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More Hardness Discussion
Sorry for having been too brief! Confused

Under scaling effects I have meant effects which occur when we scale down from macroscopic scale to microscale. At microscale the thin metallic material (e.g. a burr) cannot be regarded as homogeneous because it is composed of grains, the size of which may be comparable with the thickness of the material. This leads to change in thin sheet metal behaviour.

The paper I have read claimed, that the decisive parameter for occurrence of scaling effects is the ratio of mean grain size to the foil thickness. Grains located at metal surface behave differently than the internal grains.


Thank you Jan. I think we've got it now. If the grains have a larger diameter (dimension) than the thickness of the material holding them in place then the structure, as a whole, would be weaker. Along these same lines we have satisfied our curiosity to some degree with regard to carbides present in the steel. While apex widths of sharpened knife edges are easily sub-micron, the carbides that may be present in the steel could be from 2 - 50 microns in diameter. It appears that different steels have different types and sizes of carbide content. We're on thin ice here research wise because our sources for this information are less than ironclad. Perhaps someone who has better knowledge and metallurgical resources than we have could shed a brighter light on this carbide size thing.

Our information indicates that a vanadium carbide has a Mohs value of 9.5. That's harder than most of the abrasives we use to grind a knife edge. No small wonder then that folks complain about sharpening these carbide laden steels. This and the mental image we now have of a 5 micron vanadium carbide particle perched within or atop a .5 micron thick piece of steel.

Indeed, vanadium carbides are usually stated to be HRC 82, and I've always gotten the best edges with diamond plates.

I'm not sure how grain size affects the burr, and I've never seen anything stated in writing about it, which seems pretty odd now that you mention it.

I can say with certainty that my 52100 is very fine grain, and should likely be close to size 15, or about the limit of the capability of measuring grain size.

My edges are easy to deburr, but in my experience, simple carbon steel at high HRC is all like that.

Take an 01 file for instance. Large grain, still sharpens and deburrs easily IMHO.
Now its getting good!  

The Mohs scale is, imho, too imprecise to use in a discussion of knife materials.  Vanadium carbide is harder than aluminum oxide and silicon carbide.  However, cubic boron nitride and diamond are harder than vanadium carbide.  The abrasives will cut the material around them, and will abrade the carbides to some degree, but not efficiently.  The carbides can be cut efficiently with cbn and diamond.  However, diamond has a drawback.  It doesn't work well at high speed.  Uncooled diamond on steel will degrade by diffusion into the steel.  

Verhoevens book has many pictures of steels and carbides as examples.  Sizes range from sub micron to 50 microns or more in the various steels.  CPM/powder processed steels have carbides in the 3 to 6 microns.  Carbide type plays a role in size, but processing is the largest influence.  Chromium carbides are generally regarded as larger than vanadium carbides.  That is not true, particularly when they are in the same alloy.  Its widely believed because molecules of Cr23C6 and Cr7C3 are bigger than VC.  However, carbides are rarely molecular in size.  

Some steels in the heat treated condition have no carbides.  I have to elaborate now.  There are what are known as primary carbides or undissolved carbides.  These are present in the steel even when heated to the hardening temperature, thus the undissolved designation.  These are the type of carbides most often discussed.  They make the biggest difference in wear resistance.  

Then there are tempering carbides.  These are extremely small, single digit nanometer size in some cases and dimensions.  They can also be extremely hard, with tungsten carbide tempering carbides harder than vanadium carbide.  These form when quenched steel is tempered.  Tempering carbides are pretty much unavoidable.  They are generally not considered for wear resistance, as the volume % is fairly small in knives and the small size reduces their effectiveness for such purposes.  

Carbide volume % varies widely as well, depending on steel.  Steels with less than about 0.6 weight % carbon will not have much if any undissolved carbides when heat treated.  Low alloy and plain carbon steels above 0.6 wt % carbon have increasing amounts as carbon increases, up to 20 volume % or a bit more.  Common CPM/powder steels will have about 15% by volume, give or take a point or two.  Some will go higher, up to 20-25 % in some steels.  These are on the border of what is practical for knives.  

Wall of text to be continued.
This is getting good! I've simply never thought of this particle size thing in relationship to the thickness of the edge. I've also never considered how hard these carbides are to the things we're trying to grind them down with. I know this much from personal experience. You better be using an abrasive that is a lot harder than the thing you're trying to grind down unless you've you've got a lot of time and nothing else to do. That picture of a boulder sitting on top of a pedestal makes an impression. Raises all sorts of questions for me.
Well that is good stuff me2 and thank you for the time devoted in putting that post together. Your remarks concerning carbide size are particularly helpful.  Sorry if our photograph left you with nightmares Bud. While we're certain that it represents an over exaggeration of the situation, it is an attention grabber. It's useful for us to take a moment and think about the dimensions of things that we are dealing with. Often it leads to a better understanding. If we better understand a principle or problem then we can deal with or take advantage as the case may be. 

We're not really so interested in burrs here as opposed to edges. We talk and ask about burrs in this thread only because it would seem that there would be a relationship between the metal that composes the burr and the eventual edge that the burr was directly attached to. Both burrs and edges seem to act in a contradictory manner to the steel they were spawned from. Perhaps that can be explained by plastic deformation or as Jan offered as a possibility, scaling effects of foils or...something else entirely.

A closing thought concerning carbides in steel: As some of you know, we stumbled into this edge testing business by accident. It was originally our intent to build a better knife sharpener. One based on the principles of the fiber optic polishing machines that we once manufactured and sold. Part of that design process required that we construct a series of solid custom abrasive wheels for the newly designed sharpener (edge grinder). We leaned toward a bonding agent/matrix (for the abrasives) that we had experience with but we knew that the agent was an abrasive in and of itself. In fact, we found that the abrasive particles in the agent were considerably larger than some of the finer abrasives we wanted to use. So we had to manufacture our own bonding agent that contained abrasive particles no larger than .3 micron. We find it curious that this same consideration wasn't given to CPM powder development (for knives) given that the business end of the bonding matrix (the edge) was bound to be much thinner than the carbides used. Not a criticism by any means because they must have their reasons but still, a question.
Mr. EOU exclaimed, "We're not really so interested in burrs here as opposed to edges."

Mr. EOU, have you done much experimenting with your NERM (Nifty Edge Rolling Machine) with harder steel edges such as RHC 62 and the like?

If the ductility of burrs is explained simply as a quality of thinness, it would seem reasonable to assume that this ductility would decrease rapidly in the edge as the actual edge is much thicker, unless there are other factors in play.

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