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That is a neat micrograph Doc Drew, looks like a lot going on. Maybe they could point out what normal is supposed to look like, on the left? Those lower right, and other areas, lines, maybe grit marks from prepping the sample. Thanks again for putting the info out to see.

Tom: here is Adam's response

The microstructure with nodules (not flakes) of graphite looks like ductile iron (a form of CAST iron). The product has been wrought (rolled and twisted), but I would not call the starting product WROUGHT IRON as that generally describes iron silicates in ferrite and does not have the graphite structure seen here.

Craig: Adam is preparing images upon which he will indicate the important findings.
Hard to know what is 'normal' since no one has ever published photomicrographs of crolle Damascus before but everything to the left of the crack is normal. Adam is still working on those lines in the lower right.
He did show me a bunch of images with the yield stress indicators.

Lots more to follow gentlemen, but please keep those thoughts and ideas coming!

Thanks Doc Drew. Possibly, the parent material was not intended to be a form of cast iron. If those are graphite pockets, maybe those were inclusions that the damascus manufacturing process was intending to mitigate.

VERY interesting.

This was a good idea, Drew...

The graphite inclusions are illuminating.

I believe I'm correct in describing them as inclusions, correct me if there's a better term.

As uncombined Carbon, they contribute nothing to an Iron crystal structure and thus are a major flaw in the material.

It appears that the crack runs through a giant such inclusion, and that the area shown southwest of the crack where the inclusions line up would be another potential failure point.

I think we need to get it out of our heads that Damascus is 'steel'. It ain't. It's a composite structure. Even 'pattern welded steel' isn't a great description.

Just another quick take Shotgunjones. Maybe that gray area at the start of the crack is not carbon, it doesn't seem to etch the same. I'd wonder if the acid prep dissolves iron oxide, possibly a tiny pit, but not likely the origin of the crack.

The xray, micrograph and modern blacksmithing, I think show pattern welding to be more of a mono steel than series of connections. If this sample is a steel, in all likelyhood the carbon has migrated and evened out through the piece. The color differences may be other alloying elements besides iron and carbon.

A row of tiny carbon inclusions may not be a big concern. If this is a crolle pattern, that line may actually spiral through the material and may not be sitting as a sort of perforation line in the barrel. Just thoughts.

After some research, I think the starting material is best described as a 'ductile iron'.

References show it as a form of cast iron, but... and here's the interesting point:

It's also known as 'nodule cast iron', the main feature being Carbon nodules in the form of round congregations rather than flakes... the main result being the INHIBITION of cracks, because cracks are more likely to start at a sharp point than at a point with a radius.

This is interesting as can be. What a guy can learn with a computer...

It still isn't steel though... in steel the Carbon and Iron form an actual crystal, and there are variations thereof. A mono-steel I would expect would have to be heated above it's critical point to become homogenous, am I wrong?

Another question craigd, and I think you've done some work with steel and iron... why would not the excess Carbon 'burn out' of the iron during the welding process? It does so with steel during forging, and the forger has to limit the time carbon steel is held above a certain temperature to avoid this.

I'd suspect since iron and steel were forge welded together, they easily exceeded the critical temperature of the steel. That may not be really important here. The original smiths may not have had any intention to harden the barrel, and the carbon percentage in the steel, not the inclusions, may not be high enough to harden anyway.

Steve C. had a good comment about loss of material due to the forging process. It may be important to control times and temperatures, but I think most of the material might be lost as scale due to oxygen exposure. Maybe.

I agree that the intent wasn't to harden, in fact the 'ductile iron' was used to provide what it's name implies, ductility.

The failure modes, fluid steel vs. pattern welded, seem to show that the intent was achieved.

I've been following this thread closely. I wish that I could look over Adam's shoulder and ask him a bunch of questions.

I have been doing some micro analysis of steels myself. Well�� trying to. I have no formal training at this work and am trying to learn all that I can. I don't yet have Adam's experienced eye.

I have a damascus barrel section that I bought from Peter Dyson. Below is one of the images of it, that I shot through my microscope. Tomorrow, I will be spending a lot of time viewing steel samples that I have prepared. I'll try to post some comments and perhaps photos.

Steve, thanks for your knowledge and picture, it is great. Hopefully Drew will get more results and information.

Not just for us Damascus shooters, but to everyone who shoots old and older guns, this is one of the most interesting posts to come along.
Thanks.