much harder than that so the hardness of the steel should not really be a factor. Or not enough to dull the abrasive and cause burnishing

I don't think that it's that the carbides themselves are brittle. You just need a matrix that is strong enough to retain them. These high hardness/high wear resistance steels (REX 121, 15V, Maxamet, etc) have high attainable hardness that helps retain those carbides in the matrix(high hardness = higher edge strength). Diamond/CBN are harder than all types of carbide found in steel (see this thread: https://www.bladeforums.com/threads/carbide-hardness-chart.1705186/). They're hard enough to abrade the matrix PLUS the carbides inside it, resulting in a keener apex that is longer lasting. Abrasives that are not as hard as the carbides in a given steel abrade the matrix but not the carbides. This reduces the amount of material retaining the carbide and in theory could lead to "carbide tearout."

 

[ToddS]

Unrelated to the carbides is the hardness the matrix can be heat treated to.

Is there any evidence that the matrix of those steels can be any harder than say O2? Intuitively, the Rockwell hardness (which is measured over a much larger length scale than the dimensions of the carbides) is some sort of average of the matrix and carbides, but I haven't been able to find any research on this.

 

[Vicv]

I don't think that it's that the carbides themselves are brittle. You just need a matrix that is strong enough to retain them. These high hardness/high wear resistance steels (REX 121, 15V, Maxamet, etc) have high attainable hardness that helps retain those carbides in the matrix(high hardness = higher edge strength). Diamond/CBN are harder than all types of carbide found in steel (see this thread: https://www.bladeforums.com/threads/carbide-hardness-chart.1705186/). They're hard enough to abrade the matrix PLUS the carbides inside it, resulting in a keener apex that is longer lasting. Abrasives that are not as hard as the carbides in a given steel abrade the matrix but not the carbides. This reduces the amount of material retaining the carbide and in theory could lead to "carbide tearout."

That's my point. Even diamond isn't abrading the carbides. They're just breaking or tearing out. And when they do I'm not worried. The abrasive grit is bigger than the carbides. So the scratches are bigger. So carbide tearout/breaking isn't an issue. If I was trying to sharpen to 6k....then I could see the issue. But these steels work their best with coarse edges......as do most cutting tasks

 

[tomhosang]

That's my point. Even diamond isn't abrading the carbides. They're just breaking or tearing out. And when they do I'm not worried. The abrasive grit is bigger than the carbides. So the scratches are bigger. So carbide tearout/breaking isn't an issue. If I was trying to sharpen to 6k....then I could see the issue. But these steels work their best with coarse edges......as do most cutting tasks

I think you missed the point. The diamond 100% abrades the matrix AND the carbides. It shapes the carbides. It does not tear them out. That only happens when you use abrasives that aren't hard enough (AlOx, SiC, etc.).

 

[Twindog]

Is there any evidence that the matrix of those steels can be any harder than say O2? Intuitively, the Rockwell hardness (which is measured over a much larger length scale than the dimensions of the carbides) is some sort of average of the matrix and carbides, but I haven't been able to find any research on this.

Maybe this will help.

https://www.industrialheating.com/articles/92818-alloy-carbides
"While carbides are harder than the surrounding matrix (martensite/austenite), they do not have an appreciable effect on Rockwell (macro) hardness at this percentage."​

If carbides did affect hardness, all Rockwell testing would have to account for the carbide load.

But there are other points of view:

https://www.quora.com/Why-does-the-...end-on-the-shape-and-distribution-of-carbides

"The hardness of steel (resistance to deformation) will depend upon the strength of the matrix, and also the presence, strength, size, volume fraction and distribution of second-phase particles."​

 

[Vicv]

I think you missed the point. The diamond 100% abrades the matrix AND the carbides. It shapes the carbides. It does not tear them out. That only happens when you use abrasives that aren't hard enough (AlOx, SiC, etc.).

No it doesn't. That's the point. I saw it in a paper by Larrin Tomas on Knifenerds. I've seen it elsewhere as well. Like Roman Landes. Since I don't have a good enough microscope to see that I have to take those metallurgists at their word

 

[Mr.Wizard]

That's my point. Even diamond isn't abrading the carbides. They're just breaking or tearing out.

This is not correct. From https://scienceofsharp.com/2019/11/03/carbides-in-maxamet/ and the image below you can see that the carbides are cleanly cut by diamond to be flush with the matrix (upper part of the image), while the matrix is eroded by alumina leaving them proud (lower part of the image.) ( ToddS if you don't want me embedding your images like this even with direct links to your site please let me know. I have the utmost respect for your work.)

However I agree with your point "And being that these steels seem at their best at ~300 grit levels anyway the abrasive is bigger than the carbides so who cares." I don't understand (but am open to learn) the benefit of very high carbide steels if a highly refined edge is the goal.

 

[DeadboxHero]

I made that statement to point out that the HRC of the knife is different than the carbide volume. There seemed to be some confusion from some folks that something like S90V at 59-60rc was a "harder steel" than AEB-L at 65rc.

In fact, S90v is softer steel with production heat treatment protocols. It's just filled with harder particles at higher volume than most stainless steels that makes it more wear resistant than AEB-L at higher hardness.

While the steel has different phases and particals with their own independent hardnesses that make up the Microstructure the Rockwell Hardness measures the combined resistance to permanent plastic deformation of the whole microstructure together.


When Rex121 is not heat treated it has the highest volume of Carbides, yet it's only 45rc, it's also not as wear resistant since the matrix doesn't support the carbides
(All steels have high carbide volume before HT)
By dissolving some of those carbides to put carbon in soultion and making a tempered martensite matrix after HT we get much higher HRC (70-71 depending on the protocol)

The point here is
it's the overall microstructure and it's constituents that are more important than isolated hardness values yet the undissolved primary carbides left over after austenitizing, their type, volume and size had the biggest effect for wear resistance over just hardness yet it's a cooperation since hardness is needed to support the carbides in the matrix.

So Rex 121 at 71 is more wear resistant than 68rc

Is there any evidence that the matrix of those steels can be any harder than say O2? Intuitively, the Rockwell hardness (which is measured over a much larger length scale than the dimensions of the carbides) is some sort of average of the matrix and carbides, but I haven't been able to find any research on this.

 

[DeadboxHero]

You have some reading to do.

Here is a thread I made on carbide hardness.
https://www.bladeforums.com/threads/carbide-hardness-chart.1705186/

The hardest Carbides found in common knife steels are ~3000hv
SIC can range near that.

CBN is ~4500hv
Diamond is ~10000Hv
(These values range since they are brittle and difficult to measure and anisotropic)

Abrasive hardness overmatch has been critical in my experience. Yet, that doesn't mean that Diamond is twice as good as CBN cause it's harder. It's not that simple.

This is a complex subject that is more involved that just hardness of these things but also the tribological effects of how they cut and wear each other and the other properties of the abrasive grains themselves.

No it doesn't. That's the point. I saw it in a paper by Larrin Tomas on Knifenerds. I've seen it elsewhere as well. Like Roman Landes. Since I don't have a good enough microscope to see that I have to take those metallurgists at their word