Crack propagation in forge welded blades

Alex Topfer

Alex Topfer

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I realised I didn't know how this would work, and couldn't find an answer with a quick search

So given a san mai knife made with the warikomi method, mild steel cladding and spine, what would happen when a crack starts propagating back from the edge? Does it continue through the forge weld into the mild steel, and does it require the same force to travel through the mild steel parts?

From what i found looking at papers i would expect it to require more force for the crack to continue, but I'm not confident with my understanding of the physics here
 
sorry, i should have been clearer. this is a hypothetical, not a specific piece

my understanding is most splits in san mai are caused by unequal changes in dimension during quenching, the core expands more as it converts to martensite and this leads to stresses that split the san mai, usually along the weld line. that makes sense to me
i'm wondering what happens if you have a crack propagating from the edge up into the blade. like someone tries to pry something open with it and cracks it
 
The edge is more likely to roll or chip rather than split.

For it to crack it would probably have a poor heat treatment.

Any mild cladding would prevent it from breaking. The weld is not likely to split unless there is a lot of lateral movement.

Hoss
 
In warikomi, the crack would stop when it hits the softer spine layer.
That is the whole purpose of the multi-bar construction. The only fully hardened part is the edge.
The blade might bend, the edge might chip or crack ... but the blade will remain intact and still be an offensive weapon.

Warikomi means "interrupt". and that is what it does. It interrupts a crack from propagating. It is done with some form of awase (cladding) and multiple pieces of steel with different carbon content ... and thus different hardness once HT is done.

Warikomi gets applied to several forging techniques.
1) Wariha Tetsu Kitae - this is a simple method of splitting the edge of a bar of soft steel and inserting a piece of high carbon edge steel. The billet is forge welded together and then forged into a blade. It gives a blade with a hard edge and soft spine.
2) Kobuse San Kitae - This is the "hot Dog" construction where you roll a piece of soft steel into a "U" and insert the hard core/edge steel.
3) Hon San Kitae - This is the classic four bar warikomi where you have an edge piece of high carbon steel, a spine of soft steel, and side awase (cladding) of medium carbon steel. The cladding can also be seven or eight layers of soft ad medium steels or suminagashi.
4) Shihozume Kitae - This is a five-bar construction where there is an inner core of soft iron. The edge and spine are high carbon, and the cladding is medium carbon. This is the classic "samauri sword" construction method.
5) There are several other methods of awase involving folding vbbbbbf multiple layers and welding it up.
6) And then there is Masamune's seven pieces welded together called Soshiu Kitae. This consists of three core layers and four cladding layers (two each side) all of different carbon content. To say the least, it is a complex billet to weld up and even more complex to forge into a sword.
 
So the difference in hardness is what stops the crack? Presumably because the more ductile material needs more force for the crack to continue

Thank you for the help
 
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