Carbon migration

[Corradobrit1]

What exactly is carbon migration during the forging process and is it a good or bad thing? How does it manifest itself in the forged blade? Are some steels more prone to this phenomenon? Just curious

 

[Luftmensch]

Wait for one of the heavy hitters. I am sure you'll get a good answer from them. Until then this might tide you over:

Carbon migration is the movement of carbon from a high carbon steel to a low carbon steel during welding. This can be in sanmai blades - from the high carbon core to the low carbon cladding. Or, during pattern welding where carbon can move between folds. At the interface between the steels you will end up with a region where there is a gradient of carbon. I believe it is unavoidable but a blacksmith can control the outcome from negligible to irreversible. In the ideal case, the mixing region is very narrow. In the worst case (and probably theoretical), say with thin steels and high temps/time, you homogenise the carbon content between the steels.

As with many things in metallurgy the degree to which the carbon moves during welding depends on temperature and time.

On clad knives, the more visually distinct the boundary between the cladding and the core steel, the less migration has occurred. To the extent that some welds look like they have a hairline crack/line running down the length of the weld. I have been told that in Japanese blacksmithing, this is considered a sign of skilful welding - not sure if this is true or is indeed a 'good' thing... but it can be pretty!

 

[dan]

I study materials science so this is right up my alley. I can't comment on how it affects the final product but I could give some physical insight.

What exactly is carbon migration during the forging process?

Luftmensch gave a pretty good description here. It's where carbon atoms in one region of a steel move to another part of the steel. It shouldn't happen significantly in monosteel knives, since it requires a difference in carbon concentration to happen. Monosteel knives should have uniform(ish) composition and concentration throughout the bar.

However, consider a three-layer knife. Each layer has a unique composition. Say the center layer is carbon steel and the outer layers are iron. Take a simple carbon steel, 1095 carbon steel. It is 0.95% carbon and the remaining ~98-99% is iron (not including contaminants). Say the outer two layers are both iron and have no carbon. When the layers are forged together, they are heated up to a high temperature and as Luftmensch mentioned the layers start to melt together. Carbon migration is when the carbon from the central carbon steel layer starts to spread out into the carbon layers. This occurs as the layers melt together.

You already have a sense for why this happens. Say you've got water and you drop a spoon full of salt into the water. If you quickly go in and scoop out some water from the same area you dropped the salt into, it'll be really salty water. If you wait some time, the salt spreads out across the entire volume of water. Sampling the water a few hours later, you'd notice it tastes much less salty. It's the same concept (diffusion).

...is it a good or bad thing?

This one is tough for me to say, since it's a very complicated effect. As carbon migrates, the effective composition of the steel changes. The organization of the atoms change. It could be good or bad depending on the situation...

How does it manifest itself in the forged blade?

Luftmensch gave a great answer to this in terms of aesthetics. I defer to blade makers to how it affects the properties.

Are some steels more prone to this phenomenon?

The closer in carbon concentration the different layers, the less it happens. The greater the difference in carbon concentrations, the more it happens. There's also a time dynamic.. if you haven't allowed enough time for the physical interfaces between the steels to melt together then it won't really happen at all.

 

[refcast]

I guess I'll ask a subquestion, too. There was a forum post a while back where a user wondered why their cladding line on a iron-clad carbon knife had a thin darkish layer. It was a Heiji Iwasaki carbon, and I've rarely seen that delamination kinda-look anywhere else.

 

[RDalman]

Imo, carbon migration is something unwanted and one wants to minimize. Having some firsthand experience I think with good later forging temps, it mainly happens in the forgewelding stage, and in good execution very superficially. So if one forgewelds thicker pieces and works it out into knife dimensions, there's typically not much migration into the iron and doesnt affect the core steels performance. Example when I make a san mai with iron/steel/iron in starting thickness 10/5/10 mm, and work down to a 3-4 mm knife, welds are very clean migration wise.

 

[ian]

I guess I'll ask a subquestion, too. There was a forum post a while back where a user wondered why their cladding line on a iron-clad carbon knife had a thin darkish layer. It was a Heiji Iwasaki carbon, and I've rarely seen that delamination kinda-look anywhere else.

So, I don’t know anything, but is that what it looks like when one uses a layer of nickel in between the iron/steel as a barrier to the carbon migration? For some reason I remember seeing pictures that looked something like that, but maybe I’m making this up.

 

[Corradobrit1]

The metallurgy is fascinating. Being a chemist means I nerd out on this stuff. There's so much going on than many think and it really helps me to appreciate those bladesmiths who consistently put out quality blades that are more than just pretty faces.

 

[dgib7994]

So, I don’t know anything, but is that what it looks like when one uses a layer of nickel in between the iron/steel as a barrier to the carbon migration? For some reason I remember seeing pictures that looked something like that, but maybe I’m making this up.

Layer of nickel shows up in clad blades as a shiny line when the rest of the blade is etched in ferric chloride to turn black. I don't think it would be visible unless an etchant were used, though of course I may be wrong

(found this pic on google)

I have heard many times that a pure nickel shim will have a mitigating effect on carbon migration, as well. Though I can't find any scholarly resources on this or any specifics on why. I certainly like the etched nickel shim look, though : )

 

[Luftmensch]

I guess I'll ask a subquestion, too. There was a forum post a while back where a user wondered why their cladding line on a iron-clad carbon knife had a thin darkish layer. It was a Heiji Iwasaki carbon, and I've rarely seen that delamination kinda-look anywhere else.

Forgive the shameless cross-posting. I just started a thread to illustrate what I mean HERE. Perhaps this is similar to what you saw in the Heiji Iwasaki carbon?

 

[F-Flash]

Ive seen videos, where japanese blacksmiths use some kinda powder in between steels, when they laminate two steels together. Could this be the tiny lamination line? I have absolutely no idea what the powder is, so please someone with more wisdom give us your knowledge.

 

[Matus]

That powder is borax. It prevents oxidation of the steel so that the two steels can be weld forged. It has nothing to do with mitigating carbon migration.

 

[Sharpchef]

That powder is borax. It prevents oxidation of the steel so that the two steels can be weld forged. It has nothing to do with mitigating carbon migration.

This is not true! Japanese Bladesmith don`t use Borax... The europeans do. Japanese use some mixture out of sand and some other mystical ingredients .... I made one Damascus piece with "Japanese forge welding powder"... ****ing difficult, borax is much more easy stuff...

Greets Sebastian.

 

[Matus]

Well, maybe they did not use borax in the past, but as you said yourself, it is much easier. So while there still may be some that out of tradition use something else, I am pretty sure most of them use borax, the same way they use all possible modern tools to make their knife making more efficient and predictable - from belt grinders to sandblasted kasumi and CNC cut & pre-ground blanks Carter uses it too and he learned forge welding in Japan.

Borax mixed with iron oxide, to be exact

 

[Luftmensch]

sandblasted kasumi

 

[user 29468]

This is not true! Japanese Bladesmith don`t use Borax... The europeans do. Japanese use some mixture out of sand and some other mystical ingredients .... I made one Damascus piece with "Japanese forge welding powder"... ****ing difficult, borax is much more easy stuff...

Greets Sebastian.

in Europe ( and probably everywhere ) we also use sand, mostly in blacksmithing , where Hi Th° is not a issue ( iron/iron welding , we don't care if it burn) , Japanese blade-smith use sand ( who melt (?) at lower Th° possible , and add some things to lowering the melting TH ° ( sorry for that poor English ) ) .

So Yes you can weld with sand , but do not come to cry if you have Huge C migration after that ;D it will works but it's way too hot for steel.

 

[inferno]

This is not true! Japanese Bladesmith don`t use Borax... The europeans do. Japanese use some mixture out of sand and some other mystical ingredients .... I made one Damascus piece with "Japanese forge welding powder"... ****ing difficult, borax is much more easy stuff...

Greets Sebastian.

I heard its colombian coke they use to weld them.

 

[Corradobrit1]

Whatever it is its in that lunch box Shiraki is dipping into.

 

[DevinT]

Some Japanese use a mix of borax, boric acid, and iron oxide for flux.

Hoss

 

[Bert2368]

I heard its colombian coke they use to weld them.

I had no idea that the Colombians mined coal suitable for metalurgical coke, let alone had coking ovens!

 

[RDalman]

Yea what devin said.. Atleast that's what tanakas mix looked like when he did forging demo by cleancut. Rust with some borax mixed in. I would prefer straight borax. Sometimes they mix in iron shavings also to help fill/grab. That's also common for horse shoeing and mix like that is commercialy sold.

 

[Luftmensch]

Does the addition of flux leave any visible remnants? I assume not...

 

[inferno]

Its just there to prevent oxidation. and if the 2 pieces are now 1, then obviously there can be nothing between them.