Stress Fracture, and Test Break

Shawn Hatcher said:
James/Darrin - I've read in the past that a hardening line (or temper line/quench line/hamon/etc) can be a stress riser. Is that what you guys are referring to? Are there steps that can be taken to reduce the stress in this area?
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I'm not sure it would be called a stress riser exactly, but the transition can be a weak point. I'll leave more technical descriptions of the two different structures on either side of the line, and how it can be done properly, to others. Suffice to say I'm convinced that hardening the entire blade to a fully martensitic state, tempering normally, and if need be tempering the spine/tang further results in the strongest blade.

There's no question that hamons and temper lines are beautiful.
 
james terrio said:
I'm not sure it would be called a stress riser exactly, but the transition can be a weak point. I'll leave more technical descriptions of the two different structures on either side of the line, and how it can be done properly, to others. Suffice to say I'm convinced that hardening the entire blade to a fully martensitic state, tempering normally, and if need be tempering the spine/tang further results in the strongest blade.

There's no question that hamons and temper lines are beautiful.
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I agree on all points.
 
The reality is that the end user will probably never push the blade to the point of failure and if they do, will most likely never note it as a "weakness" of differential hardening. That doesn't mean that it is not there. It also doesn't mean that there is no place for a differentially hardened blade from a performance standpoint. A beautiful hamon was once the signature of a high performance blade. We know more now, but I think the notion of a dif. hardened blade being this ticking timebomb is just not true.
 
Rick Marchand said:
...I think the notion of a dif. hardened blade being this ticking timebomb is just not true.
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You're right. I never said it was. But every once in a while... blammo! (of course, HT cracks could happen to any sort of blade)
 
I agree that a knife this size does not benefit much from a differential heat treat. I have had very few failures in quite a few blades, those are odds I am very happy with. I saw an opportunity to share and learn and I have added a Low Temp quench during my Normalizing cycles. Thanks for all the input.

Here are some photos of the new blade for the customer. I look forward to finishing this one, it will have a Bog Oak and Curly Mahogany handle!

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Looks great. I would be interested in hearing more about Rick's suggestions. I am not quite sure how and why?
 
Looks good Ryan, thanks for opening your self up, and making yourself vulnerable. You could have kept your mouth shut and no one would be the wiser. Because you showed us, lots of people will learn some thing. It just goes to show that you are a stand up guy!
Thanks
Cody

P.s. after reading this thread, I had to go break the tip off one that I had ready for post ht grinding just to check!
 
RyanW said:
Rick, I was talking with another maker and think I will add a quench at the lower temps during the Normalizing cycles as you suggest. Probably not in Water though ;)
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Derp! I had water quench on the brain Yes... Quenching from black (800-900) is safer in oil.

I water quench between tempering cycles.
 
Shawn Hatcher said:
I've read in the past that a hardening line (or temper line/quench line/hamon/etc) can be a stress riser.
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Technically a stress riser is caused by a change in geometry, something like a notch or groove or hole or change in width or thickness. A temper line represents a change in the steel grain. A differential heat treat will lock residual stresses into the steel and my theory is that the residual stresses in the vicinity of the end of the heat treat, perhaps aggravated by the small choil notch, are what caused the fracture. And once you have a crack starting, it is easy to make the fracture go all the way through the material. (I did some fatigue testing of large steel bolts one time, once we got a fatigue crack that went partway through a 1.5" diameter bolt I could clamp the bolt in a vise break it in half.)
 
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