Luis H:
Your first sentence has a little bit of truth to it, but everything else is totally incorrect!
For starters I have two sets of European double tanks sets with tapered pipe thread an no burst disc. The Europeans have never seemed that as concern about fire danger and SCUBA tanks.
The aluminum alloy strength is compromised at a very low temperature (I am fairly certain below that of Teflon). Steel is a bit better, but not great.
Even a compromised O-ring seal will not relieve the pressure as you are hoping since it will not be extruded.
I don't have the time to get more into it, but your statements are IMO dangerous.
IMHO, it is OK to take a risk, but do it knowingly. Don't kid yourself with a false sense of security.
The failure would not be by extrusion but by degradation at temperature which can lead to a completely different failure mode.
Neoprene and nitrile O-rings have a service temp of 160-180F, Buna-N is 250F, Silicone and Viton is 400F. From past experience I know that at about 100F, or less, above the service temp these rubbers will either soften/melt, harden and crack, or coke. Any of the three will lessen, if not eliminate, the sealing properties
As the worst case is a full tank in a fire we will only talk about instantaneous heating of the metal. This eliminates the alloy age effects on aluminum alloys which is quite low at 200-250F for a period of time. Usually 1.5 to 2.5 hours for usage such as in tanks.
So, will the o-ring degrade prior to the alloy losing its ability to withstand the internal pressures? For aluminum that creep temp is between 600F and 1,000F. For steel it will be between 1000F and 1400F+ depending on the alloy.
The problem will be the race between the increase in internal pressure, creep strength of the alloy, and degradation of the o-ring. I think the o-ring will lose. As you say "The Europeans have never seemed that as concern about fire danger and SCUBA tanks" which would seem to support this opinion.
PS, no tank should ever be used after being in a fire.