couple things I can say here....don't anybody take it personally....just a giving my opinion.
The break does not show typical evidence of failure due to heat-treat. I can't find my pics right now...but there are telltale signs. I have broken, repaired and tested-to-failure more than my fair share of knives and especially kukris...and there are a few common traits.
Failures in kukris come from any of the following:
1 - incorrect heat-treat (edge too hard/brittle or too soft)
2 - stress risers in the blade (cracks in the blade)
3 - stress risers in the tang (failure happens inside the handle/bolster)
4 - abuse (pushing the materials beyond their intended abilities)
#1 (above) = you can test for this by etching the blade with FeCl or vinegar to see the quench-line...shows soft spots...and sometimes you can get a feel for if the edge has been overhardened...or if it has extended to the cho or tang.
The "proof" of this is a chipped blade or rolled edge
#2 (above) = stress risers can come from imperfections in the steel, impurities, improper forging, heat-cycling as well as too aggressive of a heat-treat. If the kami can see it, he'll toss it. If he can't see them, can't test for it.
Also, stress risers can come from areas in the blade that have had notches cut into it...such as the cho.
The "proof" that this is the case is usually a dark spot in the cross-section at the break.
#3 (above) = stress risers are more typical here where the tang meets the handle. Normally, this is not a problem that manifests itself. Only under severe conditions can the stresses put on the tang result in cracks rather than bends.
The "proof" here is the same as #2 - part of the cracked cross section is dark instead of grey.
#4 (above) = it is very easy to abuse kukris....to make them work harder than they should. The knives are Tough and can really take it. So we work them harder and harder, expecting more and more until they fail. Or, as some do, just wail on them from day one until they begin to fail.
The "proof" here is bent or twisted blades, busted tips, clean breaks and so on.
It is my opinion that the failure was due to a combination of factors:
#1 - (not a factor)
#2 - stress riser from the cho (evident in all blades anyway...but "magnified" in the CAK)
#3 - (not a factor)
#4 - while not intentionally performed, batoning of any knife is generally considered "hard use"...."severe conditions".
A few lessons I have learned over time regarding kukris:
1 - hollow grinds are more prone to failure IF the heat-treat is off. If the heat-treat is on and usage is not severe, the grind can last a long time. Best grind is convex....flat grind is a good 2nd best.
2 - "clean breaks" are not evidence of heat-treat failure....either stress risers or abuse...or both. Like I said...all kukris have
some stress risers in them from the cho and the tang...it is inevitable. The question is whether or not the heat-treat, the grinding and the weight and balance of the knife will "hide" the stress riser - never letting it become a failure - or magnify it.
3 - the "splashing on the tang" is conjecture....and was offered by a kami as a speculation. It's not been my experience that this results in failure. I have made hundreds of knives with "hardened tangs" (aka full tang knives). This is not the real problem.
The problem is the ratio of blade to tang in terms of quantity of steel. A CAK is at least twice as thick/heavy as a regular kukri and the blade-to-tang ratio is diminished.
Which leads me to:
4 - the Chiruwa Ang Khola (CAK) is so thick and heavy that any stress risers become Magnificent Failures under hard use or severe conditions.
Think about it this way. You have a 1/4" diameter rod of steel that is 24" long. You whack at a 2x4 with a chopping motion - hitting it with the end of the rod. You get it to bend a little, here-n-there...but it mostly just springs back after it hits the 2x4. Minimal damage.
Now take the 1/4" rod and weld a handful of 12" long 1/4" rods to it so that you have one half of the rod at 2" diameter and the other half 1/4" diameter.
Swing that in a chopping motion and you will bend the 1/4" rod no problem.
And where does it bend? - where the size changes from 1/4" to 2".
This is exactly what happens with a heavy knife such as a kukri. The heavier the blade gets, the more stress is put on the "non-blade" parts of the steel. Even small stress risers can become Spectacular Failures with enough weight/force applied.
Conclusion:
I do think that the CAK is the most indestructable of HI's kukris....but hopefully you can now see why I say the failures are 'spectacular'. Other knives chip...or crack...but the CAK doesn't mess around...goes all the way around the bases to home plate.
Which is why everyone should follow Uncle Bill/Rusty/Yvsa's advice on new CAK ownership - go out and chop the hardest wood the knife will ever see. That will tell you if it's a good blade right upfront....no "waiting" for the magnificent failure later.
Dan
p.s. I reserve the right to edit my commentary later..as I remember stuff...and if I find any errors/omissions.