Slag in autogenous orbital welds 316L tubing?

[scott1971]

A borescope video log of the welds in a 316L sanitary tubing system was sent to a third-party inspector and they have rejected 17 welds due to "slag." Based on my limited understanding of common welding practices, I thought slag was the result of using a filler material or welding dissimilar materials. The welding we use in my industry is normally automatic orbital butt-fusion (GTAW/TIG, no filler). Is slag even possible in this type of weld? If not, what could the inspector be mistaking for slag?

 

[weldtek]

Slag as you infer, is normally associated with slag bearing welding processes. The only possibility that comes to my mind is silicon islands, which have in the past been confused with slag. Depending on the silicon level in the base material, this may, or may not be the cause of the indications reported.

 

[metengr]

I doubt that it is slag based on the stated welding process. I would suspect if the inspector is rejecting the welds based on a visual inspection of the pipe ID surface, it is probably surface porosity or suck back in the weld root.

 

[stanweld]

While the slag call is no doubt incorrect and since this was a visual examination of the ID surface, I would suspect sugarring or incomplete penetration in addition to the flaws as stated by metengr.

 

[EdStainless]

The slag that they are referring to are islands of non-metallic (oxides and such) material that collect on the surface of the welds.
These are usually a result of cleanliness issues with the tubes and/or the purging.

By the way, I would like a name other than slag for this too, but it what is commonly used.

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[unclesyd]

In autogenous welds, MIG, and TIG where no chemical flux was involved I always called it glass or glass slag.

The beads of glass had a very similar chemical composition to beads formed by trying to weld a high silicon material.

 

[Capntom]

We GTAW sch 40 and 80 pipe all the time per MIL-278. Are you back filling the ID of the tube with Argon?? If not, I would start there. If the molten pool penetrates to the ID and does not have a protective environment it will look like slag to the Inspector. Could be classified as Burn through, melt through or excessive root reinforcement.

 

[tab1999]

Most Inspection firms that I have dealt with in the past call this glassy phenomenon a type of"dross". Its may result of improper cleaning. If you clean the interior and exterior with a sort of scotchbrite that meets the D18 Sanitary Surface RA finish and follow with an alcohol solvent and a Certified grade of Argon 99.9999%. It should help. There are numerous other requirements that assist in making the inside appearance a brite-silver or a less than 3 on the AWS D18.2 color chart.
Making the purge exit hole the same size as the inlet and minimizing the gas input after verifying the Quality of purge.
( I was just reviewing these old notes and thought I'd share some of my experiences.)
note: Sugar is very distinctive w/stainless steels.
Make sure that your 3rd party Inspection company is reputable in Sanitary work. (Not new to it)

 

[scott1971]

Thank you to everyone for your input. As an update, it turns out there never was any slag or other imperfections. We inspected the welds optically with sight tubes and they were clean. Lens and lighting effects from the borescope created the illusion of imperfections. A good experience for the future should any more of my customers site faulty welds on the videotape.