I need some advice on this kukri.

Joseph Bandeko

Joseph Bandeko

Knifemaker / Craftsman / Service Provider
Joined
Dec 9, 2021
Messages
1,363
Hello,
I am currently working on this kukri:0.jpeg
And I’m running into a problem I’d like y’all’s help on. The problem is that when I really whale on it, the edge starts to deform over (and by whale on it, I mean chopping really hard into a dry hardwood knot), and it’s coming up further then just the secondary bevel, it’s more like bacon edge from a blade that‘s edge was to thin after you quenched it, and In actuality the very apex is holding up just fine.

So on to the details...
The steel is 8670 from Pops and I’ve actually heat treated it twice, the first time I normalized it at 1550f, thermal cycled once at 1450f, DET annealed at 1380f and austenitized at 1525f, quenched in parks 50 and tempered at 350f. The second time I went ahead and just started over (though a skipped the thermal cycle), this time normalizing at 1575f, annealed at 1380f, austenitized at 1550f and tempered at 320f.
The second time it held up better to testing, but is still taking deformation.
As for measurements, the edge has a slight convex to 0.028” behind the edge (that’s after the second heat treat, the first time it was a little bit thicker), and it’s 20.75“ overall with a 15” blade.
I already feel like the edge is thicker than it should be, but that’s just me.

So what is it that I need to change? Is the heat treat the problem? Is the geometry just to thin? Any Input/advice I can get is greatly appreciated!

Edit: Upon further inspection of the edge under a microscope, the edge has taken some minor chipping, also the edge was sharpened to about 25 DPS (maybe even a little less) so I should probably increase that angle a little bit.

Joseph
 
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First thought is your hardness may be to high for the steel and impacts that the edge is experiencing. If you line up your heat treat to Larrin's results with Pop's 8670 the blade is most likely pretty low in impact toughness. Now if you thickened edge geometry you can work around lower toughness.

Only times I have experienced an edge like you are describing was with high hardness and thin edge.
 
Seedy Lot said:
First thought is your hardness may be to high for the steel and impacts that the edge is experiencing. If you line up your heat treat to Larrin's results with Pop's 8670 the blade is most likely pretty low in impact toughness. Now if you thickened edge geometry you can work around lower toughness.

Only times I have experienced an edge like you are describing was with high hardness and thin edge.
Click to expand...
I would think if it’s deforming it needs to be harder if anything, plus it seems to be holding up better after the second heat treat and it is harder this time.
 
For the 2 knives I had that I rippled the edges on one was 62 and the other 63 Rc. Once I sharpened with a little more obtuse geometery I no longer had issues.

For a chopping tools I do feel at a point increasing yeild strength at cost of quickly dwindling toughness is not best option for design. I could be wrong and I don't know where this point lies.
 
Should I put it back in the oven and temper it slightly higher, say 350f (maybe a little cooler), then just keep sharpening it back thicker and thicker until it no longer takes any deformation?
 
I should probably mention that after the first heat treatment I got it shaving hair and started chopping through 2x4’s, I ended up chopping through a 2x4 15 times and it was still shaving hair and cleanly cutting paper, I probably could have chopped through more times, but I didn’t.
 
if the edge deforms it is too soft.. if it chips it might be too hard.
you need to try a apples to apples recipe.. both with the exact same geometry to figure it out..
 
So a couple thoughts, first is was this stock removal or forged? You’re spending a lot of time in the kiln for all the cycles are you applying any coating to help prevent decarb? Since decarb is obviously soft if you impact/bend it even if the core is nice and hard and would normally flex back the soft layer can cause it to take a bit of a set. I’ve made several khukuri and machetes out of Pops 8670, as well as quite a few “bush swords” and have not run into this issue, for those heavier use blades I’d temper between 375F-425F. For thickness behind the edge I think you may be a bit thin, try beefing it up to .035 then putting a convex edge on it. Also don’t forget bad chopping technique can cause issues that aren’t necessarily a fault of the blade, if you bury the blade in a knot and twist it out you are abusing the blade and could likely just be pushing a perfectly fine blade too far.
 
Joshua Fisher said:
So a couple thoughts, first is was this stock removal or forged? You’re spending a lot of time in the kiln for all the cycles are you applying any coating to help prevent decarb? Since decarb is obviously soft if you impact/bend it even if the core is nice and hard and would normally flex back the soft layer can cause it to take a bit of a set. I’ve made several khukuri and machetes out of Pops 8670, as well as quite a few “bush swords” and have not run into this issue, for those heavier use blades I’d temper between 375F-425F. For thickness behind the edge I think you may be a bit thin, try beefing it up to .035 then putting a convex edge on it. Also don’t forget bad chopping technique can cause issues that aren’t necessarily a fault of the blade, if you bury the blade in a knot and twist it out you are abusing the blade and could likely just be pushing a perfectly fine blade too far.
Click to expand...
It was forged. Yes, I have been putting pops noscale on it throughout heat treat and there is no decarb near the edge.
I will stick it back in the oven tomorrow and temper it a little higher as well as thicken it up a bit behind the edge, then do more testing.
When I was testing I did intentionally chop into the knot not perfectly straight, so there is that, but I just really want to make sure that the end user can use it really hard without problems.
I will report back tomorrow after testing.
Thank you all for your input!
 
It is a kukrui. not a fillet knife. You are chopping hard wood knots, not slicing meat.
Two things that you should change:

1) Temper it at 400° and see if it works better.
2) Forget the thin edge. Make the edge a full apple seed grind (convex) and take it back enough to support the edge. Raise the edge angle about 5° and take the convex back about .2" not .02". I'm willing to be that this is the problem.
 
OK, I retempered it at 375f, then put an apple seed edge on it and just kept pulling it back getting it thicker until I reached 0.040” BTE, where it still took a little deformation, but only when I was pushing it harder then any end user would probably be using it, so I’ll add a little, say another 5 thou and call it good.
As for the very apex, after I retempered it and put an apple seed edge on it, I at first was doing about 27ish DPS and it was taking some micro chipping so I increased the angle a little more and there was no chipping or rolling.

I am a little disappointed that I wasn’t able to go thinner then this, but it is what it is and the blade still performs well.
And thank you to everyone for helping me!
 
Keep in mind khukuri are used like axes and are often have similar geometry. Thin is not better in this case.
 
Hey fellas thanks again for your help! Here's the final product!
0 (1).jpeg
CI5A6082.JPG
I've got some more photo's over here with a link to the giveaway I'm doing on it:

Instagram giveaway!

Hello all, I’m giving away this 20.75" long brut de forge Kukri in 8670 with black, blue, and red micarta with white liners on my Instagram! Thank you to all my customers and supporters for making this possible! https://www.instagram.com/p/Cnp_Voqs0pe/ (I wasn't able to edit the photos and bring...
www.bladeforums.com www.bladeforums.com
 
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