Precipitates in 316 Stainless Steel

[Spainman007]

I am a welding engineer wrestling with an undetermined weld process problem involving 316L stainless steel.
Our company runs many mechanized longitudinal seam welders, GTAW-based, for the creation of custom tubes, tanks, etc.

We are intermittently experiencing what appear to be precipitates of some sort in autogenous, mechanized GTAW welds in 316L SS, .048” thick.
We are NOT experiencing this with 304 stainless of the same thickness, joint design, run the same day/time, etc.
Relative Humidity is @ 48%, temperature around 75-80F.

I have not done any metallographic studies but would welcome input on suspected causes. We have material certifications for this 316 material and am wondering if there might be a particular constituent percentage, that, while within spec limits, is something we may want to limit. The indication presents as a lifted (~.005-.010") "pad." The overall size of the defect in the first photo is approximately 1/16". Two photos are attached.
Any feedback is greatly appreciated.

Spainman007

 

[EdStainless]

These are on the welds?
They are slag, either from the shielding, or surface, or the interior of the 316.
Because of the Mo in 316 it is more sensitive than 304.

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P.E. Metallurgy, Plymouth Tube

 

[Spainman007]

EdStainless:

Yes, they present on the face of the weld. We are using certified materials that are very well cleaned and prepared, so I am again looking for a culprit constituent; possibly more than one. You noted molybdenum making the weldment more sensitive. Do you mean it's more sensitive to slag formation because of the moly? Some of the research/application papers I have been reading indicate that some of the very low percentage constituents, when at their limit extremes, can increase the potential for voids, inclusions, and cracks. We see this intermittently, so I am hoping to identify the specific constituent or constituents and work with our suppliers to bring in materials within logical limits. Any additional input is appreciated.

 

[metengr]

This looks more like process related versus material. The GTAW torch arc length is varying.

 

[EdStainless]

I would feel that it is more likely weld method, gases, or surface cleanliness.
For it to be internal then you would have to have some real crap steel.
What is the S, Al, Cu, Ca, .... of which only S will be reported.

What you have isn't a void or precipitate in the structure. It is purely a surface anomaly.
Some surface conditions make the material almost impossible to clean. If it was pickled and not rinsed well there will be traces of Fl on it and you can not remove it.

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P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

This document might be worth a gander:

http://www.swagelok.com.br/assets/cat/eng/Orbital Welding in Critical Systems.pdf