I would invite you to share a cite. That is at odds with findings in reliable sources as well as the design requirements of the cylinders. Generally test pressure stresses the walls to 70% of yield, and tensile is maybe 110% of yield. So you typically hit tensile somewhere between 6000 and 7000 PSI, and then the walls burst, which is what the navy tests back in the 1970s found also.
That's the center of the curve over the population of cylinders -- test 100 cylinders for actual burst pressure, and you'll get a distribution of pressures, some lower, some higher. How far are you going to push it? For me, it's 2475, and if I want more air, I have a selection of larger cylinders in my garage from which to choose.
It would be good if you would share some of your "reliable sources". But, I will give you a hint. The first two numbers in your post (in bold) are not correct.
The hydro test pressure is actually intended to be just below the yield strength of the steel. That is confirmed by the fact that there is often some residual permanent expansion. By definition that means that the steel is exceeding its minimun yield point and it has some small amount of plastic deformation.
If the cylinder returned back to zero expansion (during the hydro test) then you could say that it only experienced purely elastic deformation and the yield strength was never reached. This occasionally does happen, but it is not common with any cylinder (even a brand new one). Actually, new or not, age is irrelevant for a steel cylinder.
Saying that the ultimate tensile strength is maybe 110% of the yield strength is very wrong. That would be a relatively brittle metal alloy which is not what you want in a pressure vessel. You want to design a pressure vessel with very ductile alloys that will yield and deform and not fracture and fragment (if design stresses are exceeded).
The most typical alloy used in 3AA scuba cylinders is AISI-4130 (per CFR-title49-vol3-part178.37).
Here are some properties that can be confirm with just a simple Google search.
4130 (Chromoly) Normalized Alloy Steel
Minimum Properties
Ultimate Tensile Strength, psi 97,200
Yield Strength, psi 63,100
Elongation 25.5%
Rockwell Hardness B92
4130 (Chromoly) Annealed Alloy Steel
Minimum Properties
Tensile Strength, psi 81,200
Yield Strength, psi 52,200
Elongation 28.2%
Rockwell Hardness B82
Chemistry
Iron (Fe) 97.3 - 98.22%
Carbon (C) 0.28 - 0.33%
Chromium (Cr) 0.8 - 1.1%
Manganese (Mn) 0.4 - 0.6%
Molybdenum (Mo) 0.15 - 0.25%
Phosphorus (P) 0.035% max
Sulphur (S) 0.04% max
Silicon (Si) 0.15 - 0.35%
Per CFR-title49-vol3-part178.37, alloy 4130 may be used in the normalized condition.
You can see that the ration of ultimate to yield (in the normalized condition) is:
97,200 psi/ 63,100 psi = 1.54 or 154%
It is easy to look up the Code of Federal regulation (CFR-title49-vol3-part178.37), but here are some relevant quotes.
For the wall thickness design below it refers to the relationship between the "test pressure" and the allowed stress, as a percentage of the ultimate tensile strength (this is a typical structural design criteria).
(f) Wall thickness.
(2) For cylinders with service pressure
of 900 psig or more the minimum
wall must be such that the wall stress
at the minimum specified test pressure
may not exceed 67 percent of the minimum
tensile strength of the steel as
determined from the physical tests required
in paragraphs (k) and (l) of this
section and must be not over 70,000 psi.
(3) Calculation must be made by the
formula:
S = [P(1.3D^2+0.4d^2)]/(D^2- d^2)
Where:
S = wall stress in psi;
P = minimum test pressure prescribed for
water jacket test or 450 psig whichever is
the greater;
D = outside diameter in inches;
d = inside diameter in inches.
Here is were it specifies the material condition for the 4130 alloy.
(g) Heat treatment.
(5) Steel 4130X may be normalized at
a temperature of 1650 °F instead of
being quenched and cylinders so normalized
need not be tempered.
Here is the test pressure and the reference about permanent expansion
(i) Hydrostatic test.
(3) Permanent volumetric expansion
may not exceed 10 percent of total volumetric
expansion at test pressure.
(4) Each cylinder must be tested to at
least 5⁄3 times the service pressure.
If you have a reference for the statement below, I would be interested in seeing it.
So you typically hit tensile somewhere between 6000 and 7000 PSI, and then the walls burst, which is what the navy tests back in the 1970s found also.
From the information I posted above, I would calculate that the minimum burst pressure would be about 5,800 psi, but that is all based on conservative minimum numbers. That being said, I don’t expect it to exceed your stated 7,000 psi, but I would like to see some references. My calculation is just an estimate based on minimum published material strength.
There is a very detail Navy specification for steel 72 (Mil-C-24447), but it doesn't say anything about burst pressure. You can get a copy of the Mil spec at VDH.
