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Jerry Knox




Location: Palm Bay, Florida, USA
Joined: 12 Jun 2007

Posts: 53

PostPosted: Tue 23 Sep, 2008 1:01 pm    Post subject: Carbide-Edged knife         Reply with quote

Hello everyone. I just successfully completed the first stage of a project I have been thinking about since I was about 12 years old and I am so excited I have to tell someone!

The inspiration came from the maquahuitl, the "sword-club" of the Aztecs, which was a sword-shaped ironwood club edged with cemented-in obsidian blades. I learned about them from the historical fiction novel Aztec by Gary Jennings (which was awfully adult-themed for my age, but I read it anyway) and knew that i wanted to make one for myself, but one that would not be so brittle as to be useless against steel.

I remember asking my father, after reading up on materials, why no one used tungsten carbide for knife edges, since it was extremely hard, but still ductile enough to be used in machine tools. His answer was "I don't know." Since then I have asked that question to various of my professors and colleagues in the engineering field, and always gotten similar answers. So i decided to find out for myself why it was a bad idea. Big Grin

Understanding material properties better now, I know that in order to have a slicing-sharp edge, the carbide would have to be too thin for use in a sword (it IS much more brittle than steel), so my first project was a small knife.

For the body of the blade, I used aircraft aluminum, because it was what i had, and for the edge, I used a micrograin solid-carbide planer blade. I wanted to use steel so I could braze the edge in, but aluminum was what i had, so i had to go with glue, which was more Aztec anyway. Since I set the carbide into a deep, tight groove in the aluminum, the glue was really only forming a fillet at the juncture of the two materials anyway, so I wasn't too worried.

The result was better than I expected. Aside from suffering scratches on the soft aluminum body of the blade, the knife has suffered no damage while cutting into, among other things, a carbon steel razor blade, a glass bottle, and a red brick. It is also excellent for trimming my nails! I will not be striking the knife, or getting it hot, because I am sure that it is too brittle and the glue will not take too much heat, but for everyday use, it is pretty cool to have a knife that will literally cut anything except diamond.

I'll post pictures later today. Unfortunately it is very modern looking, because I got excited when I realized the blade was working and just slapped on a handle I had lying around, but I hope that my next one can be a cool-looking friction folder with a deer antler handle. A friction folder that doesn't rust, or dull, and that will cut stone Big Grin

Cheers!


Last edited by Jerry Knox on Wed 24 Sep, 2008 6:39 pm; edited 1 time in total
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R D Moore




Location: Portland Oregon
Joined: 09 Jun 2007
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PostPosted: Tue 23 Sep, 2008 3:49 pm    Post subject:         Reply with quote

Jerry Knox wrote: "...but I hope that my next one can be a cool-looking friction folder with a deer antler handle. A friction folder that doesn't rust, or dull, and that will cut stone "

Be sure and patent that! Big Grin Big Grin
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Thomas Jason




Location: New Joisey
Joined: 28 Jul 2004

Posts: 230

PostPosted: Tue 23 Sep, 2008 3:54 pm    Post subject:         Reply with quote

I think Warren Thomas does something similar with his titanium knives. Only I don't think his Tungsten Carbide is a solid piece, but more of a coating.
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Jerry Knox




Location: Palm Bay, Florida, USA
Joined: 12 Jun 2007

Posts: 53

PostPosted: Wed 24 Sep, 2008 6:38 pm    Post subject:         Reply with quote

Well, the camera we have (or maybe the operator) apparently can't resolve features smaller than a barn, so the pictures are pretty bad. Instead, here is a drawing of the knife and a detail of the blade. Light Gray is aluminum, red/orange is epoxy, and black is Tungsten Carbide.

As a side note, I shaped and sharpened the carbide with hand grinding on a diamond bench stone. It was much easier than I thought it would be. The carbide ground very similarly to flint, but smoother, if possible.



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Dan Howard




Location: Maitland, NSW, Australia
Joined: 08 Dec 2004

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Posts: 3,197

PostPosted: Thu 25 Sep, 2008 3:12 pm    Post subject:         Reply with quote

Wootz workers have been doing this for centuries. Its mythical cutting ability is the carbide. The pearlite wears more quickly than the carbide leaving microserrations that are wonderful for cutting flesh.
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Jerry Knox




Location: Palm Bay, Florida, USA
Joined: 12 Jun 2007

Posts: 53

PostPosted: Fri 26 Sep, 2008 6:04 am    Post subject:         Reply with quote

I thought about trying to patent it, but there have been brazed-on edges on planer blades for years, so it's probably already someone else's idea.

All hardened steel, in fact, is like you describe Dan (although wootz is indeed among the highest-carbide content steels). It is a matrix of iron and carbon of various structures surrounding cementite(Fe3C), which is a hard ceramic, just like tungsten carbide, except slightly softer in absolute terms. The main difference in "solid" tungsten carbide (WC) is the incredibly high percentage of carbide (usually well over 50%) compared to even the highest-carbon steel, which is never more than ~2%. The cobalt used as a binder for the WC blade is very ductile, holding the grains together in a metallic net of sorts, and even giving some ductility to an otherwise brittle crystal structure.

Theoretically, one could do the same with cementite, but it is much less stable than WC, and would tend to lose carbon atoms during the the high-temp process of being formed. The benefit of steel is that the iron component dominates the material's properties, allowing extreme springiness in certain structures. The tradeoff is much lower hardness, of course.

I hope to make my next knife using mild steel or bronze for the blade, and to braze the edge in using silver solder. My hope is for a very primitive look, while still retaining the space-age performance. If I use steel, I'll probably finish the blade with some sort of stabilized corrosion process, maybe browning, to highlight the shiny black finish of the carbide edge.

I hope to post again soon on my progress!

P.S. the total cost of the first knife was about $20. more than half of that the carbide. these things are really cheap to make, and for me, without any heat-treating experience, they are the easiest way to get a good hard cutting edge without having to send the blade out to an independent heat-treater.
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M. Eversberg II




Location: California, Maryland, USA
Joined: 07 Sep 2006
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PostPosted: Sat 27 Sep, 2008 1:51 am    Post subject:         Reply with quote

When you do your more "primitive" looking knife, post up a few pictures. I'm sure you'll likely have a few people asking to have some made, myself included (assuming I have a job by then Razz)

M.

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Kjell Magnusson




Location: Sweden
Joined: 10 Jun 2004

Posts: 123

PostPosted: Mon 29 Sep, 2008 12:34 pm    Post subject:         Reply with quote

Jerry Knox wrote:
All hardened steel, in fact, is like you describe Dan (although wootz is indeed among the highest-carbide content steels). It is a matrix of iron and carbon of various structures surrounding cementite(Fe3C)


Not quite. The classical heating-quenching hardening of steel transforms the microstructure from a mixture of ferrite and cementite to martensite. Some ferrite or cementite may remain afterwards, and if the quenching was too weak then some ferrite and cementite may form again afterwards, but on the whole the process should result in a considerable reduction in the total amount of cementite in the material (quite possibly reducing it to zero). So we can have hardened steel with no cementite in it.
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Jerry Knox




Location: Palm Bay, Florida, USA
Joined: 12 Jun 2007

Posts: 53

PostPosted: Tue 30 Sep, 2008 7:29 am    Post subject:         Reply with quote

Thanks for the correction Kjell. I was thinking of tool steels and high alloy steels, which typically have extra carbide- formers for increased wear resistance, and forgot about "plain carbon steel", which is actually a far more common material!

I made another blade using a piece of stainless sheet-steel instead of aluminum, so was able to make it much thinner than the prototype. It cuts much better owing to its improved geometry. And, with the brazed-in carbide, it is also much stronger and nicer to look at.

I think I have settled on my "primitive" setup. I'm going to use low-carbon steel for the blade, and braze the edge in with silver-solder for a decorative hamon-like look. I am thinking about cold-forging the blade to shape too, so it looks extra-authentic. I have had good results with hammering mild steel to shape while making sword fittings, so I think I should be able to get almost any shape i want, and have much more control over the geometry than if I ground it out.

I'll try to get decent pictures of the new blade, and when I've got the final friction folder ready, I'll post that too.

Thanks for the comments!
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Kjell Magnusson




Location: Sweden
Joined: 10 Jun 2004

Posts: 123

PostPosted: Tue 30 Sep, 2008 5:52 pm    Post subject:         Reply with quote

I must say I'm really looking forward to those pictures.
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