Forming SS Pipe

[Canyonman44]

Hi Folks. I was hoping to get some info on forming SS pipe at home. I want to use seamless SS pipe to form pulsejet engine shapes without welding. I guess my welding isn't great as they always fail at or near the welds near the constriction. TIA, Ken

[nova_robotics]

There are a whole bunch of things that could be going on. Are you married to stainless? Reheat cracking in heat affected zones of 300 series is a thing. This isn't my field, but stainless probably wouldn't be my first choice. If you reeeeeeally want stainless then 309 or 310 might be an option.

Have you tried any post weld heat treatment to relieve stresses in the heat affected zone? This might not work on some stainless. Again, not my field.

Which stainless are you using? Is the material very thick? Thinner might be better where huge temperature gradients are involved.

[CanBeDone]

. . . as they always fail at or near the welds near the constriction. TIA, Ken

My first reaction: fatigue failure due to chromium carbide precipitation?
Questions to be asked: is the SS magnetic? If yes, it's a ferritic steel, and welding it is for experts only - amateurs like me will only make a mess of it.
Is it non-magnetic? If yes, the most common SS would be SAE 304 - too much carbon (C=0.08% max) unless it isn't the much rarer SAE 304L (C=0.03% max). Both are readily weldable, but again, the higher carbon content opens up the risk of chromium carbide forming. Whether this is due to your welding (quick with a low heat input is good, slow, with the concomitant wide heat affected zone, is bad) or because your constriction gets red hot anyway, and is shaken severely by the the valve flutter, what does it matter? Poor toughness at red-hot temperatures is not good for long life, and coarse precipitates at high temperatures just means that. So, unless you can exclude carbide embrittlement, yours is the wrong choice of steel, and probably not your welding skills.

My understanding is that you need a sizable amount of molybdenum in your steel to reduce high temperature embrittlement. SAE 304 /304L may contain up to 0.75% Mo, but that is not mandatory. Consider SAE 317L instead. Without having access to testing fatigue limits at red-hot temperatures, that is what I would opt for, with C=0.03% max and Mo=3.00..4.00% that sounds much more appropriate.
I would also be cautious on forming your constriction by forging, cold or hot, as the subsequent heat treatment can, but not must, lead to grain coarsening and carbide formation, unless you have a metallurgist, and the appropriate laboratory facilities, at your back with experience of the particular steel you are heat treating. I am not, and thus I have to stay general. And I would not touch SAE 309 or nSAE 310 either - too much carbon.

[NortonDommi]

I have little knowledge of welding SS except for a few exhaust systems and they were back purged while welding. As to forming depending on thickness you may be able to use hydraulic pressure or if you know a guy explosives?

[CanBeDone]

I see, I will have to take you by the hand and manoevre you through the reef. My standard problem - I assume too much engineering / science background. So, let's first establish whether my suspicion is right. Can you provide a photo of the failed pulsejet? Fatigue failures are quite obvious if you know what to look out for.
The second question: is your pulsejet magnetic, or is it not?
Third question: do you have access to nitric acid? I would also like to see a photo of what a few drops do to your pulsejet, that would give me an idea of whether a sizable amount of molybdenum is present or not.
Fourth question: what do you use to weld? TIG, MIG, Stick, Acetylene? Argon? CO₂? From your answer, I think I can assume you are not familiar with pre-heating welds, or what a heat-affected zone is?
The above give me an idea of what went wrong, and thus in which direction to move. But if you happen to have a test certificate of your SS, that would even be better . . .

[piper184]

What diameter and wall thickness of pipe are you planning on using?
Also, what stainless formula/number?
How much constriction are you looking to accomplish?
I've seen metal-smiths do some amazing things with thin material, bordering between art and voo-doo. Depending on what you want to accomplish and what you are working with it might be possible to do by hand with some homemade dies/tools.
Please post some pictures or drawings of what you are going for.