How low is it practical to go in microns for stropping?

[Jason B.]

Using basic abrasives carbide size will limit how refined you can go. Once the abrasive reaches the same particle size as the carbide then you will no longer be able to effectively sharpen the knife. So lets say you are using waterstones on a steel that has vanadium carbides with an average carbide size of 2u. Once you reach around an 8k ish stone , you wont be able to further refine the knife effectively.

Switch to diamonds/CBN , or enhance the waterstones with a spritz/drop of some , and the problem is immediately solved.

That's a good theory but you lose effectiveness long before the carbide size comes into effect.

The volume and carbide type are much larger contributing factors. Once the volume hits 4%+ with an optimized HT all but diamond CBN and to some extent SiC are about the only options. Waterstones below 1k have a fighting chance because they tear at the matrix of the steel but above 1k the Vanadium carbide volume yields too much resistance to the softer Aluminum Oxide abrasive most commonly found in waterstones.

A good example of this was a K390 Mule team I sharpened up the other day. With 9% Vanadium and 1% Wolfram it stopped my 1k Shapton Pro in its tracks, actually started polishing the bevel.

 

[yepimonfire]

That's a good theory but you lose effectiveness long before the carbide size comes into effect.

The volume and carbide type are much larger contributing factors. Once the volume hits 4%+ with an optimized HT all but diamond CBN and to some extent SiC are about the only options. Waterstones below 1k have a fighting chance because they tear at the matrix of the steel but above 1k the Vanadium carbide volume yields too much resistance to the softer Aluminum Oxide abrasive most commonly found in waterstones.

A good example of this was a K390 Mule team I sharpened up the other day. With 9% Vanadium and 1% Wolfram it stopped my 1k Shapton Pro in its tracks, actually started polishing the bevel.

Does anybody know the carbide and grain size of steels like Aus 8a? I know I've been able to get them consistently sharper then any other steels and I've always heard that it had to do with it being extremely fine grained.

 

[skyhorse]

Try Simichrome-it's finer than Mother's.

It's a great compound! I started using it the late 1960's and still use it.:thumbup:

 

[Jason B.]

Does anybody know the carbide and grain size of steels like Aus 8a? I know I've been able to get them consistently sharper then any other steels and I've always heard that it had to do with it being extremely fine grained.

Please define what carbides you are referring too. Aus8 is a good basic steel, not sure how fine grain it is as that is a very misused term but it will easily take a very sharp edge by most any means.

 

[ToddS]

This image exposes the grain structure in a polished swath near the edge of a carbon steel straight razor.
It should give some sense of the scale of grain structure vs the edge geometry.
I assume the "large" darker grains are carbides (in the 2 or 3 micron range) - I don't have the ability to confirm this easily.

 

[Obsessed with Edges]

Does anybody know the carbide and grain size of steels like Aus 8a? I know I've been able to get them consistently sharper then any other steels and I've always heard that it had to do with it being extremely fine grained.

AUS-8A doesn't have enough hard carbides (chromium- or vanadium-carbides) to be significant, so far as sharpening is concerned. Anything from decent Arkansas stones and up should handle it pretty easily. As far as the 'grain size' goes (carbides or not), that's more dependant on heat treat; so long as that's competent, it should sharpen up as well as any comparable mid-grade stainless steel. I suspect it's relatively straightforward and simple composition has more to do with it's reputation for sharpening up easily, as with any similar stainless steel. It's carbon content is a bit higher than something like 420HC or 440A, and that should help it hold a fine edge better, by comparison to either of those. It has a tiny bit of vanadium in it, which at those small levels, is added mainly for 'grain refinement' (VERY common in cutlery steels, in using a little bit of vanadium for this purpose). Not enough vanadium (or carbon, which makes up the other half of 'carbides') to contribute any uber-hard carbides, though.


David

 

[BluntCut MetalWorks]

The polished surface at 10kV need to etch with Nital (or equiv), otherwise not much to see in term of microstructure. I suspect black/grey figures outlines are just residual of cleaning agent. For simple carbon 10xx with good ht, grain size around 9-10 (11um), look like irregular honeycomb (cross sectional view).

This image exposes the grain structure in a polished swath near the edge of a carbon steel straight razor.
It should give some sense of the scale of grain structure vs the edge geometry.
I assume the "large" darker grains are carbides (in the 2 or 3 micron range) - I don't have the ability to confirm this easily.

https://scienceofsharp.files.wordpress.com/2015/02/blade_fibx_04.jpg

 

[North Arm Knives]

Hey Todd,

Sorry for the late response. I was Out in the bush the last 2 days.

Impressive images. It's refreshing to see you have proof to backup what you were saying near the beginning of the thread. A lot of people on the internet seem to just make claims when they don't have a clue. That's why I challenged you.

It would be fascinating to have another reputable source or two who has done similar measurements. I'm sure it's quite a difficult thing to measure! I wonder what the engineers at some of these razor blade companies have to say... that would be interesting.

I've measured approximately 20 different brands of razor blades and the average edge width is about 100nm.

 

[samuraistuart]

Concerning Aus8a. I am no expert. Many of you I am sure are educated on these topics more than I am. But here is what I see with a steel like Aus8a. The carbon percentage is right at the eutectoid point. That means there is only enough carbon (.7-.75) to make a fully hardened martensitic structure. That is to say, there isn't any carbon past the eutectoid point to even form carbides. The Cr content is right at 13% and slightly higher...just enough to make it stainless. Not enough carbon to form chromium carbides anyway. There is a TOUCH of Moly, and a TOUCH of vanadium. My guess here.....the V for grain boundary pinning and Moly for added strength. Nickel is NOT a carbide forming element, but does toughen a keen edge VERY well and there is quite a bit of Nickel (.5). But there is a whole point of Silicon in Aus8a. Because there is no excess carbon for the silicon to really bond with, it is there to aid in de-oxidation and gives added overall strength to the steel itself.

Like AEB-L in a way. Enough Cr to make it stainless, and only enough carbon for max hardness (not enough for significant primary carbide formation)

 

[ToddS]

The polished surface at 10kV need to etch with Nital (or equiv), otherwise not much to see in term of microstructure. I suspect black/grey figures outlines are just residual of cleaning agent. For simple carbon 10xx with good ht, grain size around 9-10 (11um), look like irregular honeycomb (cross sectional view).

You are correct that the usual way to expose the grain structure is a delineation etch like Nital.
However, the contrast you see in my image is purely due to grain structure. There is no "residue" in a FIB polished surface.

 

[ToddS]

Hey Todd,

Sorry for the late response. I was Out in the bush the last 2 days.

Impressive images. It's refreshing to see you have proof to backup what you were saying near the beginning of the thread. A lot of people on the internet seem to just make claims when they don't have a clue. That's why I challenged you.

No problem. Glad to find people who are able to make intelligent and constructive criticism.

It would be fascinating to have another reputable source or two who has done similar measurements. I'm sure it's quite a difficult thing to measure! I wonder what the engineers at some of these razor blade companies have to say... that would be interesting.

The reference I mentioned in Verhoeven's manuscript is one, and if you search on Gillette patents you will find similar and smaller dimensions described. It's really no secret or new discovery.


https://www.google.com/patents/EP2429777A1?cl=en&dq=EP2429777+A1&hl=en&sa=X&ei=lhTYVITCD4rLsASK9IEY&ved=0CCEQ6AEwAA

In an embodiment, the cutting edge (12) comprises a wedge-shaped tip that has an included angle of less than thirty degrees and a tip radius of less than about 1,000 Angstroms.

 

[North Arm Knives]

Cool, I'll have to look into those

The reference I mentioned in Verhoeven's manuscript is one, and if you search on Gillette patents you will find similar and smaller dimensions described. It's really no secret or new discovery.

 

[BluntCut MetalWorks]

I see something could be some sort of microstructure. Todd, if you don't mind, please draw outline(grain boundary) for a few grains. thanks.

You are correct that the usual way to expose the grain structure is a delineation etch like Nital.
However, the contrast you see in my image is purely due to grain structure. There is no "residue" in a FIB polished surface.

 

[Sadden]

This image exposes the grain structure in a polished swath near the edge of a carbon steel straight razor.
It should give some sense of the scale of grain structure vs the edge geometry.
I assume the "large" darker grains are carbides (in the 2 or 3 micron range) - I don't have the ability to confirm this easily.

Todd this might just be my favorite picture of yours

 

[ToddS]

I see something could be some sort of microstructure. Todd, if you don't mind, please draw outline(grain boundary) for a few grains. thanks.

It is very difficult to see the original grains when there is nothing decorating the grain boundaries. The roundish gray (cementite?) grains would likely be at triple points of the original grain structure. I would guess that not being able to see the original grains is a sign of successful heat treatment.
There is a subtle difference in sub-grain texture from one grain to the next - although not nearly as obvious as in a pearlite structure.
I have marked lines approximately where I see them, but this is mostly a guess from experience.