Edge carbides

I am writing this as a mental rant to generate discussion. I am not sure what I want out of this except maybe a collaborated wiki post which is the closest thing we could get to a peer reviewed journal paper.

Below is an image I found on bladeforums. It is endorsed by Lagrangian (ie. Anthony Yan).  Pretty fascinating although it is a single piece of evidence and I don’t even know what kind of steel it is, etc. A couple of amateur initial observations:

1. The matrix of martensite and retained austenite is clearly visible
2. Primary carbides on the order of 1-5 microns clearly visible
3. Secondary carbides in the nanometer range are not visible
4. The edge topography “seems” to be a function of the primary carbides.
5. The microchips seem to have exposed primary carbides left behind embedded in the martensite
6. Additional independent micrographs are needed to validate what we “think” we are looking at

After a quick reminder from my research chemist daughter about metals and the different types of bonding within the “electron clouds” of steel as well as some research into Electron Microscopes (SEM) (Cornell has great equipment and information) I think the following:

There are quite a few different bonding interfaces within steel, namely, interstitial carbon within the iron atom, intergranular (the bonds which hold an individual grain together), intragranular (bonds between grains), bonds within a carbide, bonds between carbides and grains, bonds between carbides, etc. I understand that the mechanism for what will most likely bond with something else is a function of the electron energy level (electron volts – eV) as well as other things like heat treat temperature, quench and the rates associated with them. While electrons are all the same, their energy levels are not. I’m pretty sure that the SEM takes advantage of this and uses this phenomena to generate images (oversimplification). It would seem to me that the SEM data could be used to quantify various bond strengths.  Cornell now has a new type of SEM called the Electron Microscope Pixel Array Detector (EMPAD). Per their press release:

The electron microscope, a powerful tool for science, just became even more powerful, with an improvement developed by Cornell physicists. Their electron microscope pixel array detector (EMPAD) yields not just an image, but a wealth of information about the electrons that create the image and, from that, more about the structure of the sample. “We can extract local strains, tilts, rotations, polarity and even electric and magnetic fields,” explained David Muller, professor of applied and engineering physics, who developed the new device with Sol Gruner, professor of physics, and members of their research groups.

It also seems to me that as we abrade and burnish the edge with the Wicked Edge System down to 0.1 diamond films that we “must” be defeating every one of these different bonds to remove metal. Being that the pressure (force/area) we exert on the very edge defeats these bonds, it would seem to me that we are introducing residual stresses into the matrix which cannot be good (albeit unavoidable). I wonder if the sharpening process itself sets the edge up for failure.

It amazes me that after quite a bit of research into the science via the peer reviewed journals on metals that the physics about what is going on in not fully documented. I pretty much take the various forums with a grain of salt because it usually gets into bar-style talk about the evidence of failure modes.

I clearly understand that all of these edges are sufficient for what we use the edge for (kitchen for me), however, the failure mode of chipping is what interests me. There are millions of kitchen knives out there and it seems like every one I get to look at with my USB scope at 250x have chips. We all think we know why but do we really know why. What can be done to improve it ?

If I had a chance to do my engineering career again, I would consider the Metallurgy department at Cornell – LOL

Etching-guide

My questions arose when reading the document “Metallurgy of Steel for Bladesmiths – by John D. Verhoeven”. It is a complimentary work to the “Experiments in Knife Sharpening – by John D. Verhoeven” which is on the WEPS download page. It is 200 pages, fascinating reading, and gets into the why’s and wherefores of carbides and everything else. I put it on my Google drive (link below) with full sharing (hope it works).

I am personally looking for a new 8″ chef knife with a wide body (2.375″) and custom fit handle. Right now I am thinking of 440C or CPM154 steel with full cryo (Jay Fisher has me believing) and my budget for this custom knife is around $500. I am going to do this once and I want to do it right. It comes down to the proven reputation of the knife maker relative to the quality of the steel he buys (not all 440C is the same) AND his specific heat treating details. This is what drives the grain sizes and carbides (some are good, some not so good). I have inquired on Bladeforums and just get a lot of trust me responses. Since I trust you guys I wanted to solicit your advice. What do you think ?

https://drive.google.com/drive/folders/0B5t7FJ9Zmj1hZ2hiZUx1Y3JKMTA?usp=sharing

1 user thanked author for this post.

Marc H

As an example of the excellent explanations in Verhoeven’s Metallurgy paper  —  here is his description on where the primary carbides end up in the steel matrix. My thought is that if the energy levels (bonding) are lower than those within the steel matrix (and also lower than within the carbide) then the carbide will be more likely to “rip” out whole vs. being sharpened/smoothed itself. I concur that we all pretty much know this but my interest is in knowing exactly why.

1 user thanked author for this post.

Mark76