Hardened Tip on 25" Kobra?

Dave Rishar

Dave Rishar

Joined
Oct 25, 2004
Messages
3,178
Here's an odd one for you...

I've been using my 25" Kobra for blackberry clearing duties over the last few weeks. (I know, it's not made for that, but it's very efficient.) I was recently sharpening up the old Kobra again for a rematch this weekend. My reprofiling/resharpening procedure consists of some judicious file work, a bit of time on the Sharpmaker, and maybe a stropping if it needs it. (Yes, you can sharpen a 25" Kobra on a Sharpmaker...email me if you need details.)

The tip portion still had minor dings in it from the last outing. I ironed out the ones I could with the chakma, then started in with the file. The funny thing is, the file started skating off the tip like it was the sweet spot! I tried a few more times, then tried using (much) harder pressure, but nothing changed...the tip was at least as hard, if not harder, than the sweet spot. My Black Diamond kept skipping right off. I performed a few experiments to verify; the file was not loaded, there were no high spots (even though they feel different, I made sure just the same)...the last 1-2" of that tip's edge are just very hard. What's stranger, the first few sharpenings I did with a file didn't show a hardness like that...it's almost like I had to eat my way down to it. When filing along the edge, there's a definite grab-skate-grab-skate (worse) on both sides. The different hardness zones on either side seem to be very well matched with one another...it's the placement that's odd. It's just weird.

Any thoughts on this? Just for the record, I'm in no way displeased with this blade; I can't say anything other than good things about it. It's performed jobs above and beyond what it was designed for with ease and continues to do so. It is a magnificent blade. I'm just surprised that it's hard (if indeed it is, and I'm not just mistaken) in such an odd area.
 
I don't know. I'd say it was a flaw.
But maybe since the Kobra is not intended for wood cutting there's a hard tip for armor peircing....

munk
 
Satori said:
Here's an odd one for you...
.....the file started skating off the tip like it was the sweet spot!.............the last 1-2" of that tip's edge are just very hard.
What's stranger, the first few sharpenings I did with a file didn't show a hardness like that...it's almost like I had to eat my way down to it. ......
Click to expand...

Not odd at all.
I've etched over a dozen hi blades,
in virtually all
the hardening zone extends well onto or completely covers the tip.

When the edge is /forged/ (thanks stevomiller),
the high heat drives out the carbon from the surface of the steel.
Smiths leave the edge thick because of this.
The initial sharpening of the edge removes the decarborized layer;
mostly, usually.
If the heat is a little higher,
a little longer,
then the decarborization goes deeper;
all the soft steel may not be removed for a sharpening or two,
or more.
Thus stories of blades that get harder/sharper with use.

The patterns of hardening vary quite a bit.
Some can be seen as two or three pours.
Some as a single pour.
Some multiple pours overlap,
some barely touch side to side.
Some are regular in width along the edge.
Some are very irregular.

To be hardened properly,
the steel must be heated into a very specific temperature range.
Not too low -or- too high;
and every alloy is at least slightly different.
The rate of cooling from this critical temperature determines the final hardness.

The smaller the cross-section of the blade, the faster it cools.
I assume the sweet spot is hardened first.
As the sweet spot is hardened (maybe a second of time passes)
the other areas of the edge are already cooling.

I can imagine at least a half-dozen minor variations in the quench pours
that alone or in combination could detectably give a different hardness.

So, everything you describe is within the realm of variation.

Pretty cool, hunh?

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Just what Ddean said. Carbon loss actually happens more during the forging process than the hardening process though. The steel spends time at a higher temp and for a longer period time while forging than during the heat treating.

stevo
 
I've etched a handful of HI blades too. I found that the hamon line extended over the tips on my 18" Kobra and Pen knife, but not on the Gelbu Special or Ang Kholas.
 
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