I am concerned that aluminium alloy 6061 pressure vessels are being subjected to a paint drying process after their hydrostatic test, possibly reaching a temperature of 140 degrees C? My questions are:

Is the pressure test invalidated?

Is the integrity of the vessel affected by the temperature?

Comments gratefully received.

Thanks Ron,

Dave

Thanks Bob,

Dave

Precipitation hardening temperature for a typical 6061 Al alloy is at 160 deg Celsius.
I would say perilously close to your 140 deg Celsius paint curing temperature.
You will need to seek professional advice on this one I suspect.
My gut feeling is that it will undertake metallurgical changes.
You should be able to assess this by destructive testing of 2 samples to get an initial feel.
One prior to paint curing and one after, polished and etched samples should show any structural changes, also you would then have two sacrificial bodies and you could undertake perhaps a tensile and impact test on them.
A good materials lab should be able to do this for you easily and provide a report on any changes.

Paul has given an excellent answer. I would suggest a check on a sample of material before and after the curing process. A hardness test is probably the simplest indicator of whether the ageing has been compromised.

How long ageing takes is strongly dependent on the temperature and the alloy. In this case we are concerned about over-ageing, where the strength can drop sharply. The majority of the strength in the weld region is likely to be lost during welding, and is recovered by fresh heat treatment.

Whilst not wishing to condemn without the full details may I ask why a company is undertaking such work without the competence re metallurgy to hand?