SpudFiles

It was meant as something of a joke...I do not understand Romanian.

Good luck, play safe.

Al

Welds are a risk. Firstly, the weld itself is an unknown quantity. Next is the alloying of the base material(s) and any filler used, which leaves another unknown. Hydrogen absorption into the weld puddle is another risk. Without subsequent heat treatments (for which the welds themselves pretty much need to be a known quantity), there are differing crystalline structures of metal, and embedded stresses, from the weld bead, the heat affected zone, and the base metal(s). Certain blends of stainless also work harden, so anything that's been cold worked, drilled, tapped and the like could also be a source of problems.

This does not, of course, mean that any welded join will instantly explode. However, it is why, for example, welded tube is downrated by greater than 50% in pressure rating. And when you're pressurising with a compressible fluid, you're making a potential bomb. Yeah, I know, shooting vegetables at high speed isn't overly safe either, but there's no point taking unnecessary and potentially fatal risks.

The only way to be sure is to test, and the easy way to do that safely is by hydrostatic testing (which also has the advantage that you don't need to pump for hours, incompressible fluids go up in pressure *fast*). As gipetto says, test to well above the pressure you want to use. 5/3 sounds reasonable.

I just want to note that a proper weld joint is MUCH stronger than any threaded arrangement. Note only that but a proper weld is stronger than the base metal in most cases. The primary concern is most certainly HAZ and proper execution. The chamber on my air rifle is a section of 3/4 hydraulic pipe that i chopped one end off and tig welded the head of a grade 8 bolt in the end. If you trust your ability you will be fine.

PracticallySane wrote: ↑

Tue Dec 29, 2020 5:42 pm

I just want to note that a proper weld joint is MUCH stronger than any threaded arrangement. Note only that but a proper weld is stronger than the base metal in most cases. The primary concern is most certainly HAZ and proper execution. The chamber on my air rifle is a section of 3/4 hydraulic pipe that i chopped one end off and tig welded the head of a grade 8 bolt in the end. If you trust your ability you will be fine.

may share a photo, is it stainless ?

If I had the money, I'd make 3 of every cannon I have and test two to failure under different conditions. That way I'd really know the safety factor in a practical way and discover the weak spots and failure mode. Unfortunately, you could buy a nice car for what all that would cost, so it is what it is. Build strong and use conservatively.

Moonbogg wrote: ↑

Wed Dec 30, 2020 3:16 am

If I had the money, I'd make 3 of every cannon I have and test two to failure under different conditions. That way I'd really know the safety factor in a practical way and discover the weak spots and failure mode. Unfortunately, you could buy a nice car for what all that would cost, so it is what it is. Build strong and use conservatively.

I follow his same philosophy, and up to now I have my two eyes and my ten fingers.

I dont know if you can make it out properly but this is a picture I have on hand.

The pipe is stainless, I am not sure what alloy...

Works fine for me. I have been up to 3500 so far. I did not do a hydrostatic test.

Different strokes I guess.


Moonbog;

The theoretical fail point and "weakest link" are reasonably easy to calculate before construction. I do hydrotest "unique" assemblies, and in some cases will go beyond 5/3 MWP, but do not test to destruction, never saw the need on something where I am the one "using it not abusing it". If lawyers and insurance companies could become involved...yeah...CYA.


Al

^ I'm just curious to know what would really happen. Maybe one cannon fails like you'd expect, but a duplicate cannon fails in a totally different way. The only way to really *know* is to destroy not just one, but several of them at least. Repeated testing to failure is necessary to satisfy my desire to know.

hectmarr wrote: ↑

Wed Dec 30, 2020 5:18 am

I follow his same philosophy, and up to now I have my two eyes and my ten fingers.

I didn't used to, but I have had a few close shaves, and each time you realize how lucky you were and do things differently next time.

Apart from being sure of your pressurized fittings, one thing that applies to everyone is use hearing protection! Hearing damage is cumulative, and once it's gone it's gone.