doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Iron, Steel & Alloy Staining

I didn�t mean for my earlier comments to sound too harsh, concerning my knifemaking brethren. They are serious crafters, earnestly seeking information on how to make the best and most beautiful blades possible. Their sincere desire for knowledge can leave them all too willing to accept information at face value, rather than questioning why.

The knife fella may be referring to AISI 13XX "Manganese Steel" or AISI 15XX Carbon Steels

I know the knifemaker discussions about high manganese steels etching darker, did not include the 1300 or 1500 series of steels. The considerations were most often between the use of 1095 and 1084 steels, in making damascus steel. Below is a link to one of these discussions.

Knifemaker Discussion

Electrochemical Corrosion of Steel:
Electrochemical Corrosion

The etching of steel is an electro chemical reaction. The etchant solution acts like countless microscopic batteries, which attach to the metal surface and conduct an electron exchange with the metal. The attachment points for these microscopic batteries, are the metal�s grain boundaries. The more numerous the grain boundaries, the more nucleation points for the chemical reaction. Steel with a finer grain structure, will etch much more quickly than a steel with larger grain structure.

As manganese promotes hardenability and fine grain structure, it is not surprising that steel with a high manganese content has the potential to etch darker. However it is not the manganese itself that affects the etch, it is simply the fine grain structure of the steel. If a steel with low manganese and a high manganese steel are heat treated to have the same grain structure, both would etch similarly.

In regards to harder steel etching darker than soft steel, it is not the Rockwell numbers that cause the difference. It is the fact that hardened steel has a finer grain structure than unhardened steel.

Sorry for being slow to continue this thread. It took me a few days to put some more information together.



Here is another reference, agreeing that chemical corrosion propagates from ferrous grain boundaries. This, from a document on the use of etching solutions to prepare samples for metallurgical testing.

National Board of Boiler Inspectors - Grain Boundaries

ETCHING:
In order to observe the microstructure, a piece of the metal is smoothly polished to a plane and mirror-like finish. The prepared surface is chemically attacked with dilute acid for a short period, a process called "etching." The grain-boundary atoms are more easily and rapidly dissolved or "corroded" than the atoms within the grains. A small groove is left at the grain boundaries. Since a groove will not reflect light as do the flat, polished grains; the grain boundaries appear as black lines, and the structural details are visible.



So to review; in finishing of damascus steel, the goal is to create a bold display of the damascus pattern. Knifemakers use destructive electro chemical corrosion to create deep etching and topography between the layers. Damascus gun barrel finishing, typically uses weaker etchant and acid solutions to create only a small amount of topography and gently develop the light vs. dark coloring to display the damascus pattern. BOTH, finishing processes, utilize electro chemical corrosion reactions to achieve the finish on the surface of the material.

Steel is the ferrous material in damascus that is very reactive to the etchant solution. As corrosion propagates at the grain boundaries, more active corrosion is facilitated by a ferrous material with a small grain structure. The carbon in steel also tends to precipitate into the grain boundaries, as cementite. The particles of cementite in the grain boundaries, provide more anodic connections for the electro chemical corrosion reaction. Steel being very reactive to corrosion, will create the colored (brown/black) layers in the damascus pattern.

The other layers in the damaacus pattern need to be of a light color. There are two ways to accomplish this. Simply use a ferrous material that has a larger and less dense grain structure. Or, use a ferrous material that has alloys that resist corrosion. As noted in the earlier comment about the steels that knifemakers use for damascus steel, 15N20 steel, with a corrosion resistant 2 percent nickel content is laminated with the much more reactive 1084 steel.

Damascus steel gun barrels have wrought iron, as one of the ferrous materials used. Wrought iron lacks the alloys necessary to create fine grain structure and it also lacks enough carbon to precipitate out of the ferrous grains to create a high number of anodic connections to facilitate electro chemical corrosion. But more importantly it contains silica, which acts as a resist to the etchant solution, inhibiting the effects of electro chemical corrosion.

Artmetal.com - Wrought Iron

The slag in wrought iron also provides natural corrosion resistance. Let's face it, nearly all ferrous metals rust, but wrought iron does a better job at handling it. As corrosion progresses, the fibers tend to disperse the rust into an even film, which gives the metal a natural brownish appearance. This film repels the scattering spotty corrosive attack t that other metals endure.
Because of its corrosion resistance, wrought was the metal of choice in earlier years for marine use, bridges, and girders. In fact, in extremely corrosive areas, an architect may still specify the metal.

Real Wrought Iron Company

The important corrosion properties of wrought iron are therefore due to its impurities in the form of slag. It follows that iron and steel without the slag will not exhibit the same corrosion resistance, and this is what is found in practise. Furthermore the slag has additional advantageous in relation to traditional forging techniques, especially fire welding.



So finally�

My THEORY is that the wrought iron layers of gun barrel damascus are always white. Because wrought iron is resistant to the electro chemical corrosion process. The steel layers are always the layers that bear the color (brown/black). Because the grain structure and carbon content of steel promote the electro chemical corrosion reaction.

I would really like to hear from the guys who finish damascus gun barrels. Which material do you perceive etches light and dark? Do you do your finish work with a plan to turn either the wrought iron or the steel dark? Are you able to reverse the colors on the barrel by changing your process?

As always thank you Steve.
This is a 25X 10% Nitrol etched section clearly showing the grain structure of the darker stained steel. The black globules are mostly silica, and the metallurgist thought they "stretched" in the weaker iron component in response to the shearing forces at the time of the burst

Interesting picture Doc Drew.

By 'stretched' are you looking at the roughly 45* dark streaks in the light colored band? Maybe I'm seeing it wrong, but that may not be signs of the burst as it does not seem to cross into the dark colored band. Also, those specs you showed earlier had a very similar strength of wrought and low carbon steel. Again, I'm probably seeing it wrong.

Also, maybe of note, if that is a depiction of steel grain in the darker colored band, it seems that the nitol etch is darkening more than just the grain boundries.

Back a few years ago, was there the conclusion that the dark globules might have been carbon? Just wondering.

Thanks for the topic guys.

Craig: I only play a metallurgist on DoubleGun The "stretching" may have also occurred during the rolling, twisting and hammer welding of the rod.

Re: tensile strength
AISI 1005 Low Alloy Steel: 40,000 psi
AISI 1010 Low Carbon Steel: 53,000
Wrought Iron: 34,000 - 63,000 psi; average 52,000 psi
Siemens (Open Hearth) Low Carbon Steel: 55,000 - 62,700 psi
Bessemer or Decarbonized Steel: average of 63,000 psi

BUT Yield Strength for Wrought Iron is 23,000 - 32,000; Bessemer 33,000; 1010 44,200.

The "wrinkles" are plastic deformation (stretching) before ultimate strength is reaching and the barrel bursts



This is a 500X SEM of an inclusion which was predominantly silica with phosphorus and sulfur ie. slag.

I am certainly no expert at examining micrographs, but I understand that it is an exceptionally challenging thing draw conclusions from. Must take a lot of experience to understand what is being viewed.

I know that metallurgists have always had difficulty with measuring grain size in a polished and etched sample. The sample only allows viewing the 3 dimensional grain on a single plain. It�s sort of like sawing through a block of frozen soup and trying to determine the size of the pieces of vegetables by looking at what has been revealed in that single view. You don�t know what has been removed by slicing through the sample and you don�t know what lies below the viewed surface.

Drew is correct that the silica stretching may have occurred during any part of the mechanical distortion of the wrought iron material. It could go all the way back to the shingling process to make the wrought iron.

It seems to me that making a determination of what happened to a piece of steel by looking at a micrograph, is like trying to describe all of history by looking at a single snap-shot in time.

I've used several processes, the iron always stains white. In my previous post about how the layers were arranged in the billet determining what color the spiral riband edge weld was/is based on a comparison between regular Damascus and three-iron Washington, Where the weld stains dark.

Regards
Ken

Thanks guys.

What I was thinking, was that the silica/slag stretching seemed to be what typical wrought iron might look like, and likely the dark/light difference goes to some deeper level than 25x. To me, the various micrographs that Doc Drew has shown, seems to show that potential weak spots, inclusions, are managed. Thanks for your time folks.

OKOKOK. I hope no pirate is reading this or my whole article might be "borrowed"

During the tensile testing at METL, it just so happened that a Twist barrel segment was machined right where a flaw was visible on eyeball inspection of the fracture surface. The inclusions are likely slag (silicates, phosphorus, sulfur and metal oxides) based on the appearance and Energy Dispersive X-ray Spectroscopy (EDX) spectrum. And I have a picture but ain't showin' it but will show this one

1895 Ithaca Crass with damascus barrels that were being cleaned up for reblueing. Initially just a small pit on the surface of the barrels, quickly grew into what you see here as polishing continued. Inside the barrels are bright and smooth, giving no indication of the large inclusion. Courtesy of Robert Rambler.

And the plot thickens regarding a distinct boundary between the wrought iron and "mild steel" alternees.
From METL's metallurgical study:

"The microstructure was banded, but there were not clear �iron� and �steel� microstructures. The overall material appeared to be a single piece of metal with segregations of inclusions. It is likely that the manufacturing process had combined the two original �iron� and �steel� materials into a more homogenous structure, but that without melting the combination was restricted to mechanical mixing and possibly diffusion.
The banded regions are likely predominantly comprised of the original chemical compositions, but during manufacture experienced changes."