Cracked Dhankuta test-to-failure thread

Daniel Koster

Daniel Koster

www.kosterknives.com
Knifemaker / Craftsman / Service Provider
Joined
Oct 18, 2001
Messages
20,978
"Snoopy" sent me his cracked dhankuta kothimoda (which Bill replaced a la H.I.'s warranty). Little did he know what I had in mind for it.... :eek:


Good lookin' little knife with a beautiful handle. Shame I had to break it.....



But I've always wanted to "peek inside" a khukuri to examine its grain structure. Unfortunately I don't have anything to compare it to at the moment....but that will come soon I hope.


Anyway, in the meantime...enjoy some pics and ponder what you see.




Here is a pic of the knife after I broke it. I don't have a "before" pic (I think David does - do a search if you're curious). The red arrow indicates where where the crack began - and ran to the spine. The crack that I put in it goes from the arrow to the edge.


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Here's how it went down:

I chopped completely through a 2x4 - no visible damage. Take that all ye small-khuk-can't-be-choppers nay-sayers.....! :p

Went through the 2x4 again.

One more time just for kicks.

No damage....no movement....nothing.

I figured, I was going to have to submit it to some "Cliff Stamp" level tests....so I picked up a big piece of cherry and used it to baton the knife halfway into a 2x4. Nothing. 3 more times.....nothing.

Ok, losing patience....


So, for the heck-of-it...I put the tip into my vice and tried to pull/bend it. Wouldn't budge...started eating up my vice. I got the piece of cherry and beat on it. Then I got the hammer out (for sure this would work, I figured) and hammered it with a thin piece of cherry to keep from marring the face.

Best I got was a nice vibration in the handle....:(

Lil' brat....


I took it out of the vice, laid it at a 45 degree angle on the floor and whacked it bare with the hammer.

Ting!

Finally. Of course, this last test has no practical application to real world chopping....but I had run out of ideas.....:(



Anyway, once I had it cracked, I could now take some zoomed in pics of the blade structure.



Here's a pic of the knife end of the break:


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You can see how the "previously cracked" part is darker than the rest. My theory on this is that the crack came in the forging. Somehow there was a flaw in the steel that caused the crack or delamination, etc. Then under quench and heat-treat, the crack expanded and filled with scale - which is the dark stuff you see.




Pic of the tip end of the same break:

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Here's a closeup of the transition (on the broken tip end). Also shows the grain size. Looks even - no "striping" or "banding" - which would indicate an uneven temper.


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Another pic showing the tip - again on the tip end. Clean, even grain structure.




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Neat stuff, eh?


Well, not being smart enough to just leave well-enough alone.....:footinmou.......I decided to to ahead and try to break it again.


I went through all the same stuff as the first time. Same results...EXCEPT it wouldn't break on the last one (holding it 45 against the concrete floor and hitting it with a hammer).

So, I took it to my bandsaw and slowly (:() cut a slot in the spine about 1/4" deep and about 1 inch away from the previous break.

Went through all the tests....from 2x4 (harder to cut all the way through now, but still....it's the idea that counts, right? :D)

Anyway, nothing would make it budge.

So I laid it on the floor and broke it as the first time. Much to be learned here.


Here's a pic of the break:



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(you can see the cut marks from my bandsaw on the left)





And here's a life-size picture of me holding the broken part - to give you an idea of just how much I was able to zoom in (helps to have a good camera).


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It might just be the lighting...but it seemed to me that the grain was slightly larger in the center of the blade and more refined more toward the edge.

This would be consistent with what I've said in other threads - that the hardening is happening at a certain depth within the steel. Plenty deep for significant reshaping/resharpening. But the softer center and spine go a long way toward preventing breaks....makes for a more pliable blade (vs. a fully hardened blade).

Like I said....there's more than meets the eye here....I have more questions than answers now....:(



Eventually, I'd like to send the steel off for testing....

I also want to etch the blade to see what temper line might show. I'll post pics when I get that done.


Lastly, I plan on re-forging the blade to a new shape, tempering and then breaking it again to see what the resultant grain structure is.

Fascinating stuff....
 
Interesting stuff

Is the knife a 12" knife and would it have broken under normal use?
 
Yes - it is the 12" Dhankuta Kothimoda. I'll post a pic of the "remainder" to show the handle.


I spent a few hours testing this knife and I think I would describe 70-80% of what I did as "normal use". Just the banging in the vise and on the floor (as last ditch efforts) fall outside that.
 
Interesting Dan. I'm curious about something. It appears to me from the pics that the blade looks to be hard all the way through? Hard to tell from a pic though.:rolleyes:
I wonder what would happen if you took the edge of a sharp file and checked the hardness about every 1/8" or so all the way from edge to spine?
If it's hard all the way up then not only was there a forging flaw but in all probability a slip of the kami's kettle when he was pouring the water to harden the edge.
I would have expected the grain pattern at the edge to be a bit finer and shiner than the rest and it doesn't appear to be so. It all looks the same to me, am I not seeing something that's there?

Also if it's hard all the way up it may have been that the kami had the steel too hot and a cooling crack could have formed at the top when quenching, much like you said, only it could've been sound before the water hit it?
 
Ok, as promised here are more pics. Just snapped 'em....





Here you can see a picture showing both breaks.


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Now, a word about the black lines....I was considering just regrinding the edge and testing it to see if it holds an edge. I may still do that....but I'm leaning more toward just tearing it apart and reforging a tip to it and heat-treating it over again.





Here's a closer pic of the 2 breaks.


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You can see the cut I made with the bandsaw (using a bi-metal blade 18 tpi, mind you). I also made a cut in the edge to see if that would help "guide" a second crack...no luck. Went where it wanted to.





And finally, a pic of the beautiful handle.

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There is a crack in the handle - but it was there before. No amount of wrenching the handle seems to open it. I'm guessing it's inconsequential to the testing.

The only damage I could see to the handle was that the buttcap came loose.
 
Yvsa - see my comment about the 2nd break. I couldnt' see it in the first, but in the second one, it appears to have a slightly more refined grain like this:


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The red is where it is more refined. The center being just a hair more coarse.

Have a second look. I've been staring at it for a long time......:o




I did take a file to it, before and after - to check the hardening. Even throughout. Again, see pic above for reasoning. That's part of what led me to that conclusion.



Perhaps the kami water hardened quite a bit of it - since it is, after all, a fairly skinny blade comparatively. Would be difficult to zone just the edge. That's why I want to etch it. No time tonight though....maybe tomorrow.



I should also note that this dhankuta was given a nice convex edge and fully sharpened before being employed. It is still sharp. ;)
 
From what I can tell with these old eyes it does appear so Dan. I sure wouldn't have expected a break where it did on either of them, a bend of course if properly hardened, but definitely not a break!
And if I had my drathers I had rather a knife break down toward the point than at the tang. At least you still have a handle to control the blade you have left and some semblance of a point.

Was the saw beginning to stop cutting where you stopped on the second break or was it continuing to cut somewhat freely?
It would sure be interesting to have a Rockwell test done on the blade!
 
Rockwell test - that's what I want to send out for testing. I have a knifemaker buddy who has a tester. Going to send both pieces if he'll take 'em.


A break is not a problem.....a bend would have been nicer of course....but I didn't have the strength to try to bend it...!!! :eek:

I ended up hitting it with quite a slam with the hammer....several hundred pounds of force on it, I'm sure. Any knife would break...some thinner ones would bend, of course.

(after all....I was quite determined to break it.....intentionally)


I found the "before pic" of the crack in the blade. Here it is:

Kuk1.jpg
 
Yes, Bruise...see above. Going to do that tomorrow.
 
Just for kicks, could you grind and polish one of the breaks and etch that too? It would show the depth of the hardening. Maybe the end of the remaining blade?

Thanks,
Steve
 
It seems that without the extreme abuse, this knife might have lived a long and productive life. Even had the tip come off, the blade would have continued to be usable. Quite a fine thing, considering it was replaced.

You've also shown that you learn more from failure than success...

Thanks Dan!
 
Great stuff Dan! Please keep the dissection going- it's in the name of science and education for all of us, so don't feel bad about deliberately destroying a blessed... err, keep at it. I want to see what's going on inside there too.

Ad Astra :footinmou

and what Nasty said- "You've also shown that you learn more from failure than success..."
 
It's easier to destroy somebody else's knife (vs. one of your own)....:D




KM - compared to these pictures of another knifemaker's grain, I'd say there were right on:


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This is a picture of fairly large grain in a knife blade.


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This one is a close-up of the transition between hardened and non-hardened steel. The dark spot in the middle is the quench line. The hardened grain is on the left side. Looks to be about the same size.



Look back at the picture where I'm holding the blade.....at that same relative size, the grain is quite small.
 
Great pics Dan. You should post a link or the pics over in the shop talk. Let some of those anvil and hammer guys take a look at it. I am interested in seeing the Rc results myself. What kind of hammer did you use and how hard did you smack it? And no matter what you do with it that horn handle has to end up on something and not wasted!
 
ferguson said:
Just for kicks, could you grind and polish one of the breaks and etch that too? It would show the depth of the hardening. Maybe the end of the remaining blade?
Click to expand...
Yeah...I can do that. I'm curious too, obviously.....



Nasty said:
It seems that without the extreme abuse, this knife might have lived a long and productive life. Even had the tip come off, the blade would have continued to be usable. Quite a fine thing, considering it was replaced.
Click to expand...
I think that's right, Nasty. I'm sure, with some batoning and prying, it would have eventually failed, and that's why I supported Bill replacing it. That's also why I made the request to have it sent to me. So that I could see if it would pass "regular use" testing....which it did. It took extreme measures to break it.

I was real impressed, though....I hadn't used a 12" knife to chop with since first using my Pen Knife. Had my 14" BDC for light duty stuff....figured I didn't need the Pen except for camp duty. But this Dhankuta really chopped well. I don't think you'd want to use it on a 6" log, but I know I could if I had to. ;)
 
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