Concern about discoloration i.w.o thin Invar weldments 2

[MotoGP]

Greetings to all erudite Material Engineers:

One part of my assigned duties takes me to a welding shop where welders seek qualification to participate in the manual, plasma-arc welding of thin (1.2mm thick) Invar sheets that are utilized for the primary containment of cargo on liquefied gas carriers. The gap between the Invar sheets, prior to welding, is to be from 0mm-to-0.3mm. Acceptable weld-depth penetration is determined as being from 0.6mm to fully penetrating the Invar membrane sheets, and the bead crown should maintain a 4mm-to-6mm mm width.

The welds are visually inspected, "PT"'d, and also cut apart so that macro-etch testing specimens can be harvested and subsequently fondled-over.

All-in-all, the only area that I cannot seem to find any joy in is the subjective interpretation of the discoloration that I find present on both sides of the Invar "membrane" test specimens. There presently exists no written guidelines on my project assignment that deal with this issue. As such, should I consider it NOT to be an issue?

The discoloration that I have noticed reveals as a range from none-at-all, to areas of dark brown/dark purple/black on some specimens. The latter is what I am most concerned about. Discolored areas are noted as residing parallel to the weldment toe, and, quite often, seems to "bleed" (sorry, I do not know the correct language to be used here)in way of the surrounding base metal, as much as 10mm (or, so I have catalogued). On the reverse side of the welded Invar, discoloration never seems to go beyond a color similar to that of brown sugar.

Again, the parameters of the weld are:
Thickness of assembly: 1.2mm
Welding Current: 40 - 60 A
Arc Voltage: 7 - 10 V
Argon (shield) Flow: 7 - 10 litres/min
Welding Speed: 140 - 200 mm/min

The material will be exposed to LNG most of its intended 40-year life...temp's o/a -160 deg. C...only areas in the upper part of the tank will see the dynamic loads imposed by "sloshing".

In short, does discoloration forebode disaster? If so, under what criterion?

Thanks very much for your time and consideration of my enquiry.

 

[unclesyd]

Invar oxidizes very easily and doesn't like to be overheated. The Invar will spatter if overheated and readily oxidize. Invar is considered a difficult material to easily weld.

It seems that your are also getting oxidation due to the lack of shielding. A contributing factor could be that the welder is dragging the torch or hoovering over the weld bead.
I would work on both aspects, especially the shielding.

We made a lot of Invar-36/316 SS welded parts and we quite successful using GMAW with welders that were certified to weld Titanium.

 

[EdStainless]

Disaster, probably not, but it isn't good.
Shinney or a clear yellow-to-straw color are what you want on these welds. Anything darker indicates excessive oxidation and the possiblity of poor weld properties.

Cleaning, gas quality, gas coverage are all issues.

Since Alloy 36 rusts so easily I would be worried about these locations. Even short term exposure to humid air could result in rusting of the heavily oxidized areas.

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Corrosion never sleeps, but it can be managed.

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