Does metal thickness correlate with maximum PSI pressure?

SpaceTimeTravelr · Jan 7, 2016

4140 Steel has a Tensile Strength of 95,000 psi (pressure pounds per square inch) and a Yield Strength of 60,200 psi.

Is is possible for me to build a barren made of 4140 Steel to be able to withstand pressure of 150,000 psi - 200,000 psi by making the barrel thicker?

By making the steel thicker, would it be able to withstand more pressure.

wesg · Jan 7, 2016

Yeah, that's pretty much how it works.

billzweig · Jan 8, 2016

Increasing the thickness or changing the material tensile strength will have the same effect on increase in pressure rating.

GGaskill · Jan 8, 2016

By making the steel thicker, would it be able to withstand more pressure.

Only to a limited extent. Play around with this thin wall/thick wall stress calculator and see what happens.

The strength figures you are quoting for 4140 are in the annealed state. It can be heat treated to much greater strength and usually is for high pressure cartridge barrels.

steve-l · Jan 8, 2016

Out of curiosity, tensile and yield strength numbers are in PSI or KSI, those define the area of the applied force, but not the thickness. I assume these numbers are for some standard test piece. Does anybody know what that standard is?

Royldean · Jan 8, 2016

steve-l said:
Out of curiosity, tensile and yield strength numbers are in PSI or KSI, those define the area of the applied force, but not the thickness. I assume these numbers are for some standard test piece. Does anybody know what that standard is?

Take a round (or square) bar of exactly 1 square inch cross section, and anchor one end. On the other end, place a 100 lb-f load (pure tension). The stress in the bar will be exactly 100psi, REGARDLESS OF LENGTH of the bar. Does that answer your question?

johnoder · Jan 8, 2016

Note the standard Tensile Test Specimen has the reduced area .505" diameter - fairly close to exactly 0.200 square inches. Hanging 100 lbs on that section will stress it to 500 psi.

https://www.google.com/search?q="St...lwDrv8NJM:&usg=__auofRfpmQbkKxPfBWbEeQ2zMkHw=

steve-l · Jan 8, 2016

Thank you John and Roy. So in relation to the OP's question, he would have to calculate the hoop stress and then increase the cross sectional area of the cylinder to make certain the material yield strength is not exceeded plus some consideration for cyclical fatigue. Am I correct?

steve-l · Jan 8, 2016

One more small point. I don't understand the OP's 150-200 KSI requirement. Perhaps I don't understand the subject well enough, but the last time I checked, which was several years ago, the maximum chamber pressure allowed by SAAMI was about 55 KSI. It was my understanding that that limit was the maximum pressure the propellant could withstand before it quit burning and detonated.

paracongo308 · Jan 8, 2016

steve-l said:
One more small point. I don't understand the OP's 150-200 KSI requirement. Perhaps I don't understand the subject well enough, but the last time I checked, which was several years ago, the maximum chamber pressure allowed by SAAMI was about 55 KSI. It was my understanding that that limit was the maximum pressure the propellant could withstand before it quit burning and detonated.

I believe it has been raised to 64-65 thousand psi about 40+ years ago. All modern magnum cartridges develope 60,000+ psi.

JNieman · Jan 8, 2016

Maybe it's for a high factory of safety

JLarsson · Jan 8, 2016

I'm curious what the OP's application is, because there is more to a firing system than the barrel. In a standard, cartridge-based firearm, pressure is limited also by the brass casings used. Steel casings may (I don't know) raise that pressure limit, but their limit would still (I would think) be lower than any barrel in which they are fired.

So, what I'm saying is, "What's it for?"

mmack · Jan 9, 2016

back in my days working in a machine shop that produced hydraulic cylinders we used the same base material and adjusted the wall thickness to suit the PSI required to operate the system .

imported_brian_m · Jan 9, 2016

JLarsson has the correct approach. If this project really is for a barrel then many other factors need to be considered as well as the strength of the breech/barrel, most of them will be more limiting than the barrel strength.

abarnsley · Jan 9, 2016

Max Chamber Pressure - SAAMI Specs

And ALWAYS remember that the cartridge brass, is thinner and weaker than the barrel.

What takes many guns apart instantly, is a failure of the rear of case..

steve-l · Jan 10, 2016

abarnsley said:
Max Chamber Pressure - SAAMI Specs

And ALWAYS remember that the cartridge brass, is thinner and weaker than the barrel.

What takes many guns apart instantly, is a failure of the rear of case..

You allude to that case failure causes the chamber to rupture. No case can withstand chamber pressure without chamber support. Case failure can only contribute to chamber rupture, not cause it. Yes, a cartridge case does reduce the overall chamber end stress experienced by the bolt, but that partial relief is actually quite small in stress percentage, as this help is caused by the cartridge wall to chamber wall sticktion under chamber pressure. Chamber failure cannot ever occur because of case failure. Chamber failures usually occur because the experienced pressure exceeded the design limits of the chamber or bolt.

imported_brian_m · Jan 10, 2016

Chamber failure itself may not be caused by case failure but the case failure itself will almost certainly cause considerable damage to the firearm and likely the shooter. All regular gunsmiths have seen this, blown out magazines, split stocks or grips, destroyed trigger mechanisms, blown out extractors and ejectors, etc etc. Now imagine this failure taking place at a chamber pressure over 100,000 psi.

The OP is talking about trying to contain a pressure of 150,000 to 200,000 psi. This may be possible in an industrial pressure vessel or in a specially designed test breech and barrel assembly but it is going to be a tricky task in any practical firearm barrel and we should not be giving a novice encouragement to attempt these pressures without warning against all of the practical failure modes.

What this boils down to is that if these pressures were possibly the major firearms manufacturers would be using them.

GGaskill · Jan 10, 2016

Having fixed the thin wall/thick wall stress calculator to accept 6 digit pressures, it is interesting to note that as you increase wall thickness, the stress approaches the chamber pressure. So with very thick walls, a chamber pressure of 150KSI is going to have an inner surface stress of 150KSI, which is much too close to the maximum material strength and probably in excess of the yield strength. Not a good idea.

johansen · Jan 10, 2016

You will need two or three shrink fit tubes to achieve 200kpsi with 100kpsi steel. This technique was used to make brass and cast iron cannons, should be easy to find books detailing the math needed to calculate the interference fit required.

Alpacca Fortyfive · Jan 10, 2016

There are several parts to answering the op's question.

First the question itself, yes a thick walled cylinder can be designed that will resist those sorts of pressures.

The resistance to failure of the inner surface of that cylinder depends upon the three dimensional relationship of the stresses. Look up von Mises failure criterion, but be warned that you'll need to seek help from someone experienced in stress modelling.

There is then the question of how you are going to load, seal and fire the cylinder.

As previous posters have noted, conventional case heads and primers do not handle more than about 65k psi reliably.

Thirdly, what are you going to use for a projectile and how are you going to stabilise it? At the pressures you are thinking about, conventional bullet cores will be deforming plastically and rifling will not last for very many shots. aerodynamic stabilisation in the form of fins or diabolo type wasp waisted slugs, comes with major increases in drag, compared to spin stabilised projectiles.

So expect either very short accurate life of the very heavy barrel, or to loose any gains in muzzle velocity, very rapidly due to increased aerodynamic drag.

Does metal thickness correlate with maximum PSI pressure?

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