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"Dual Phase" Steel Gas Cylinder

10/18/2017 12:51 PM

"Dual Phase" steel gas cylinder according to prEN10338.

At the level of dome of the half-shell forming the upper part of the bottle which is provided with a hole and welding a flange for receiving the gas distribution valve. This welding operation has the consequence of subjecting the material to high temperature constraints. In short, substantially at the level of this welding of the collar, an area that has been thermally affected is distinguished.

The tests demonstrated that it was in this thermally affected area of the dome that breakages of these DP steel cylinders occurred, whether during burst tests or fatigue tests.

This is not the case for conventional steel cylinders, for which the rupture occurs at the level of the circular welding joining the half shells.

Thank you for sharing solutions or proposals, knowing that DP steel cylinders do not undergo heat treatment.

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#1

Re: "Dual Phase" Steel Gas Cylinder

10/18/2017 1:24 PM

Pictures would be immensely helpful for us to understand the problem.

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#2

Re: "Dual Phase" Steel Gas Cylinder

10/19/2017 4:14 AM

Good morning everyone,

Thank you to find here enclosed pictures of normal rupture and breakage of DP steel cylinder.

Greetings.

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#3
In reply to #2

Re: "Dual Phase" Steel Gas Cylinder

10/19/2017 5:46 AM

As a former user of gas cylinders I am intrigued by your testing processes. The "cylinder" on the right, which presumably was once in the shape of a cylinder, is now tending to the spherical, suggesting that your test pressures are way above normal service pressures.

Testing to destruction may serve a purpose, but does it matter which way a cylinder ruptures when exposed to grossly excessive pressures? Does it matter which way the cylinder is constructed if they fail at the same pressure?

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#5
In reply to #3

Re: "Dual Phase" Steel Gas Cylinder

10/19/2017 9:28 AM

Dear All, These are the destructive tests required by the construction standard ISO 4706 and EN 1442 which stipulates that the rupture must reach the circular weld of the pressed parts at a pressure greater than 67 bar with a volumetric variation greater than 20%. Greetings.

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#6
In reply to #5

Re: "Dual Phase" Steel Gas Cylinder

10/19/2017 9:42 AM

Well, they've still failed.

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#7
In reply to #5

Re: "Dual Phase" Steel Gas Cylinder

10/19/2017 9:46 AM

Well, the rupture has reached the circular weld. Does that make the test a success or a failure?

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#8
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Re: "Dual Phase" Steel Gas Cylinder

10/19/2017 9:49 AM

<...Does that make the...> failure <...a success or a failure?...>!

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#12
In reply to #8

Re: "Dual Phase" Steel Gas Cylinder

10/19/2017 1:18 PM

Indeed. I would like to know what the text of ISO 4706 actually says

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#13
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Re: "Dual Phase" Steel Gas Cylinder

10/19/2017 1:43 PM

https://www.iso.org/standard/39102.html

There you go, I googled that for you.

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#15
In reply to #13

Re: "Dual Phase" Steel Gas Cylinder

10/19/2017 4:54 PM

Alas, that is not the full text, merely the invitation to spend CHF 138 for the pdf file.

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#4
In reply to #2

Re: "Dual Phase" Steel Gas Cylinder

10/19/2017 9:24 AM

That's not a test. The vessel has burst, so it is only good for weighing-in, as scrap metal.

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#9

Re: "Dual Phase" Steel Gas Cylinder

10/19/2017 10:58 AM

This is most likely caused (for the dual-phase steel) by transformation of the crystal phases in the HAZ during welding. No amount of annealing will correct it, as this has been changed to single phase, in my humble opinion.

This explains why the failure mode is predominantly at the fixture weld.

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#10

Re: "Dual Phase" Steel Gas Cylinder

10/19/2017 12:08 PM

Welds and their heat affected zones are always highly stressed, usually with some areas with residual stresses at the yield strength of the material. Some methods, such as vibro-lax mechanically vibrate the material while the weld solidifies and claim to reduce the residual stress levels.

In my experience, the only way to get residual stresses down is to either normalize the assembly or anneal the assembly after welding to relieve the residual stress. Preheating the weld joint before welding can also reduce residual stress, but it will not eliminate it.

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#11
In reply to #10

Re: "Dual Phase" Steel Gas Cylinder

10/19/2017 12:44 PM

Yes, but I think they losing the dual phase form in the HAZ to begin with?

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#14
In reply to #11

Re: "Dual Phase" Steel Gas Cylinder

10/19/2017 3:09 PM

The dual phase results from a heat treat process on the steel, though I don't know if it is done before or after forming of the cylinder. The process involves quenching an alloyed low carbon steel to form martensite islands in a ferrite structure. Cleanest would be to heat treat after welding, but perhaps not possible due to geometry.

Second choice would be to heavy up the metal in the heat affected zone to accommodate the loss of material properties from weld heat, but that makes the forming process more difficult and expensive.

In any case, the original material properties and microstructure are effectively gone in the HAZ and the benefits of using dual phase steel are lost as far as making a light weight cylinder. You could potentially reduce the extent of the HAZ with TIG, plasma or laser welding, but the process and fitup requirements may end up pricing the fabrication out of reach.

After some additional thought, it might be possible to make a joint arrangement where the top ring overlaps the HAZ and acts as backing for the area when under pressure. Then the HAZ becomes a backed diaphragm with some of the force transferred to the backing ring.

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#16
In reply to #14

Re: "Dual Phase" Steel Gas Cylinder

10/20/2017 12:36 PM

Very good!

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