6351 question.

[broncobowsher]

In another thread someone was stating that the 6351 had issues with cyclical loading, how many times the tank is pressurized and depressurized. After time the tanks cracks and fails. Add drama about it blowing up on the person filling it.

My understanding is the issue is from sustained load cracking, which sounds like having it filled all the time.

Not that I have any 6351, nor will I ever pick any up. I am just trying to clarify what the failure mode really is. Cyclical or sustained loading? One of us has it wrong. I have been proven wrong before.

 

[Wookie]

SLC is caused, as you say, by filling and leaving them filled. Although primarily noticed in Scuba cylinders, it is observed in all pressure vessels under load for a prolonged time. And it isn't just gas cylinders or aluminum. In the aerospace industry it is called sustained load strain, and it occurs in many titanium alloys. Welding titanium is a bugger, the best way is to heat the base metals very hot before welding and let it all cool off together. Much like many aluminum welding techniques.

 

[runsongas]

sustained load cracking

Sustained-load cracking (SLC) in ruptured scuba cylinder made from 6351 aluminum alloy

Sustained load cracking in aluminium cylinders manufactured from aluminium alloys HE30/AA6082 and AA6351 – IMCA

 

[Compressor]

Nice discussion above; it begs the question: should one fill cylinders close to time needed to dive?
Answer seems to be no.

Many dive centers not to mention other facilities have tanks full at all times. This includes their cascade systems. I am not aware of tanks exploding.

 

[Wookie]

This includes their cascade systems. I am not aware of tanks exploding.

A steel cascade cylinder is so massive and strong in comparison to an aluminum scuba cylinder that the comparison does not really exist.

 

[Compressor]

Thank you @Wookie.

 

[Wookie]

To answer your other question, 6061 does not seem to be subject to SLC at scuba pressures. There are other alloys of aluminum that are. Scuba Cylinders were not made from the alternate alloy.

Steel pressure vessels of high strength that contain hydrogen are subject to SLC. Those cylinders are no longer authorized for storage of hydrogen. Steel cylinders are still authorized, but it's a lower strength steel, which would suffer a ductile failure instead of a brittle failure. Of course, the filure of 6351 aluminum is a slow brittle failure, if there is such a thing.

 

[Bob DBF]

Of course, the filure of 6351 aluminum is a slow brittle failure, if there is such a thing.

More like, the aluminum slowly gets brittle and ultimately results in a failure.

 

[rjack321]

Of course, the filure of 6351 aluminum is a slow brittle failure, if there is such a thing.

SLC in 6351 scuba tanks is partially blamed on the lead content in this particular alloy. Essentially the lead molecules were breaking away from the rest of the molecular structure one atom at a time under load.

 

[OTF]

Good info here. I believe I was the poster in the other thread you were referring to, and I was incorrect that it was a cycling fatigue issue. Thanks for the links on sustained load cracking. I'll go add an edit to my post to avoid confusing future readers.