Ask a metallurgist: Why is a Miyabi 5000MCD67 easy to bend?

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besserbissen

besserbissen

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A question for the metallurgists: What has gone wrong when a Damascus blade with a core layer of ZDP189 (HRC66) bends relatively easily?
That can't be normal, can it?
We are talking about a Zwilling MIYABI 5000 MCD 67.
 
If it's bending at the spine, most likely because the cladding is soft stainless. A benefit of San Mai construction, they can take quite a bit of bending. Do you have pictures?
 
How thin the material is effects how easy or hard it is to flex.

How hard the steel is effects the point at which it will stay permanently deformed.

It having softer cladding comes into play here as well. Making it act as if it were a softer material overall, and permanently deforming at a point when monosteel zdp wouldnt have.
 
If it came like this from the factory it might have been faulty heat treatment that led to a warp.
 
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Jovidah said:
If it came like this from the factory it might have been faulty heat treatment that led to a warp.
Click to expand...
Warps can happen in any heat treatment. Nothing about that necessarily means it was faulty.

That said. If it came from the factory with a warp it is definitely bad qc. That should have been corrected before it left.

But thin san mai blades can just easily be bent. Its just part of having softer cladding. And a thinner spine.
 
besserbissen said:
I would expect the blade to spring back to its original position after the load.
Click to expand...
This is simply not a characteristic of soft-clad (whether stainless or iron) blades. Monosteel blades will do that, as well as blades made with a hardenable cladding, or hardenable damascus.
 
The knife is not mine, but the question came up in the German "Messerforum.net".

Every now and then I get Kai Shuns for sharpening and thinning and none of them bent so easily or were bent.
 
blokey said:
Really depends on the lines, different Miyabi lines can have quite different grind in my experience
Click to expand...
Yeah I agree, the birch ones are nice and thin but the "artisan" line is quite thick. I suppose I'm generalising about what comes through the shop, more birch Miyabi's and classic Shun's than other lines from each company.
 
besserbissen said:
My experience has been different:
The Shuns are much thinner directly above the cutting edge than the Miyabis.
Click to expand...

The part that is going to affect how much it bends is the thickness at the spine. I have no recent experience with either. But I generally prefer monosteel knives and one of the main reasons is because they are less bendy.
 
Is it completely unproblematic if the soft outer layers force the brittle core into a permanent bend?
At least with soft steel, the point at which it permanently bends is a strong load: one side is compressed, the other is pulled apart.
 
besserbissen said:
Is it completely unproblematic if the soft outer layers force the brittle core into a permanent bend?
At least with soft steel, the point at which it permanently bends is a strong load: one side is compressed, the other is pulled apart.
Click to expand...
It shouldn't be an issue. Bends should be easily corrected with hand pressure or a straightening stick.
 
besserbissen said:
Is it completely unproblematic if the soft outer layers force the brittle core into a permanent bend?
At least with soft steel, the point at which it permanently bends is a strong load: one side is compressed, the other is pulled apart.
Click to expand...
You can just hammer it straight if you're brave.
 
besserbissen said:
Is it completely unproblematic if the soft outer layers force the brittle core into a permanent bend?
At least with soft steel, the point at which it permanently bends is a strong load: one side is compressed, the other is pulled apart.
Click to expand...
The core is not as brittle as you might be thinking.

Imagine you can magically click your fingers and have the soft cladding disappear. The thin core would spring back into straight - or close to it.
If you take this thin spring and bend it back and forth, you'd have a very flexible material. So long as the force you apply isn't concentrated in one small area, the thin metal would very happily take that force and distribute it down it's length. With a spring that thin, you could very likely bend the tip to a 90° angle (relative to the handle) over the length of the blade. It would want to flex back to straight, but click your fingers again and you have the cladding back, holding the angle of the core in place.

Check out how much flex you can get at the tip of a mono-steel blade that has a very thin taper.
There's no soft cladding, so it goes back to straight.



The reason why you're concentrating on the brittleness of core steel is because we are normally referring to the stress along the edge, which is a thickness that theoretically starts in the microscopic, and gets much thicker only just a millimetre away. Any large force you place on an edge like that will not be able to spread itself out very far, and you are likely to get a brittle failure. Spread that same force out over the length of your blade core (which also has even thickness) and the steel will not have much of a problem.

So how do you straighten your bent ZDP san-mai blade?
Just bend it in the other direction - by hand and eye.

 
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