Hi, Driftwood,
Have you ever seen anything definitive on the cylinder and frame material of the SAA? I once read that they were originally wrought iron, like the percussion guns, which was why they used case hardening to prevent wear from the internal parts. (Iron can't be hardened - the carbon content is not right.)
Then c. 1900, Colt switched to steel and that is when they OK'd the guns for smokeless powder, but kept the color case hardening as a cosmetic feature.
Howdy
In his book The Colt Single Action Revolvers, a Shop Manual, Jerry Kuhnhausen has extensive notes about frame and cylinder materials.
I quote:
Frame Material Notes: (1) Early Black Powder model S.A.A. frames up to about s/n 96,000 (S.A.A. frames manufactured through about mid 1883) were made of malleable iron. (2) Intermediate S.A.A. frames between approx. s/n 96,000 and 180,000 (S.A.A. frames manufactured from about mid 1883 through mid 1898) vary but were apparently made of transitional materials generally similar to modern low-medium carbon steels. The lowest carbon content found in intermediate s/n S.A.A. frames tested to date is approx. .0155, indicating the possibility that early formulations of 1015~1018, or higher carbon type steels may have been used in frames of this era. (3) Although there are exceptions, frames manufactured after s/n 180,000 appear to have been made from medium range carbon steels. The lowest carbon content found in the after s/n 180,000 frames tested was approx. 0213, possibly indicating that 1020~1027, or slightly higher, carbon, or similar, steels may have been used in these frames.
Cylinder material notes: (1) First Generation S.A.A. cylinder material changes began to occur at about the same time that S.A.A. frames were being metallurgically updated. Cylinders prior to to approx. s/n 96,000 were made from materials generally resembling high grade malleable iron. Original cylinders from approx. s/n 96,000 to about 180,000 were made from transitional low/medium carbon type steels. These cylinders and their parent frames were not factory guaranteed for smokeless powder cartridges. Cylinders after frames s/n 180,000 (about mid 1898) began to be made from medium carbon type steels. Later versions of these cylinders were better and more uniformly heat treated. S.A.A. revolvers with cylinders of this final type were factory guaranteed in 1900 for standard factory load smokeless powder cartridges. (2) 1st Generation .357 Magnum cylinders were made from fine grain, higher tensile strength ordinance gun quality steel. These cylinder blanks, identified by a 5 pointed star imprinted on the front, turn up on other caliber S.A.A.'s made during, and after 1935 (The .357 Magnum was introduced in 1935.)
Historical Note: Disagreement apparently still exists amongst historians and collectors as to when Colt factory conversion to smokeless powder model S.A.A. revolver production was finally completed. Some believe, incorrectly, that all frames made beginning with, or after s/n 165,000 (mid 1896) are smokeless powder frames. As far as actual changeover to smokeless powder S.A.A production is concerned, three things are absolutely certain: (1) Colt S.A.A. revolvers produced up to serial number 180,000 (reached about mid 1898) were not guaranteed by the factory for smokeless powder cartridges. (2) Colt did not specifically guarantee S.A.A. revolvers for smokeless powder cartridges until 1900 (beginning s/n in 1900 was about 192,000), and (3) the smokeless powder verified proof (the letters VP in a triangle) was not stamped on Colt S.A.A. revolver triggerguards until sometime in 1901.
End quote.
A few personal comments:
When Kuhnhausen talks about Malleable Iron, he is not talking about Cast Iron. Cast iron has high strength in compression, but because of the impurities that occur in it, cast iron has poor tensile strength. Cast iron is brittle and tends to crack easily. This makes cast iron unsuitable for components such as revolver cylinders, that see high pressures when cartridges are fired. Malleable iron, or Wrought Iron has been annealed by heat treating to make it easy to work without cracking. Malleable iron has been used for centuries to make swords and knives, and was suitable for frames and cylinders of revolvers during the Black Powder era.
Note that it is the cylinder, not the frame, that is the pressure vessel in a revolver. So cylinder strength is the most important consideration regarding what type of cartridges, and how powerful they can be, to be safely fired in any revolver. Note that Kuhnhausen states not only the carbon content of cylinders made during and after 1900, but also the better heat treatment used at that time that allowed Colt to feel confident in guaranteeing the SAA for Smokeless powder in 1900.
Here is a photo of the Verified Proof for Smokeless powder VP in a triangle on the trigger guard of a 2nd Gen Colt.
Case Hardening: The purpose of Case Hardening was to impart a harder surface to the softer iron and steel frames of the SAA. Case Hardening infuses extra carbon into the surface of the metal, forming a thin layer, or 'case' of harder steel for wear resistance, while the underlying body of the metal retains its ductility. This was important for revolver frames because they needed to have the ductility to absorb the pounding of recoil while still being resistant to wear on the surface. The brilliant colors of true bone Case Hardening are a byproduct of the hardening process, they have no intrinsic value of their own. However the 19th Century gun buying public became so enamored with the colors that each firearm manufacturer developed processes to produce the most brilliant colors possible, and they jealously guarded their processes. True bone Case Hardening on modern Colt frames still fulfills the requirements of leaving the interior of the metal ductile while hardening the outer surface. Using modern steels Ruger heat treats and hardens their frames throughout the body of their frames. Interestingly enough, Smith and Wesson never Case Hardened their frames, but they did Case Harden hammers and triggers. But that is another story.
