Stainless Steel Tanks?

[GDCB]

Anybody make a stainless steel tank? If not why. Seems like a no brainer so what am I missing. For background I'm a retired industrial fabricator so I know stainless costs more. But we made just about anything from it and mild steel or aluminum. Damn durable stuff.

Carl

 

[rjack321]

None that are DOT approved. Frabrication costs would be extremely high, you'd probably be looking at $1k a tank (roughly 4 times 3AA steel)

 

[GDCB]

Why so much? Isn't it just a robot welded pipe with an end and a fitting? If the aluminum can take the pressure I'm thinking stainless would be simple. The material IS expensive and labor should be the same. Stainless is generally expensive to work with in a custom application but a production item should be similar to mild steel.
Carl

 

[spectrum]

Dive tanks are a one piece item. Made by a punch/die and or spinning process from a single plate or billet. The material needs to be maleable enough to work well in such a process.

There are some welded cylinders around (military origin I belive) and I have read here that they don't have a good reputation.

I belive a chrome molly is the common steel alloy.

Pete

 

[GDCB]

Any engineers out there want to tackle this one. My brother is a ME but we like to argue stupid stuff. Ha Ha


Carl

 

[meesier42]

This is why, SS has a failure mode, known as Chloride Stress Corrosion Cracking.

The standard 304/304L and 316/316L grades are most susceptible. Increasing nickel content above 18 to 20% or the use of duplex, or ferritic stainless steels improves resistance to CSCC. High residual or applied stresses, temperatures above 65-71C (150-160F) and chlorides increase the likelihood of CSCC. Crevices and wet/dry locations such as liquid vapor interfaces and wet insulation are particularly likely to initiate CSCC in susceptible alloys. Initiation may occur in several weeks, in 1-2 years or after 7-10 years in service.

Oh, and I am pretty sure that most tanks are Carbon Steel, not CroMoly

 

[ChrisA]

Anybody make a stainless steel tank? If not why.

The "standard answers" to your question is that..

1) The strenght to weight ratio os SS is not as good as chrommolly steel so the tanks would weigh to much. Anyone have numbers?

2) SS workability is not so good. Steel tanks are seamless and are made by the "deep draw" methood where a disk of metal is heated and then pushed through a round die with a ram, it then cools (very slowly in an oven) and then it is re-heated an pushed again each time through the process getting deeper. Large tanks take up to 12 cycles through different size dies and rams. Finally the top is heated and the tank is spun and they close the top. Final step is to machine the end

3) How much more durable do you need? Steel tanks have a 50+ year service life. How much more would a diver pay for a tank that lasts 100+ years?

4) even the best stainless will rust if exposed to saltwater and there is no oxigen. For example if there is a small crack or crevise. Stainless is only stainless because of it's oxide coating. Just like aluminum really.

I think #1 is the "killer" I want my tank to be about one pound negative in saltwater when it is empty. You have to think about "trim" and weight distribution when a diver is swimming under water. a 60 pound tank just would not work.

OK so you were n the bussines?? Maybe I'm wrong. Can you compute what wall thickness of 316 stainless steel tank would be required for a tank that could be DOD approved and expected to pass a hydro test every five years for 50+ years. I've been told that you can't make a tank like this. But I'm not the expert.

One more question: Why not a titainium tank? My gues is #2 above.

 

[GDCB]

Dive tanks are a one piece item. Made by a punch/die and or spinning process from a single plate or billet. The material needs to be maleable enough to work well in such a process.

There are some welded cylinders around (militay origin I belive) and I have read here that they don't have a good reputation.

I belive a chrome molly is the common steel alloy.

Pete

Thanks Pete. I didn't know that. I know 4130 is tough to work with and is usually resistant to the elements. I wonder if the same or similar tooling would work in 304/316 stainless?

Some of the welding cylinders I've used are very old pre WWII and used very hard. Banged around quite a bit. Still being regularly filled to 3k. In fact most industrial cylinders are charged the majority of the time. Fatigue isn't a big issue. Are scuba cylinders made so much different or are they held to higher standard?

Carl

 

[ChrisA]

This is why, SS has a failure mode, known as Chloride Stress Corrosion Cracking.

Oh, and I am pretty sure that most tanks are Carbon Steel, not CroMoly

I think PST's new e-series tanks are chromoly. I think this ishow they were able to keep the wall thickness the same but increses the working pressurs from 2400 to 3440. Note that the specs are the same from the old LP and the new E tanks all except for the working pressure. I was told they are simply using stronger steel.

That failure mode youu pointed out is easier to explain if you know that it is a very thin layer of chome oxide that provides the protection. This layer forms if the stainless steel is exposed to O2. in a small crevise sea water and salt can block the air and no O2 is in contact there. Once a crack forms it can rust through very fast.

 

[rjack321]

Go down and buy a bolt, any bolt.
SS costs roughly 4x carbon steels. A tank won't be any different.

3A tanks are carbon steel, 3AA are chromemoly steel. Modern steel scuba cylinders are either 3AA or exemption chrome moly (e.g. pst 9791).

Their are some foreign titanium cylinders, not legal for use in commerce in the US.

Edit: there are no carbon steel scuba cylinders being made. all steels currently in use are chrome moly. The steels are different between 2400 and 3442 working pressure cylinders - oddly enough the latter has more carbon.