I hope follow up thoughts are okay, not questioning your research and experiences.
Follow-ups are quite welcome! I fear that I have nearly hijacked this thread. My intentions with my first posts, were to point out that there are numerous ways that anomalies can be created in micrograph images and that at 20X magnification, it can be very difficult to make a determination of exactly what they are. Didn't mean to get too far into my work. I guess as long as it pertains to analyzing old damascus barrels, it's still good.
I'd suspect if wrought iron were used as one of the components, that the overall carbon percentage would even out to at best to what might be considered medium.
CORRECT! Carbon migration is scientifically proven. Once the forge welds are solidly completed, the carbon seeks to find a balance between the laminated layers. I have some documentation that suggests the rate at which carbon will migrate between the materials. It is a time and temperature related chemical reaction. The higher the temperature, the faster the carbon migration. However the caveat to this, is the alloying content of the steels. Certain alloys will, and can be added to the steel for the purpose of controlling the carbon's molecular bond within the iron matrix. Obviously, this will affect the rate of carbon migration.
I believe that the overall carbon content of old damascus barrels will fall in the .35 to .40 carbon range. This is below the carbon content that would heat treat well for things like knife blades, but is just enough to affect the steel's grain structure.
Jumping forward to Jawjadawg's question about damascus barrels being heat treated; I do not believe that there was anything comparable to a hardening quench and tempering process done to them. However, I have read in several writings of the cold hammering of barrel tubes after welding. Greener states that this hammering “greatly increases the density of the metal” and was done to the best barrels. The tubes were not actually “cold”, but in blacksmith terms, at a heat that was too cold to effectively move the steel under the hammer. This would be from around 1400 degrees F, down to a temperature of about 900 to 1,000 degrees F. So they were aware that the process created barrels that had a better finished appearance. It appeared to them that the steel was more dense, but actually, it was simply of a smaller grain structure. The hammering that was done to the tube was unnecessary. It was the repeated heating and cooling of the tube that affected the grain structure.
This heating and cooling of the steel is called thermal cycling, or normalizing. We knifemakers do this to all of our forged knife blades; or, it should be done. The heating to low red heat and cooling in room air to a black heat is repeated three or more times. I am convinced that this is exactly what the barrel smiths were doing during the cold hammering of their barrel tubes.
Normalizing will not soften steel to the dead soft condition of being fully annealed. But fully annealed steel will not etch well, because it causes lamellar banding of the pearlite grain structure.
Also, I think etch appearance of modern steels might be affected by added manganese, that would be interesting to know what amount may be present in historic barrels.
I have heard from many damascus smiths, that manganese will cause steel to have a darker appearance with a ferric chloride etch. However, my recent testing has not proven that conclusively. It could be that the etch coloration is more closely related to the grain structure. We knifemakers know that hardened steel etches darker than soft steel. If we make knife fittings out of damascus, we have to run them through a complete heat treatment, just like a knife blade, to get them to etch well.
I am acquainted with about 100 of the finest damascus smiths in the world. We all know how to go through the steps that we have learned, to make our damascus come out looking nice. Yet, none of the smiths that I have talked to can explain the chemical process that makes this work. That is the reason for my testing. So far, I have only been compiling test results. I have not had time to analyze them fully. At this point, I have many more questions than answers.
I also wonder what house manufactured the barrel sample you have in your possession? Was it of high quality, or one of the "belgian clunkers" from a substandard manufacturing process?
The barrel tube that I have is of Belgian manufacture. It is of two iron Crolle pattern. This I find a bit unusual, because most of the Crolle pattern barrels that I have seen were made of three twisted rods. I expect that this tube was intended for a lower grade gun. However, I view the workmanship of welding it to be superb. I doubt that they intentionally made "clunker" barrels. Some may have been made with the intention of keeping labor costs down; like perhaps with the barrel that I have. I expect that barrels of complicated damascus patterns, like Bernard and chain, were more likely to be welded by experienced smiths, due to their skill of uniform manipulation of the material. Barrels were probably graded after finishing and etching to display the damascus pattern. Barrels with very uniform patterns would be saved for best guns, while less perfect patterns were sold for use on lower grade guns. I doubt that the steel and iron used for barrels, were different for best barrels and lower grade barrels. The steel and iron would not be the major cost factor in barrel making and it would not be cost effective to run different grades of material. Too, it would be best for the barrel welders to always work with materials that they are familiar with.
When you say soft metals are harder to break, is that the result of the grains essentially stretching? Ductility?
Yes, the ductility makes breaking soft materials very difficult. I broke the piece of 1018 three times before I felt that I had a surface that I could analyze. I wouldn't have tried it at all, except that I was trying to match how the barrel material was exposed.
