edge holding vs sharpening

Some wandering thoughts on the subject of hardness and edge holding... an interesing topic that I find fascinating.
I am not at all surprised that it was a major undertaking to re-shape a D-2 edge. In my opinion, D-2 is the toughest and most abrasion resistant steel currently available with the possible exception of CPM-440V. I used it for a long time in many of my fixed blades but switched to ATS-34 because D-2 was not stain resistant and reacted badly to fruit juices, fingerprints, blood and seawater.
Abrasion is what forms a blade shape and abrasion is what dulls an edge. Abrasion can only be performed by a substance harder than that being abraded such as dirt (on an animal skin), cardboard binders(tiny particles in the glue) and sharpening stones,etc. There is no steel that even approaches the hardness of a sharpening stone by several orders of magnitude.
Even stones that appear to be "soft", like a common whetstone that eventually gets a dip worn in the middle from use is several thousand time harder that the hardest steel. It is only the binding vitrification that breaks down with use and allows fresh new particles to be presented for cleaner cutting as the old particles have their corners knocked off. This phenomenon is known as "friability" A low friable stone will not allow its rounded particles to be knocked off and hence will not present fresh, sharp points to the work in progress.
The hardness of stones cannot even be measured on the Rockwell scale which is arithmetic. (2 is twice as hard as 1, 10 is ten times as hard as 1, etc.) Stones are measured on the Moh's scale of mineral hardness which is based on talc being 1 and diamond being 10. But this is a LOGARITHMIC scale where 10 is 27,000 times harder than 1!
So if you are having trouble sharpening a blade, it is not because it is too hard.
There are two other factors to consider:Abrasion Resistance and Geometry.
Abrasion resistance refers to the steels' ability to keep those sharp, hard stone particles from digging into it and removing material. This process, as I mentioned above both forms as well and dulls a blade edge.This is a function of the materials from which the steel is made and the manner in which it is heat treated to combine and alter those materials. Hardness can contribute to edge holding and abrasion resistance mainly because of the heat treating process used to obtain that hardness. For example, D-2 steel at Rc57-58 will hold an edge four to five times longer than 440C at Rc 60-61. It is a steel designed not for a stainless nature but specifically for a tough and abrasion resistent nature.
( In the days before lasers, Commercial knife blades were stamped out of large sheets of steel. The stamping dies were made of D-2 and had to last a minimum of 5000 stampings before re-sharpening). CPM-440V is comparable.
Edge Geometry: this , I believe is the most common cause of sharpening "blues". The end user is not holding the blade at the same angle that the maker created on the edge. Perhaps the maker put too obtuse an angle on the edge or the user wants to have more of an acute angle on the edge. Either way, if the user does not follow the same original angle,
too much material will have to be removed requiring a coarse stone(diamond is great in about 280-400 grit) but will then require a finishing step to smooth out the edge), or more time on the finer stones.
I don't know if this has been helpful but I just felt like meandering along here, It's been a long week. Best to all ,BobT
 
Thanks for the informative post, Bob -- Further evidence of what a great resource these forums are.

------------------
--+Brian+--
 
Thank you very much Bob.......I understand it now.....thanks for the clear explanation.

Joe Leung
 
Bob, this kind of brings us around to my original question. In my experience, which is limited to a very narrow area of the entire world of knifedom, there appears to be more to it than just "blades that resist sharpening, also resist dulling." Most of my usage outside the kitchen has been to fillet fish. I started with a Gerber, advertised as being 59-61 RC. That blade edge would go away almost before your eyes while filleting a few fish. I got the impression it was a function of time rather the number off fish worked on! After getting home I would try and resharpen it. From the first time, until I lobbed it into 170 feet of water off Eagle Harbor (Ooops), it would show water spots on the blade by the time I got home. Even rinsing it between fish with fresh water I had taken along would not prevent spots from forming. Later when trying to restore the edge, every place where the water spots appeared at the cutting bevel, the metal would just flake out like a completely different material - it was no longer steel. Chorides perhaps? That Gerber was so hard, that it sneered at my diamond sharpener. The steel just slid around on it. It took forever to get the edge back, and in order to do it, the flaked out areas had to be taken out as well. I would go through this every time I used it in saltwater. I bought a Kershaw, and then a Buck, and they behaved pretty much the same way - got dull in a few minutes; had water spots; flaked out; took hours to sharpen. (One of those is also resting in the cold depths of Puget Sound among the fish it was intended to disassemble, and the other is in a drawer, ignored). Because I assumed a direct relationship of hardness (sharpening resistance), to edge holding, what was happening seemed impossible. Hard to sharpen blades were getting dull with minimual use. I began to suspect chemical action on the edge rather than abrassion. I think it was Joe Talmadge who thought this was unlikely - and it does seem pretty far fetched. He suspects something in the heat treat which I do not yet well understand. I now use knives in soft 440A (inelegant and utilitarian) and they hold an edge better than any of the above (which are probably 425M), BUT are a snap to resharpen using the same diamond stone. I've got a Buck lockback. It has that same difficult-to-sharpen quality. It also slides around on the stone when being sharpened. The stone doesn't seem get much bite. The same sharpener gets good bite on an ATS 34 Applegate blade, which is harder and more abrassion resistant than the 425M. It's almost as if the texture fo the steel is the determining factor. I'm going to purchase a Wegner jr in ATS 34. I've got a mountain of carboard boxes on hand. I'm gong to sharpen the Buck and the jr, and alternate knives, doing one box each, and see which one gives out first. Then I'm going to resharpen them and see if my previous observation holds. The question is, will the blade which gets dull first, resist resharpening more or less than it's more durable pal? I'll report back.

Jack
 
Just one clarification: the Mohs scale is not logarithmic; it is random. Mohs just took some minerals he had on hand and observed that some would scratch others and assigned them numbers with no attempt to make the intervals uniform.

There's a big gap between 6 (feldspar) and 7 (quartz). All steels are much softer than quartz; that's about all the Mohs scale can say about the hardness of steels. There's an enormous gap between diamond at 10 and aluminum oxide (aka emery, alumina, corundum, ruby, sapphire, etc.) at 9, much greater than the difference between 9 and 1 (talc, aka talcum powder). Silicon carbide is harder than aluminum oxide but enormously softer than diamond. Despite the advertising, nothing is "almost as hard as diamond."

I could go on all day but I think I've covered everything relevant to us knife knuts ... except that Wa****a, Arkansas, and sandstone are all forms of quartz, much softer than aluminum oxide but enough harder than steel.

-Cougar Allen :{)
 
Bob, this kind of brings us around to my original question. In my experience, which is limited to a very narrow area of the entire world of knifedom, there appears to be more to it than just "blades that resist sharpening, also resist dulling." Most of my usage outside the kitchen has been to fillet fish. I started with a Gerber, advertised as being 59-61 RC. That blade edge would go away almost before your eyes while filleting a few fish. I got the impression it was a function of time rather the number off fish worked on! After getting home I would try and resharpen it. From the first time, until I lobbed it into 170 feet of water off Eagle Harbor (Ooops), it would show water spots on the blade by the time I got home. Even rinsing it between fish with fresh water I had taken along would not prevent spots from forming. Later when trying to restore the edge, every place where the water spots appeared at the cutting bevel, the metal would just flake out like a completely different material - it was no longer steel. Chorides perhaps? That Gerber was so hard, that it sneered at my diamond sharpener. The steel just slid around on it. It took forever to get the edge back, and in order to do it, the flaked out areas had to be taken out as well. I would go through this every time I used it in saltwater. I bought a Kershaw, and then a Buck, and they behaved pretty much the same way - got dull in a few minutes; had water spots; flaked out; took hours to sharpen. (One of those is also resting in the cold depths of Puget Sound among the fish it was intended to disassemble, and the other is in a drawer, ignored). Because I assumed a direct relationship of hardness (sharpening resistance), to edge holding, what was happening seemed impossible. Hard to sharpen blades were getting dull with minimual use. I began to suspect chemical action on the edge rather than abrassion. I think it was Joe Talmadge who thought this was unlikely - and it does seem pretty far fetched. He suspects something in the heat treat which I do not yet well understand. I now use knives in soft 440A (inelegant and utilitarian) and they hold an edge better than any of the above (which are probably 425M), BUT are a snap to resharpen using the same diamond stone. I've got a Buck lockback. It has that same difficult-to-sharpen quality. It also slides around on the stone when being sharpened. The stone doesn't seem get much bite. The same sharpener gets good bite on an ATS 34 Applegate blade, which is harder and more abrassion resistant than the 425M. It's almost as if the texture fo the steel is the determining factor. I'm going to purchase a Wegner jr in ATS 34. I've got a mountain of carboard boxes on hand. I'm gong to sharpen the Buck and the jr, and alternate knives, doing one box each, and see which one gives out first. Then I'm going to resharpen them and see if my previous observation holds. The question is, will the blade which gets dull first, resist resharpening more or less than it's more durable pal? I'll report back.

Jack
 
The automatic censorship software is screwing up again -- Wa****a should read W a s h i t a.

-Cougar :{)
 
Cliff, above you said this:
**************
Along the same line, high alloy steels for example are more trouble to maintain that low alloy ones because the more carbides the bigger chunks of the steel matrix that break off. Of course the more carbides the more abrasion resistant and the longer the times between sharpening.
*************

Is there a direct relationship between carbide content and grain size? I thought that CPM steels had lots of carbides, but a small grain size. Am I in error??

Jack Goertz; try some Stellite or Talonite. It laughs at corrosion, and keeps an edge for a long, long time. Tim Flanagan's wife has a Stellite paring knife, and Tim is always taking it away from her because it cuts so well on boxes, plastic banding, etc.

Good thread. My sincere compliments to the poster that could restore a dull Moran edge with a few swipes on a stone!! I get shudders just trying to imagine resharpening one of those convex edges. Walt
 
http://www.japanwoodworker.com/

Brian, the link above is for a company that specializes in fine woodworking tools and focus on Japanese stuff, though there are lots of other things. They have a good selection of Japanese water stones that are actually made in Japan (and aren't cheap). They carry the slip stones and everything else you could want to use the waterstones in a traditional manner. I know that Norton Abrasives is also making their own waterstones now, but don't have a good idea of where to find them. Norton is a big name in this country so it shouldn't be too hard.

All,
I won't even begin to try to act like I know what really makes a knife easy/hard to sharpen. I have a couple of David Boye's (though I'm pretty sure that Matt Conable was the person that did most of the work on the ones that I have) dendritic steel knives and they are easy to sharpen and will cut all day long.

------------------
Paul
Keep Em Sharp


 
Hey Walt, you whacko boy you... hehehe...

Maintaining the sharpness of the convex edge is easy. getting the edge angle at first is really a pain in the you-know-where. My dad and I made this homemade bowie and decided on that type of edge, for better slicing ability. You should've seen how many times he almost gave up on it. I guess persistence paid off. Sharpening it though, I had to do it very slowly, as in SLOW. I can't swipe it like I do to normal everyday knives here. But that slow swiping on the stone (around 2 to 3 swipes each side alternately) brings back that hair-popping sharp edge quite fast.

I'm practicing on it everyday, and couldn't even bear to use my DMT aligner on this. Seems like convex edges needs the TLC than hand-sharpening provides.

Thanks for the compliments, but I believe it should go to the person who taught me how to do it well, Mr. Joe T.
smile.gif


Dan
 
Cliff, thanks for the info. I'll keep experimenting on the edge bevel until I can get the right angle on this @$#^$%^ convex edge, hopefully it won't turn out to be a saber grind! hehehehe....

Dan
 
I think the key to your question lies in the hone you use.....Diamonds are harder than anything on this planet (I think). I sharpen carbide for a living...make knives as an avocation.....carbide is very very hard.. i think in the rockwell around 80 or something, and the only thing......that will cut it is diamonds.....ive tried grinding carbide on my belt sander that has the $5.50 3m belts that dont get dull forever and the carbide just wears them down to the cloth underneath with little or no effect on the carbide.... I made a bunch of knives out of vascowear a while back...developed for the paper industry to slice paper...which is very hard on blades...and a friend showed me his vascowear knife after a year....and i mean this stuff is tough...harder to grind than anything i have ever seen...i will never use it again....anyhow...after a year with a diamond hone.....the knife is about 1/4 its original width.....i hope everyone gets the point.....
 
Here are some of my own random observations. First off I think edge and blade geometry play a MUCH greater role than most folks realize. I'd far rather have a knife with outstanding geometry than one made exceptionally hard or out of wonder steel but featuring poor geometry or obtuse angles.

Just to add to something Joe T. mentioned, I think there is a LOT to be said and learned about which particular steels will support a very thin edge matrix. Some will, some never will. Two good cases in point are 1095 and 420. I've got a diminutive 1095 parer that a rec.knives poster made for me that represents several interesting points. For one thing it was heat treated to be around 62Rc. Since I don't have a Rockwell tester I can't be sure of that but one can use the spine of this knife to steel other knives that I know are around 59-60Rc. Anyway, this is the *easiest* to sharpen knife that I've ever had. Why? It's also the thinnest knife I've ever seen.

The spine at it's absolute thickest is only 0.05" (aprox 1mm) Measuring 1/8" up from the edge the 2.25" blade tapers from 0.025" to 0.010". It's *t h i n*. In fact, it's so thin that it's very flexible despite being quite hard. This knife not only takes an edge sharper than a scalpel, (literally) it holds it's cutting ability for a good long time, just because it's so thin that when the edge wears away, what's left is still plenty thin enough to cut with. In fact, I'd wager that even if the edge were to wear away the first 1/8" it'd still be thinner than most conventional paring knife edges as they come from the factory. As mentioned, Alvin's custom parer was made from lowly, common as dirt 1095. I think Alvin said he got it as 1/16" stock from Brownell's.

OK, compare that one to a typical Lamsonsharp mystery stainless parer that is a whopping 0.045" as measured 1/8" up from the edge. Most folks might think the Lamsonsharp parer is plenty sharp, but note that the edge rapidly transitions to almost the full spine thickness of the Custom parer. It won't keep an edge for 5 minutes, and the steel won't even begin to support the kind of cigarette paper thin edge that the 1095 parer gets easily.

I don't know the hardness of the Lamsonsharp parer, but I'd guess it not to be more than 54-56Rc. So, it would seem like it would be easier to sharpen. But it's not. So I think, as others far more learned have pointed out, that a few miniscule Rc points are almost irrelevant to the far harder stone or hone. What is relevant is the thinness of the blade, the angle one is trying to achieve and the abilitly of the steel to achieve and support a super thin edge.
***********************************
Diamond Hones-- Hmm, I know lots of folks swear by them, and they do seem to work wonders on some otherwise hard to hone stain-resistant blends, but personally I doubt that I'll ever buy another one. Before I knew that the weakness of diamond hones was the adhesive that bonds the diamond dust to the metal surface, I went through about $100 worth of the things by pressing too hard on them to reprofile fat factory edges.

If you look at silicon carbide and aluminum oxide being 'N' times harder than any steel, does it really matter if diamond is 'X' times harder? Of course, the trouble I've had is finding cheaper hones that will duplicate diamonds universitality when it comes to quickly putting an edge on a variety of alloys. Anybody want to tackle that particular oddity???
wink.gif
Why is it that some alloys seem to sharpen best with specific types of hones? I wouldn't think it would matter, since we're just talking about simple abrasion, but it clearly does matter, at least from what I've seen.
*************************
Convex Edges are really easy to do if you sharpen leading with the spine and sort of roll them off the hone.
******************************

more later,
mps

 
I'm about to be overwhelmed. If I understand all this (or at least some of it) there is a host of operators, internal to the steel and external, which "could" contribute to the ease of sharpening - and I suppose edge holding qualities. The hardness of the sharpening medium shouldn't matter provided its harder than the steel. Courseness of grit could. The blade bevel and thickness clearly can and do. Inernal to the steel is another world. As I understand it when the liquid steel cools Fe, etal, form (precipitate out) carbides which are clusters of C atoms hugging up to metals atoms. These take time to form, so the cooling rate is controlled to make sure we get some. As to what causes big or small carbides, I have no idea. But it seems to me that if big ones form they would do so at the expense of the number of carbides present. Perhaps fewer big carbides, as opposed to lots of small ones, could have an effect on what the sharpening medium sees. Then there is all that other stuff in there. Someone told me Cr prevents corrosion by sitting in the mix like blocks, which prevents cracks from running through the steel, or something like that. If there are globs of Cr mingled in there what affect would they have on sharpening or dulling? Could they act as a lubricant and protect the hard carbides from abrassion? (I had no idea how softer metals behaved until I tried drilling lead with a 3/8 hand held. Don't bother!). But I don't even know how hard Cr is compared to carbides. Cr certainly has some affect on the way steels behave, independent from its anti-corrosian activities. If steels like stainless are hard to machine are they also hard to sharpen - or easy to dull? I think I need to read a book on how steel is made and what's going on in there - one written for remedial types - with small words, large print, and lots of pictures. Can anyone recommend one.

Jack
 
I'm about to be overwhelmed. If I understand all this (or at least some of it) there is a host of operators, internal to the steel and external, which "could" contribute to the ease of sharpening - and I suppose edge holding qualities. The hardness of the sharpening medium shouldn't matter provided its harder than the steel. Courseness of grit could. The blade bevel and thickness clearly can and do. Inernal to the steel is another world. As I understand it when the liquid steel cools Fe, etal, form (precipitate out) carbides which are clusters of C atoms hugging up to metals atoms. These take time to form, so the cooling rate is controlled to make sure we get some. As to what causes big or small carbides, I have no idea. But it seems to me that if big ones form they would do so at the expense of the number of carbides present. Perhaps fewer big carbides, as opposed to lots of small ones, could have an effect on what the sharpening medium sees. Then there is all that other stuff in there. Someone told me Cr prevents corrosion by sitting in the mix like blocks, which prevents cracks from running through the steel, or something like that. If there are globs of Cr mingled in there what affect would they have on sharpening or dulling? Could they act as a lubricant and protect the hard carbides from abrassion? (I had no idea how softer metals behaved until I tried drilling lead with a 3/8 hand held. Don't bother!). But I don't even know how hard Cr is compared to carbides. Cr certainly has some affect on the way steels behave, independent from its anti-corrosian activities. If steels like stainless are hard to machine are they also hard to sharpen - or easy to dull? I think I need to read a book on how steel is made and what's going on in there - one written for remedial types - with small words, large print, and lots of pictures. Can anyone recommend one.

Jack
 
when steel is heat treated it changes molecularly....complete marstenite transformation is when it is done right... so yes....you do need to read up...its not just the same stuff only harder.....it changes from one material into another....
 
Walt you comment:

Is there a direct relationship between carbide content and grain size?
Click to expand...

Walt. my wording was fairly poor in the post you are responding to. To be more precise, what I have found with the regular stainless alloys (ATS-34, 425, 440C) and some other high alloy steels, is that the edge does not wear in an even fashion. As the edge wears down it does not wear smoothly it sort of crumbles away and it looks very ragged. What I am guessing is happening is that the carbides are breaking away from the steel matrix that is getting weakened by the stresses and thus the carbides are providing fault lines of a sort and the steel is breaking along them.

This is a very simplistic explanation as it totally ignores one very critical factor and that is as you pointed out - grain size. It may be that the very high alloy CPM steels might be very durable because of their very small grain size. I don't have one yet but I will soon and I am interested in how it performs.

-Cliff


[This message has been edited by Cliff Stamp (edited 12 April 1999).]
 
Speaking of oversimplications, in order for me to fully visualize some of this stuff, I always think of steels in terms of dirt. With dirt you've got consistancies that range from clay, (low grain size, high cohesion, hard to work), to sand, (low grain size, zero cohesion, easy to work) to soft loam, (mixed grain size, variable cohesion, easy to work), to rocky mixes, (radically mixed grain size, variable cohesion, tough to work).

I think of the carbides in steel as being rocks. Now, unlike gardening, farming or building adobe huts, presence of these rocks (carbides) is good, provided that they are the right size, and are suitably bonded to what we're trying to build (a knife). Actually, a better annalogy might be making bricks or concrete. If the rocks, (carbides) are too big then they won't stay in the mix, and if they are too small, they also won't stay in the mix. Likewise if they are too varied in size, they won't stay put either.
(My appologies to all the knife and steel pros that are probably laughing their keisters off at that analogy, but it's what helped me sorta understand some of this stuff in the limited fashion that I may.)

OK, so here's my $50 question: I know that working different kinds of soil takes some very different kinds of equipment to produce good results. Does working or sharpening different kinds of knife alloys also take different tools? For instance, is it desirable or necessary to have a hone grit that somehow matches the carbide size in the alloy? Maybe it takes bigger hone grit to work bigger carbides loose? Maybe the performance that Cliff and I have noted with certain stainfree blends is indicative of the exact opposite. Certainly some alloys simply can't be sucessfully sharpened by devices that somehow pry out the bigger carbides. (Witness the edge crumbling that he and I have commented on.)

I think there's some alloys out there that simply can't support low thin angles due to their very makeup, but perhaps perforance of these could be enhanced by going to greater measures to match optimum angle and sharpening grit size based on carbide size.

What do ya'll think?

mps
 
To follow up on what Tom Mayo said, I think it's really important to understand that the steel actually changes state. That's where the dirt or concrete annalogies get somewhat left by the wayside.

When something like water changes state, there is a deadband of latent heat where temperature can be added without any change of state. For instance, 0C water can be either water or ice. I don't know enough about arrest points and austenite and marstenite conversions to even begin to speak coherently, but from what I've seen, the answers to most knife questions not having to do with geometry lie more in the proper heat treat than anything else. I think that (in totally broad parameters) the matching of the right heat treat to the alloy may well be more important than the alloy selected.

Here's my current theory. (subject to change at anytime) : The very moment that the removal of metal ends on the abrasive, it starts all over again at the moleculal level due to such external forces as corrosion and the friction of use. Literally, I think that the very first cut through thin paper begins the breakdown of the edge, and as anybody that's ever had simple carbon steel kitchen knives can tell, one doesn't even have to cut anything for the edge to start decaying due to corrosion. I think the same thing happens with all steel alloys, just at vastly varying rates. Some so small as to be insignificant.

So if the edge is basically constantly breaking off or decaying on the molecular level, then what is edge retention? Well, how 'bout this for a working theory. Edge retention is when the edge wears or decays in ways that are either irrelevent or even benneficial to the task at hand. ie, If the edge wears properly it may to some extent be somewhat self sharpening by constantly presenting fresh hard cutting surfaces. Some alloys clearly can't do that.

But perhaps more important than the specific alloy is the level to which it was converted to the *exact* state that best represents the zenith of that particular alloy's benneficial edge decay.

I know I'm not the only one here that has experienced both terrible and quite decent blades made out of that bad old whipping boy, 440A. Likewise with ATS34. The heat treat of it seems most critical and when done badly it's no better than anything else.

Maybe one of my favorite examples of bennefical edge decay Sandvic 12C27. Looking on a bladesteel chart, I wounldn't even think it to be knifesteel. (Same for the weird mix that Victorinox uses.) None the less, somehow, the makers have figured out how to get very surprising performance out of alloys that are ridiculously low in carbon. How? Could it be that each specific alloy mix has not a specific Rc number that best fits it but a much more specific *exact* level of change of state that best represents what it's truly capable of?

mps
 
In referencing MPS's post, I have seen very different edges formed by different stones. I would like to see someone with actual knife-smithing experience discuss this subject making reference to the steel matrix, the presense of carbides and how to best chose the type of sharpening material and optimal grit size for each.

-Cliff
 
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