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I’ve been doing a bunch of experiments with burr removal and I’m puzzled.
 
Does anyone know why some burrs are so !@#$!$’ing malleable?
 
I’ve had burrs that I can bend over and over and over and over… again at 45 degrees and they are extremely resistant to fracturing.  They almost refuse to fracture and break off.  Additionally, even thought I didn’t do it, I’m sure that if I did the same thing with the blade itself it would break after only a few bends.
 
The thing that is really a head scratcher is that burrs are really very thin, but they seem to have the malleability of gold and are far more malleable than the blade itself.
 
I was reading on the Science of Sharp website where the author was talking about stropping 300 – 500 strops on the extremely thin metal on the edge of a razor blade.  300 – 500 strops on edges only a couple of microns thick!  Why is that?
 
I’ve had a lot of experience with stress fracturing metal.  It always seems to happen when I don’t want it to.  Bolts break in half, some piece of metal I’ve bent cracks or fails under normal use with what would seem to be not that much stress.  Yet with a burr, when I want it to fracture and break, that extremely thin little sliver of steel is amazingly fracture resistant.
 
I’m guessing that something happens to the metallurgy of the steel when the burr is formed.  Perhaps Mr. Jan or Mr. Mark can chime in and help explain this odd metal behavior.

Malleability is material’s ability to form a thin sheet by hammering – deformation under compression. Ductility is material’s ability to be stretched into a wire – deform under tensile stress. Increasing levels of carbon decrease steel ductility.

Both properties are aspects of plasticity which occurs in metals due to metallic bonds. Outer electrons are delocalised, shared between atoms, what allows that metal atoms can slide past one another.

Mechanical properties of metals depends on dimensions when the piece is smaller than some 1 mm. It is called size effect. The grains located at metal surface behave differently than the internal grains. When the thickness of the piece of metal is reduced to several tenth of μm, than the role of surface grains increases what leads to different properties compared with bulk material.

This is my understanding of burr behaviour in some cases. The topic is complicated and so no guarantee is given for the correctness of this post.

"Mechanical properties of metals depends on dimensions when the piece is smaller than some 1 mm. It is called size effect. The grains located at metal surface behave differently than the internal grains."

That sure makes sense considering what I see happening.  What was really puzzling me was that the burrs could be bent back and forth 30 times and still not fracture, which seems amazing for such a thin little sliver of steel.  I'm just about 100% sure that if I did that with the blade itself it would simply snap in half after only a few bends.

I really don't understand burr formation.  It almost appears that metal has been "smeared" off the edge of the blade forming a thin foil.  That foil is, many times, very resistant to stress fracturing. It sure seems that the role of the surface grains is what I'm seeing.

Thanks Mr. Jan.  That explanation makes sense, and helps to explain why such a thin sliver of steel can be so tough to remove.

Yes, Mr. Grepper. Strength and ductility of steels are controlled by dislocations. Dislocations are places where atoms are out of the position in the regular crystal lattice.

When a stress is applied dislocations are generated, move through the grains and pile up at boundaries of internal grains. This causes that the surface grains have lower resistance against deformation.

I have noticed the same thing, and it gets frustrating, especially when it is on one small section of the blade!
I've considered taking a mapp gas torch to a blade when this bending back and forth over and over occurs without the burr actually falling off.
Just a quick pass of the torch to see if it ignites the burr. Of course this would temper the edge left behind where it is the thinnest, so, probably doesn't help the edge much... just a curiosity.

Mr. Jan, do you have any references for the size effect you mention above?  I have not heard it discussed in terms of dislocation pile up on the interior grain boundaries.  In relation to the burr, I'm not sure it applies if the burr is only several 0.1ths of a micron, but I have not seen anyone measure a large burr for thickness.  It's typical for the grains to be bigger than that, and typical grain size in inexpensive kitchen knives is likely a dozen or more microns.  

In terms of why do the burrs bend back and forth, that's what thin materials do.  Think of fibers in fiber glass.  They have 0% ductility, being glass, but can flex back and forth without damage, though for them it has to be a relatively large radius.  The thinner the burr, the more it can flex back and forth.  How many times can you fold a piece of paper back and forth before it tears on it's own, not with you pulling it apart?  Now apply that to something that has a tensile strength at least an order of magnitude greater, maybe 20 to 50 times greater.  It's really a wonder we ever get rid of burrs in the first place.

Some types of steel and heat treatment combinations will have burrs that are very rigid and break off without much effort.  Others will have burrs that hang on like grim death.  On steels I know are in the latter category, I either cut them off and start over, or try not to form them in the first place.  Usually, I don't know until it's too late, so I end up doing both.

Jan

This thread just struck us right in the mid-section and here's why; We were very recently exploring some potential new burr removal techniques and came across a very peculiar situation. We had been looking at the relationship between grind techniques/materials, burr formation and burr removal. This research fits well with our general sharpening philosophy here in that we feel "it's all about the burr".  

In this instance we were using our Kally 1SM with a 180 grit ceramic belt with a custom angle guide attached. We were sharpening a brand name stainless steel knife of some reasonable quality. This is an "edge trailing" methodology. We produced several easily detectable and substantial burrs on this knife and then conducted our experiments. After a while we changed things up and decided to begin with a really dull edge. We ran a ceramic rod, 90 degrees to the edge, straight down the edge. This was interesting in itself because our optical microscope indicated that the ceramic rod had created a burr (or something that certainly resembled a burr) on both sides of the edge. Then we returned to our grinding and here is where the mystery began. No longer could we raise a substantial burr. At first, we thought we weren't raising a burr but on closer inspection discovered exactly the kind of burr that you gentlemen are discussing. Flimsy beyond belief and removed only with great difficulty. Additionally, and once the burr was removed, our edge tester readings were 50-60 points higher than we had been achieving previously. This all remained the same through two additional grinding efforts. It was if we had, literally and in some way, managed to change the very character of the steel.

We don't have an answer, perhaps a few theories, but mainly, just this perplexing story. You may say that this is much like the patient who told his doctor that he had broken his arm in seven places. The doctors advice to the patient was to "stay out of those places". In keeping with the doctor's advice we may not run ceramic rods down the edge of knives any longer but it does make us wonder how this might relate to the topic that is being discussed here and how all this might tie to general sharpening/burr removal knowledge and technique.

That is quite interesting. I have noticed the 2 sided burr in the past. I have also used ceramic rods to remove an old edge and start fresh. I have some knives to sharpen that are in that exact condition now.

Mike, thanks for sharing with us this mysterious story. It is a very good food for thoughts.

The often cited statement by Wolfgang Pauli (Nobel Prize in Physics 1945) says "God made the bulk; the surface was invented by the devil".

Jan