Welds sweating

[weeeds]

We have a carbon steel vacuum tank with stainless steel and carbon steel connections. Under a high vacuum condition we have witnessed the carbon-carbon welds and HAZ "sweating". The stainless-carbon welds do not exhibit this same phenomenon. The welds are fillet only and FCAW. Has anyone had a similar experience? What could be causing the sweat?
Thank you.

 

[unclesyd]

What is the process material in the tank and what temperature does it operate at?

 

[weeeds]

unclesyd
The tank is empty. Is is simply under high vacuum, ~50 microns, and is sitting at room temperature.

 

[unclesyd]

A little different than my original thoughts.

Could this be cooling from expansion at a nozzle entering the tank? If not too hot feel the line and tank for a quick comparison.

Adding to this is the tank normally cooler that the surrounding area?


To check for any physical problems a quick test during a period of sweating is to wipe the sweat off and quickly apply some Dye Penetrant Developer or if not available some chalk dust like that used in chalk line.

 

[Goahead]

Could you perform leak testing with a sensitive method like Mass Spectrometer testing while Helium is flown on the exterior of the tank in correspondence with the sweating spots?

http://www.welding-advisers.com/

 

[weeeds]

We performed a Helium Leak Test at the outset and found no leaks.
Things have evolved somewhat in that I was told the welds in question were actually phisically colder than the surrounding metal. This would explain the localized condensation. As things stand right now the sweating has stopped and we have reached some kind of steady state condition.
So now my question has changes to "what would cause carbon steel welds, subject to full vacuum on one side, to be phisically colder than the surrounding material?"

 

[pecidar]

I'm just a lowly BSME (not a PHD or anything so I'm sure someone will find fault in my theory here) just using common sense I think you are seeing a thermal conductivity issue. I don't think this has anything to do with the welds exactly but rather with the fact that the weld creates a thicker cross section than the nearby base metal of the tank wall. When creating a pressure differential between outside and inside of tank, there must also be a temperature differential going along with that between the outside and inside of tank. Thermal conducticuty and heat transfer are based on (affected by) material thickness. I would suspect that the condensation and temperature difference is only noticeble for short period after applying vacuum (or any outside temp changes) and eventually it goes away as the temperature gradients stabilize and reach equilibrium.
(Just a thought.......maybe ????)

 

[weeeds]

Thank you for your comments pecidar.
You may be quite right as the phenomenon has not returned. I suppose that the HAZ would be also affected by the colder weld metal.
We will be building more of these tanks so I will keep my eyes open and let you know if similar issues come up.