Carbides and cutting edges

[dkb45]

I think I may have a misunderstanding as to how carbides work. To my knowledge, chromium carbides, which are abundant in steels like D2 and 440C can be worn down like the matrix by most normal abrasives, leading to a uniform edge.

On the other hand, taking a steel like S30V with vanadium carbides to something like silicon carbide leads to the matrix being abraded but not the carbides, with leaves a toothy but not as aggressive edge, because the carbides are plowed out instead of worn down. Take the same steel and use CBN or diamond, the whole structure is worn down, leading to a uniform edge which is more aggressive because the carbides are worn down too.

Am I right, partially mistaken, or totally insane? Are vanadium steels best sharpened by diamonds? Should I always use diamonds or can less hard media be used in the stropping stages to add more aggression?

I realize this could easily be a maintenance question, so mods feel free to move to where it needs to be. I kind of have two threads mashed together here.

 

[TravisH]

Diamond and CBN simply make the task easier/faster. Other abrasives work; they just take longer.

It is the coarseness of the grit that determines if material is 'torn out' or 'worn down'.
60grit SiC will tear up an edge right quick but I don't think .5 micron/30,000 grit diamond spray is 'tearing' anything out.

Stropping is not normally associated with making an edge more 'aggressive' but more 'refined'. Stropping is normally done with very high grit compounds/polishes.

 

[FortyTwoBlades]

I think I may have a misunderstanding as to how carbides work. To my knowledge, chromium carbides, which are abundant in steels like D2 and 440C can be worn down like the matrix by most normal abrasives, leading to a uniform edge.

On the other hand, taking a steel like S30V with vanadium carbides to something like silicon carbide leads to the matrix being abraded but not the carbides, with leaves a toothy but not as aggressive edge, because the carbides are plowed out instead of worn down. Take the same steel and use CBN or diamond, the whole structure is worn down, leading to a uniform edge which is more aggressive because the carbides are worn down too.

Am I right, partially mistaken, or totally insane? Are vanadium steels best sharpened by diamonds? Should I always use diamonds or can less hard media be used in the stropping stages to add more aggression?

I realize this could easily be a maintenance question, so mods feel free to move to where it needs to be. I kind of have two threads mashed together here.

Pretty much along those lines. Steels with lower vanadium carbide content can be successfully sharpened on silicon carbide or aluminum oxide stones, but high vanadium steels like S90V are best sharpened with CBN or diamond or else you end up with unsupported carbides that tear out, leading to premature dulling. That doesn't mean that you can't get them sharp using silicon carbide or aluminum oxide stones, but you lose a lot of the advantages of the steel and can end up having lower edge retention than non-vanadium or low-vanadium steels as a result.

 

[davek14]

This is a good thread. There's been much debate about carbide tear out with much impassioned debate on both sides. If diamond stones can wear the vanadium carbides even with the matrix that would explain a lot. Everyone is right!

Ankerson?

 

[FortyTwoBlades]

The largest culprit is usually folks trying to use natural stones on high carbide steels, since silicon dioxide isn't even hard enough to really handle chromium carbides effectively. Works fine on low alloy steels, but for general sharpening duties it's better to step up to at least silicon carbide or aluminum oxide. There's a number of very good reasons why those quickly became the preferred choice for sharpening stones once they were developed, even though steels at the time were largely simple carbon steels.

 

[Ankerson]

This is a good thread. There's been much debate about carbide tear out with much impassioned debate on both sides. If diamond stones can wear the vanadium carbides even with the matrix that would explain a lot. Everyone is right!

Ankerson?

I don't use diamonds generally, never had a problem sharpening any steel using Silicon Carbide.

Opinions do vary though.

This is K294 (A11) at 40 Micron (400 grit SIC), like CPM 10V.

[video=youtube;EytTUsg3J6s]https://www.youtube.com/watch?v=EytTUsg3J6s[/video]

 

[davek14]

The largest culprit is usually folks trying to use natural stones on high carbide steels, since silicon dioxide isn't even hard enough to really handle chromium carbides effectively. Works fine on low alloy steels, but for general sharpening duties it's better to step up to at least silicon carbide or aluminum oxide. There's a number of very good reasons why those quickly became the preferred choice for sharpening stones once they were developed, even though steels at the time were largely simple carbon steels.

Where would fine emery cloth stand in that assessment.

Or coarse

 

[FortyTwoBlades]

It all has to do with the hardness of the abrasive relative to the hardness of the carbides (and their volume) in the steel. Unless I'm mistaken, emery is just aluminum oxide. Coarse vs. fine doesn't affect the hardness of the grit, that's just the size of the abrasive grains.

 

[Ankerson]

CPM S125V 400 Grit Silicon Carbide, 15 DPS.

[video=youtube;K_R1yxULRuQ]https://www.youtube.com/watch?v=K_R1yxULRuQ[/video]

 

[dkb45]

So I'm not crazy, and the perceived improvement in the edges from aluminum oxide or silicon carbide to diamond is not just psychological? Now is there any point to stropping with anything other than diamonds when you sharpen on diamonds? Obviously the edge gets sharper, but the few times I have dulled my vanadium steels the extra sharpness seemed to go pretty quickly.

 

[FortyTwoBlades]

CPM S125V 400 Grit Silicon Carbide, 15 DPS.

[video=youtube;K_R1yxULRuQ]https://www.youtube.com/watch?v=K_R1yxULRuQ[/video]

I don't think that the common assertion is that you can't sharpen a high-vanadium carbide knife using silicon carbide. It's that you'll see the edge degrade at a more rapid rate than it would if sharpened using a suitably hard abrasive like CBN or diamond since the vanadium carbides aren't able to provide a longer-lasting edge if they get torn out prematurely. Obviously this would also depend on the mediums used, so you could slice millions of tomatoes and not see any difference due to the near zero wear or strain put on the edge when cut.

 

[Ankerson]

I don't think that the common assertion is that you can't sharpen a high-vanadium carbide knife using silicon carbide. It's that you'll see the edge degrade at a more rapid rate than it would if sharpened using a suitably hard abrasive like CBN or diamond since the vanadium carbides aren't able to provide a longer-lasting edge if they get torn out prematurely. Obviously this would also depend on the mediums used, so you could slice millions of tomatoes and not see any difference due to the near zero wear or strain put on the edge when cut.

Interesting.... Sure it is, that's exactly what people say all the time.

I can and have cut over a mile of cardboard with the high vanadium steels sharpened with silicon carbide and still sliced phone book paper, that's more than once.......

And that's without touching up the edge along the way or making crap up.

Knowing how to sharpen goes a long way.

 

[FortyTwoBlades]

Interesting.... Sure it is, that's exactly what people say all the time.

I can and have cut over a mile of cardboard with the high vanadium steels sharpened with silicon carbide and still sliced phone book paper, that's more than once.......

And that's without touching up the edge along the way or making crap up.

Knowing how to sharpen goes a long way.

Perhaps you misunderstood what I was saying--have you done comparisons of cutting performance with a given knife sharpened on silicon carbide vs. diamond to see if there was any difference? Without comparison it doesn't say anything more than "yup--it's sharp!" Measure the cutting force required for a particular target (maybe a piece of twine) after doing a certain number of cuts and see if anything jumps out.

Obviously vanadium carbides represent only a portion of the blade's composition...I could be way off here on account of not being a metallurgist, but S125V is 3.25% carbon and 12% vanadium, and with the chemical formula VC, vanadium carbide couldn't exceed 3.25% even if you were able to somehow convert all of the carbon into vanadium carbide. I'm wondering how pronounced the effect is, because obviously you cannot abrade a material with another substance that is softer than it is, so the silicon oxide physically cannot be abrading the vanadium carbide itself. It would, however, abrade any chromium carbides that might be present, as well as the regular steel component. So the question is, if there's an effect, by how much is it?

 

[Ankerson]

Perhaps you misunderstood what I was saying--have you done comparisons of cutting performance with a given knife sharpened on silicon carbide vs. diamond to see if there was any difference? Without comparison it doesn't say anything more than "yup--it's sharp!" Measure the cutting force required for a particular target (maybe a piece of twine) after doing a certain number of cuts and see if anything jumps out.

Obviously vanadium carbides represent only a portion of the blade's composition...I could be way off here on account of not being a metallurgist, but S125V is 3.25% carbon and 12% vanadium, and with the chemical formula VC, vanadium carbide couldn't exceed 3.25% even if you were able to somehow convert all of the carbon into vanadium carbide. I'm wondering how pronounced the effect is, because obviously you cannot abrade a material with another substance that is softer than it is, so the silicon oxide physically cannot be abrading the vanadium carbide itself. It would, however, abrade any chromium carbides that might be present, as well as the regular steel component. So the question is, if there's an effect, by how much is it?

The stones I use are actually Silicon Carbide, industrial grade silicon carbide that is extremely hard with very low percentage of fillers.

They are not your typical normal stones you find in the big box stores etc.

I actually have diamond stones, never noticed any difference personally, the reason why I don't use them.

 

[knarfeng]

Not to argue one side or the other, but perhaps this chart would help...

 

[BluntCut MetalWorks]

Just a simplified and easy(a bit off when dealing with high alloy but hey it's good enough for me) to calc carbide volume ...

125V carbide volume calculation is something like this: (3.25%C - 0.55%C matrix) / 6.67%C max => around 40%. Yes, it's a whopping ~40%.

In perspective D2 carbide volume = (1.55-0.6) / 6.67 => ~14.6% (Where 24% of 14.6 is VC, balance Cr7C3 + Cr23C6 + Fe3C)

Also in atom count perspective of 3.25% mass C is equivalent to atom count of 15.11% Fe. So even with carbide in form of MC and M2C, carbide volume easily be 25%-40%
http://www.ptable.com/

...
Obviously vanadium carbides represent only a portion of the blade's composition...I could be way off here on account of not being a metallurgist, but S125V is 3.25% carbon and 12% vanadium, and with the chemical formula VC, vanadium carbide couldn't exceed 3.25% even if you were able to somehow convert all of the carbon into vanadium carbide. I'm wondering how pronounced the effect is, because obviously you cannot abrade a material with another substance that is softer than it is, so the silicon oxide physically cannot be abrading the vanadium carbide itself. It would, however, abrade any chromium carbides that might be present, as well as the regular steel component. So the question is, if there's an effect, by how much is it?

 

[Carboniferous]

Just a simplified and easy(a bit off when dealing with high alloy but hey it's good enough for me) to calc carbide volume ...

125V carbide volume calculation is something like this: (3.25%C - 0.55%C matrix) / 6.67%C max => around 40%. Yes, it's a whopping ~40%.

In perspective D2 carbide volume = (1.55-0.6) / 6.67 => ~14.6% (Where 24% of 14.6 is VC, balance Cr7C3 + Cr23C6 + Fe3C)

Also in atom count perspective of 3.25% mass C is equivalent to atom count of 15.11% Fe. So even with carbide in form of MC and M2C, carbide volume easily be 25%-40%
http://www.ptable.com/

It's all about the units on that percentage. One part of what per 100 parts of what? One mole of something per 100 moles total, one gram of something per 100 grams total, one unit volume of something per 100 units volume total, one mole per 100 grams total, one gram per 100 units volume total, etc.

 

[FortyTwoBlades]

The stones I use are actually Silicon Carbide, industrial grade silicon carbide that is extremely hard with very low percentage of fillers.

They are not your typical normal stones you find in the big box stores etc.

I actually have diamond stones, never noticed any difference personally, the reason why I don't use them.

It doesn't matter how "industrial grade" the silicon carbide is. It's still silicon carbide (Knoop hardness 2480.) Which is softer than vanadium carbide (Knoop hardness 2660.) So it cannot abrade it. It can abrade the other components of the steel, though. Chromium carbide is only 1735 and martensite is 500-800.

Out of curiosity, what make/model are these stones?

 

[Ankerson]

It doesn't matter how "industrial grade" the silicon carbide is. It's still silicon carbide (Knoop hardness 2480.) Which is softer than vanadium carbide (Knoop hardness 2660.) So it cannot abrade it. It can abrade the other components of the steel, though. Chromium carbide is only 1735 and martensite is 500-800.

Out of curiosity, what make/model are these stones?

Congress Tools

 

[dkb45]

Would y'all quit fighting? You are derailing my thread that was already spread thin on topics!

I'm kidding of course. I am dying to know if the stropping with softer media is beneficial outside of the cool factor of the sharper edge, or if I need to quit being cheap and buy some finer diamond stuff.