work hardening with the leather honing wheel

[Ken S]

Tormek consistently states that an edge processed through the leather honing wheel will last longer than an edge straight from the grindstone. (This debate seems centered with turners.) I watched a video with another honing product which stated that removing the burr with alternating strokes not only removed the burr, but also left the edge work hardened, thus increasing the longevity of the sharp edge.

I do not know enough metallurgy to understand this, however, an edge which was both and harder would seem to last longer. Thoughts?

Ken

[Herman Trivilino]

I watched a video with another honing product which stated that removing the burr with alternating strokes not only removed the burr, but also left the edge work hardened, thus increasing the longevity of the sharp edge.

I'm not sure what "work hardened" means. It could just mean that the edge has been properly prepared in that the burr has been removed. In other words, a burr will create sometimes what could be seen as a sharp edge. Think how well it might cut a tomato! But as soon as the burr breaks off the edge becomes dull. Then perhaps a steel could be applied to freshen it up. All of this can be circumvented by leather honing, so that may be all there is to it. Perhaps barbers were seen as hardening their edges to give a good shave by applying the strop. A hard edge being one less susceptible to breaking off.

[Jan]

Work hardening which is also known as strain hardening or cold working is a metallurgical process of metal strengthening by means of its plastic deformation. Plastic deformation occurs when energy is quickly added to the metal. The work hardening of a metal increases its hardness and yield strength. The reason for metal strengthening is movement and generation of many new dislocations within the metal atomic lattice.   

Here is a classical text book example: "Strain hardening can be easily demonstrated with piece of wire or a paper clip. Bend a straight section back and forth several times. Notice that it is more difficult to bend the metal at the same place. In the strain hardened area dislocations have formed and become tangled, increasing the strength of the material. Continued bending will eventually cause the wire to break at the bend due to fatigue cracking."

https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Structure/strengthening.htm

Jan

[grepper]

Here is a great explanation of work hardening:
https://en.wikipedia.org/wiki/Work_hardening

Here's my take on the sharpening / stropping / steeling thing:

I agree Herman.  A burr can be very sharp, but is very wimpy.  A good indication of incomplete burr removal after sharpening is if the blade almost instantly becomes dull from normal use.  A burr is not the sharpened edge of a blade, but rather small metal shards and thin, possibly even rolled metal waste remaining from the sharpening process.

If the burr is not removed, two things happen; some of it breaks off into what you are cutting (broccoli and steel shards anyone?) and the rest folds down over the edge or off to one side.  This remaining burr makes the blade appear to be dull, when in fact it is not.  Once the burr is removed the true edge will be exposed and the blade will be as sharp as it can be.  You can actually see this with a microscope and verify the observation sharpness tester.

A burr is different than the true edge of a blade bending or deforming from use.  This is what happens when a blade becomes dull.  When that blade is stropped or "steeled", the deformed edge is straightened and almost magically becomes sharp again.  Contrary to popular belief, unless a blade is cutting an abrasive that is harder than the steel of the blade and therefore capable of removing steel, dullness is not caused by the edge wearing away, but simply that the thin edge of a very sharp blade folds over, gets deformed or just bashed in one way or another.  You cannot, for example, actually sharpen a blade using broccoli or a wood cutting board because sharpening requires steel to be removed to form the edge.  This is why judicious use of a knife "steel" keeps a blade sharp.  The deformed edge is straightened, and the blade sharpness is maintained.

Eventually this bending/straightening of the edge causes the very thin, sharp part of the edge to break off due to metal fatigue.  Once this happens the blade is actually dull, and no amount of stropping or steeling will help because the thin, sharp edge no longer exists.  At this point, it's Tormek time!

So, a really good question Herman brings up the relationship between steel hardness and edge retention.  Hard steel will resist deforming, but is more brittle and will chip off more readily from stropping or steeling or normal use.   Like everything there is always some trade off.  Where is the sweet spot between hardness and the ability to maintain an edge between sharpening?  I wish I knew.  I'm still trying to get a handle on that.

I understand basic work hardening, but I don't understand what it means to previously hardened steel.  I'm still trying to figure that out.

My $0.02

Some burrs:

[Ken S]

I get the impression that edge sharpness may be like a parabola. We start with a burr. As we remove the burr, the edge gets smoother, sharper and perhaps harder. If we go beyond the ideal point, we introduce metal fatigue. Does this make sense?

Ken

[grepper]

You can think of the sharpened edge and the burr as two, totally separate things.  Almost like a bunch of glue stuck to the edge.  Removing the burr does not make the edge sharper, it just exposes the edge.

Once the burr is removed, what you are left with is the actual, sharpened edge of the blade.  Obviously, a sharp edge is very thin and prone to damage as it is used.  As it is damaged and deformed the knife dulls.  Generally the damage is that the edge bends and folds to one side or the other, or just gets kind of smushed (technical term) or bashed (another technical term) or smashed or crushed or stove in one way or another.

So when you then strop or steel the blade, the deformed edge is straightened.

You are correct, Ken.  When the edge is straightened by stropping or steeling, the bending back of the metal fatigues it.  Do that enough times and it's going to fracture and break off, just like bending any piece of metal back and forth until it breaks in half.

That said, with most blades, you can do this a number of times before the metal fails and keep a blade sharp and performing well for a long time before it starts to crap out on you and actually needs sharpening again.

Now consider this:  If you take a paper clip and bend it back and forth only one degree side to side, it will take many more bends before it breaks than if it's bent 90 degrees each time.  It seems to me that a blade that is stropped before every use could be maintained much longer than if it is allowed to get really deformed because the edge does not have to be bent as far to get it straight again.

So, stropping or steeling an edge will induce fatigue and cause the edge to fail.  But... it will keep an edge sharper and useful much longer than if it is not maintained. 

This is actually a good thing because it means that all blades will need sharpening eventually.  If all blades remained sharp forever, life would be dull because we would have nothing to sharpen! 

Does that make sense?

[Jan]

Grapper, your picture of the toothy burr is an iconic photo for me. (reply #3)  What is the scale?

Now consider this:  If you take a paper clip and bend it back and forth only one degree side to side, it will take many more bends before it breaks than if it's bent 90 degrees each time.  It seems to me that a blade that is stropped before every use could be maintained much longer than if it is allowed to get really deformed because the edge does not have to be bent as far to get it straight again.

What concerns the classical paper clip experiment to demonstrate work hardening there is another important factor influencing the result achieved. The "work" should be done fast enough to produce many new dislocations which harden the material. In rheology this time factor is described by the so called Deborah number. 

The usage of the Deborah's name was inspired by the Bible verse "The mountains flowed before the Lord" (Judges 5:5). The basic idea is that everything flows, even the mountains, if you wait long enough! And vice versa. Water is a fluid, but if you jump into a swimming pool you can hurt yourself like you crash with a solid. Some fluids behave closer to solids under some circumstances.

I am not sure that it may be applied to metals. The Deborah number takes into account the time necessary for a material to adjust to external deformation. At higher Deborah numbers the material behaves in a more solid like (elastic/plastic) manner while at lower Deborah numbers it shows fluid like (viscous) behaviour.

In work hardening the "work" should be done fast enough without giving relaxation time to the newly produced dislocations and internal stresses to dissipate in the material. 

Jan

[Ken S]

Based on our forum discussion, I would give work hardening a "not proven" verdict. I do accept on faith the statements of Tormek and other manufacturers that procedures beyond initial grinding can increase the longevity of the sharp edge. Like Jan, I believe there is more to learn about burrs, especially in my case.

I look forward to continuing this learning quest.

Ken

[grepper]

Jan inquired, " the toothy burr is an iconic photo for me. (reply #3)  What is the scale?"

Jan, you are a pest!    You always ask me that!    I didn't calibrate and throw a scale under it so I can't say exactly.  Probably ~200X. 

That was a unique burr.  It was extremely thin.  Just lightly breathing on it caused it to wave in the breeze and sparkle under the light. I wish I remember the conditions that created the thing, but that was some time ago and memory fails me at this point.  Kind of cool looking though!

Ken, "procedures beyond initial grinding can increase the longevity of the sharp edge."  That's a loaded question!  Obviously after sharpening the burr must be removed.  It's easy to imagine by looking at the pictures of burrs what would happen if you didn't remove them.  All of that crud (technical term) would get smashed down over and around the edge.  Some of it gets torn off, but some does not.  Not a good thing. 

Strain hardening does work for carbon steel.  To the extent that happens as a result of stropping off a burr, or how much that changes edge retention, I have to admit ignorance.  I mean, burr removal by stropping does not flex the burr very quickly, or generate much, if any heat. 

This subject has made me understand that I don't know what metal fatigue really is either.  You know, flex metal enough and it gets brittle and breaks or cracks.  I've seen that.  I can demonstrate it and create it.  Can metal fatigue happen without strain hardening, or is the fatigue the result of it?  Or, is fatigue unrelated?

I really think that the more you know the stupider you get!  At this rate, in a few years I will be reduced to a babbling imbecile.

This is a true story:  On another forum, one member is a old sage that lives in the deep forest.  He has moose and bear in his back yard.  He has been sharpening for about as long as as I am old. And, I'm pretty old.  In a post he spoke an utterance of great wisdom.  I don't remember the exact phrase, but, basically he said, "often people's blades are sharper than their minds".  Now..., how cool is that! 

It would be great if we had a metallurgist on the forum!

[Ken S]

Grepper,

The usual punishment for not calibrating is banishment to the UP.

(I should explain my joke: the "UP" is the upper peninsula region of Michigan. The U P is cold in the winter. Actually, in the summer it is a favorite retreat for the wealthy. I live in Ohio; Grepper lives in Michigan, two states which have enjoyed a longtime very friendly rivalry.)

All good natured fun aside, we should all keep our equipment calibrated.   

Ken

ps I should point that Grepper recently told me that clicking or tapping Tormek Community at the top left of the screen automatically returns us to the home page. Clever. I may end up computer literate someday.

[Jan]

Grepper, the reason why I have asked you about the scale/scale bar of your iconic burr photo so insistently was that I planned to use it in my presentation about sharpening for school children and young students.   

Based on my experience children understand microscopic images well when I insert a circle which represents human hair cross section (diameter circa 70 μm). Especially girls like to see their hair magnified. 

Jan

[grepper]

Jan, You mean you are not just trying to pester me? 

I'm guessing that it's about 200X because that is the max optical magnification my scope will do and that's always what I use.  It also looks like other blade images I've done where I did calibrate.

The burr in that image was very long.  Easily visible to the naked eye.  It was some time ago, but "if" memory serves, some if it could have been .5mm off the edge of the blade.  It was indeed a unique burr.

How does steel do something like that?  It almost seems like it was sort of smeared off the surface of the bevel.  I remember being somewhat amazed when I saw it.

[Jan]

OK, Grapper, thanks for the additional info. I promise it's the end of your tribulation. 

The "Deburring and Edge Finishing Handbook" by LaRoux K. Gillespie mentions two major factors influencing the size of the burr:

1)   the ductility of the material,
2)   the work hardening coefficient of the material.

Ad 1) We cannot expect large burrs in brittle material which has small capacity for plastic deformation but we can expect large burrs in materials with high ductility.

Ad 2) In materials with large work hardening tendencies we can expect formation of large burrs.

In another book, the above mentioned author, defines burr as a plastically deformed material produced at the edge. 

Jan

[Herman Trivilino]

Eventually this bending/straightening of the edge causes the very thin, sharp part of the edge to break off due to metal fatigue.  Once this happens the blade is actually dull, and no amount of stropping or steeling will help because the thin, sharp edge no longer exists.  At this point, it's Tormek time!

And I would venture a guess that the vast majority of kitchen knives live out the rest of their lives in this poor condition. Most people just don't have the tools or the skills to sharpen, and are not inclined to have it done professionally.

Where is the sweet spot between hardness and the ability to maintain an edge between sharpening?  I wish I knew.  I'm still trying to get a handle on that.

It will depend on what the knife is used for, and how it's used. One type of steel may be better for your fishing knife, another for your pocket knife, another for your paring knife, and yet another for your chef's knife. And since we all use our knives in different ways, and our family members also use them in ways we don't know about when we're not looking, there's no universal determination.

[Ken S]

I agree, Herman, about "universal determination". Sadly, I must also agree with his assessment of the poor condition of most kitchen knives.

I do believe good practical suggestions, like those posted by Stig, can speed up the learning curve. If something works for a knife person like Stig, it will probably work for me.

Ken