Edge Hardening

[oldschool45]

Here is roughly what a HI Khukuri would look like if you risked derailing a train to try and snap the blade. The hard edge just snaps and the still soft spine just bends. Might happen if you tried to use a Tarwar as a prybar

 

[JParanee]

differential heat treat is a wonderful thing

 

[killa_concept]

Not too sure what I'm looking at

Also, I don't think the hardened section of a kukri or tawar would give way from the softer section from simple prying... the change in hardness is relatively gradual and not instantaneous, so it would take a lot of force in a very short period of time on a very specific area for it to happen. A train running it over? That might do it. Prying with a tarwar? No way... it would just bend in this manner (sorry for linking to this guy )

 

[warty]

what is the thing in the picture? sorry but some things are simply not easy for me to make out.

 

[oldschool45]

Warty, its a piece of 1095 steel that I was hoping to make into a kitchen knife(going for a Nakiri). I edge quenched it roughly the same way the Kami's do our blades. The blade had other "issues" so I snapped it to show what edge hardening/differential heat treat looks like when it fails. Basically the hardened edge breaks and the softer spine remains pliable. It will take another 3~5 180' bends before this spine complete fails.
Killa, the Tarwar was the only HI blade I could think of that a normal person could stress to breaking. Maybe an R series?

 

[AF]

Part of the ABS tests include a spine bend where the requirement is that the edge not crack - correct me if I'm wrong.

 

[killa_concept]

Part of the ABS tests include a spine bend where the requirement is that the edge not crack - correct me if I'm wrong.

I believe you're correct, although my knowledge on blademaking is low and my experience non-existent. If you can break the hardened portion off through bending, that means that the edge was hardened to the point of brittleness or that your deferential treat was too abrupt in it's change from soft to hard. The optimal knife has a softer back, and a harder edge - but an edge that is still "tough" (not brittle) and not abruptly hard, but gradually so from the soft section. This is usually accomplished through tempering, or at the very least, very knowledgeably-done quenching.

Maybe I'm completely misinterpreting though, oldschool: how did you manage to snap the hardened portion from the soft backing? Was it just through a couple of lateral bends? Also, how did you do your quench? From my understanding, kamis do not quench the blade through dunking, but rather - they pour the water (or oil?) onto the blade slowly and with precision learned through years of experience. The proper technique, timing and control is key to getting the right level of toughness/hardness:

 

[oldschool45]

Killa, I didn’t snap the hardened edge from the softer spine. I bent (hit it with a hand sledge) perpendicular to the hard edge/soft spine merging. I’m gonna have to link to one of Kevin Cashen’s links with graphics to give a better explanation.
Except for the cracks in the edge (probably caused by my poor grinding) the blade did exactly what I wanted it to do.
One exception to the tempering back (after quenching) is when you want a super-hard edge and don’t care if it is brittle. Just quenched 1095 has a theoretical hardness of about 67 Rockwell C scale, temper it at around 350’F and it lowers the Rockwell into the upper 50’s. If you want to make a slicing/carving/sushi knife that will take a super thin edge and hold it knowing you’re not going to baton bones with it you can do that by not tempering the blade back.
I’m not saying that edge quenching is easy or that there is one best way to do it. I fix computers for a living I mangle good blade steel for fun

 

[RandomAyes]

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