Chasing a burr on REX 121

Everything you just said I completely agree with. My point was that all that carbide makes it rather pointless to sharpen to a high polish as the carbides are bigger than the edge and will tear out.

Now I'm not going for a fine vs large carbide steel here I like both for different reasons. It's just in my experience steel type, hardness, and heat treat doesn't particularly affect my sharpening. With an India stones yes. With Japanese water stones yes. Norton corborundum stones, no. It cuts them all. But I haven't used the very top steels out there like rex 121 so I really can't say from personal experience.
And apparently Mr wizard has shown me the folly of my ways. That is interesting. I love that website. I know I had read something to the opposite and it was a reIiable source and I can not find that article now. Rats!
 
Nah, you cut the Carbides to shape in the apex if you use the right stones.

If the steel matrix is softer yet filled with hard carbides and it's being sharpened with softer abrasive it's more prone to tear out.

If the steel matrix is much harder with the same carbide volume and hardness yet also sharpened with a softer stone the carbides are broken to shape not cut.

ToddS ToddS
Did some research with a Spyderco Maxamet folder which can run 67-70rc and took SEM pictures of the edges comparing Diamond to Ceramic.

The ceramic broke the Carbides to shape

The diamond cut the Carbides to shape.

Everything you just said I completely agree with. My point was that all that carbide makes it rather pointless to sharpen to a high polish as the carbides are bigger than the edge and will tear out.

Now I'm not going for a fine vs large carbide steel here I like both for different reasons. It's just in my experience steel type, hardness, and heat treat doesn't particularly affect my sharpening. With an India stones yes. With Japanese water stones yes. Norton corborundum stones, no. It cuts them all. But I haven't used the very top steels out there like rex 121 so I really can't say from personal experience.
And apparently Mr wizard has shown me the folly of my ways. That is interesting. I love that website. I know I had read something to the opposite and it was a reIiable source and I can not find that article now. Rats!
 
And apparently Mr wizard has shown me the folly of my ways. That is interesting. I love that website. I know I had read something to the opposite and it was a reIiable source and I can not find that article now. Rats!
I've seen similar as well, maybe Verhoeven or Landes. I'll have to check my old files. It's probably going to be about the amount of force to cleave the carbide vs the amount to pull it from the matrix. Whichever is lower will happen before the other has a chance. So it would probably be affected by the sizes of abrasive, carbide, hardnesses involved of them and the steel structure, and method of abrading.
 
Landes is brilliant and wrote the book about on edge stability and coined the term. However he didn't have access to rex121 and 15v with custom protocols nor did he have access to the bonded super abrasives we have today.

I've seen similar as well, maybe Verhoeven or Landes. I'll have to check my old files. It's probably going to be about the amount of force to cleave the carbide vs the amount to pull it from the matrix. Whichever is lower will happen before the other has a chance. So it would probably be affected by the sizes of abrasive, carbide, hardnesses involved of them and the steel structure, and method of abrading.
 
Can you point me to some of these super abrasives? I'd be interested to check them out as I love sharpening equipment. As I said the only ones I know are the JKI stones and heavy handed didn't seem exactly blown away
 
I don't think Martin HeavyHanded HeavyHanded was using them for the steels we are describing here.

He can correct me, but I believe he was comparing the finish on softer HRC, lower alloy steels and the diamond was cutting more aggressively than he liked making a rougher cut.



I recommend looking at

Naniwa's resin diamond line
Gritomatic and the Venev's and Poltava stones they distribute.
Practical Sharpening and his line as well as what he distributes
And David's Matrix Stones for the Edge Pro.

Stropping on Diamond/CBN compounds also have the best effect after sharpening with any of these stones.







Can you point me to some of these super abrasives? I'd be interested to check them out as I love sharpening equipment. As I said the only ones I know are the JKI stones and heavy handed didn't seem exactly blown away
 
Yes he was sharpening on softer steel. What I gathered was they were too slow for his use for the price. For me to consider it if want them to be spectacular on softer steel.
I've heard good things about the naniwa but like the JKI stones they're too fine
Thanks I will check out the others except the edge pro as I don't use one
 
This is a Rex 121 blade made by Bluntcut. I just did a full reprofile and resharpening, which took only 10 minutes or so.

It's a 3 inch blade with a full distal taper. The steel is hardened to 70 Rc. The edge, after full resharpening, is 0.008 inches at the shoulders. The spine is 0.79 inches at the handle and 0.035 inches a half inch back from the tip.

The sharpening was freehanded on DMT diamond stones, starting with E. Coarse (because I decided to cut the edge angle down a bit), then Coarse, then Fine, then E. Fine.

A small, stiff burr came up quickly at a very low angle -- about 12 dps on my laser protractor.

The burr was easy to remove with light, short edge-leading strokes. I deburred it each time before going to the next stone. At the end, I stropped it on on a thin leather pad glued to a glass plate. The pad was loaded with 0.5 micron CBN paste. At the end, the knife cuts through cardboard with ridiculous ease. It's my dedicated cardboard blade, and using it is like doing surgery.

People think that these super hard, super wear-resistant steels are impossible to sharpen. But with a thin blade and edge profile and diamond stones, they sharpen up like a breeze. The edge stays very sharp for a long, long time.

2v2Ha2fwhxAWtWs.jpg
 
Thanks for that. The edge on mine, well, every dimension, is crazy thick. That plus the less aggressive SiC probably led to the larger and more persistent burr.

I went after it this afternoon with EF and EE DMT plates, and it got considerably sharper (not saying much without a reprofile) with less burring.
 
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.
...

Thanks for that. That reference also says
"The number of carbides present in low-temperature-tempered carburized alloy steels is typically less than 10%. While carbides are harder than the surrounding matrix (martensite/austenite), they do not have an appreciable effect on Rockwell (macro) hardness at this percentage"

Also, consistent with Shawn's observation that 15% carbide volume is a threshold for different behaviour.

...... I've noticed when the Vanadium carbide volume in the matrix blasts towards and past ~15% and you have a high matrix hardness moving toward the mid 60s HRC you'll notice a drop in the effectiveness of SiC. ..

Intuitively, at some volume percentage, those carbides have to contribute to the macro-scale hardness, independently of the matrix hardness.
 
Yes and no

The mechanicism we see from Carbides affecting the hardness with a contribution to the matrix is precipatation strengthening and solid soultion strengthing of dissolved elements.


The annealed hardness of CPM Rex121 is very high because of this at 45rc. CPM 15v annealed hardness is 25rc which is also higher than many steels.

It's not the independent hardness of the Carbides as much as it's the size and volume.

It's all about impeding dislocation movement to reduce permanent plastic deformation.

That's why the steel needs a tempered martensite matrix to function.

Cemented carbide with 70-90% carbide volume can achieve it's hardness without a hard matrix with just sheer volume and a soft cobalt binder but has consequences to the modulus of elasticity being much stiffer combined with lower edge stability. More difficult to apex without taking the apex off with the burr removal and just more prone to edge chipping, less resliance (elastic deformation) combined with all most zero plastic deformation before failure.

30-40% volume found in exotic steels is not enough volume to make the steel hard enough without a Tempered martensite matrix to raise the Yield Strength to useable levels for knife edge performance demands.



Thanks for that. That reference also says
"The number of carbides present in low-temperature-tempered carburized alloy steels is typically less than 10%. While carbides are harder than the surrounding matrix (martensite/austenite), they do not have an appreciable effect on Rockwell (macro) hardness at this percentage"

Also, consistent with Shawn's observation that 15% carbide volume is a threshold for different behaviour.



Intuitively, at some volume percentage, those carbides have to contribute to the macro-scale hardness, independently of the matrix hardness.
 
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I don't think Martin HeavyHanded HeavyHanded was using them for the steels we are describing here.

He can correct me, but I believe he was comparing the finish on softer HRC, lower alloy steels and the diamond was cutting more aggressively than he liked making a rougher cut.

I tried them a variety of steels and what I found was they weren't aggressive enough, esp on lower alloy/lower RC steel where they cut slow and loaded up rapidly.

They worked well on higher carbide steels but my opinion is they are more a niche tool. Aside from cosmetics on wide bevels or very shallow convex on these steels they hold no real advantage over a diamond plate.

For just the right application they are a very good tool if not the only tool, for everything else not so much.
 
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