347 Stainless steel Sheet metal qualification WPS

[falco]

This might be unsual and I will appreciate any opinion

We were qualifying a mmanual GTAW WPS to weld stainless steel sheet metal 0.040" thick. We use about 40 A and a travel speed range between 2-4 in/min. Gas flow used was about 20 CFH and we use also backing gas. Electrode used was 0.035" diameter and we use a 1/16 Thorium electrod as well as a size #8 cup, root opening was 0.020"
The weld is done in one single pass, The appearance of the weld was pretty good, fairly o no sign of dicolorations, no crack indications were found inboth sides (root and face). It has a good visually appearance.
When we took the radiograph there was a fine dark line (somtimes straight somtimes a little weavy) indication that appears in the center of the weld. in some coupos it is along the complete weld and in some others no.
We ask the welder to re-weld taking care of its speed as well as of its weaving motion. but the indication continously appears in the radiograph
We decide to test the weld, We cut the coupons and we remove excess reinforcement. Weld succesfully pass bend testing (root and face) and tensile testing it brokes beyon 104 Ksi in the weld but it didn't broke in the center of the weld but almost along its toe within the weld metal
We also cut soem coupons each 10 cm and we polish and perform chemical etch using ferric chloride and nitric acid as specified in AWS b2.1 and no sign of lack of fusions were detected.
So either we are performing an incorrect etching process or it might be some metallurgical condition I am not aware that is deviating the Xrays beam and givig the indication

In case someone has face a similar case, what might be wrong?

Will appreciate a lot any comments

 

[fegenbush]

What is the source for the radiography? If you are using iridium, you might be lacking detail that would be evident with selenium or actual x-rays. The sharper picture along with the (slightly) increased exposure time might tell you if you are getting an odd shadow or refraction in the film.

 

[falco]

Actually is X rays, we shot it with 70 kV 1.4 min using a D4 film. The film is very defined. We increase the kV and actually adjust the time and the indication still appears

 

[stanweld]

If visually, under magnification, the weld is sound at the same locations X-rayed, I would consider the X-ray indication as not being indicative of a rejectable flaw.

 

[vanci]

Perhaps a solidification/ fusion line?. By stating "no crack indications were found" did you perform visual inspection or dye penetrant inspection?

 

[vanci]

Perhaps a solidification/ fusion line?. By stating "no crack indications were found" did you perform visual inspection or dye penetrant inspection? If the coupon passes mechanical testing, the weld is most likely sound. Lastly, you could attempt a side bend then dye check the cross section of the weld to see if any "discontinuities" have opened up.

 

[EdStainless]

stabilized SS (321 and 347) are notorious for weld center-line indications.
We often see them, no DP indications, nothing on ET or UT, but visible lines.
If you do a micro across the weld you will see the center has different grain structure and a different size/distribution of stabilizing precipitates. I think that this is what you are seeing.

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Plymouth Tube

 

[falco]

Thanks guys,
Actually we do the Dye check using fluorscent and it was ok, even we use a 20X magnifying and there were no indications.
Ed, are you aware if there is any available and approved reference to support your point? We actually deal with auditors and people that are not familiarized with Technical specialized issues and base their judgment on what is "written", will do a micro test, Would you consider that this grain distribution on centerline may cause any issues with fatigue cracking or stress corrosion when it is in service?
thanks a lot I highly appreciate your points!
best regards

 

[Guest102023]

EdStainless, if you are talking about segregation, then the low travel speed might exacerbate the problem.

I have no experience on these stabilized SS alloys, but could it be welded without filler (assuming tight fit-up can be consistently achieved)?

 

[gtaw]

Did you note a centerline crease along the length of the weld in the “as welded condition”, i.e., with the face reinforcement intact? The slight depression can appear on the radiograph when the base metal is thin.

Solidification initiates along the edges of the weld and progresses toward the weld centerline. Assuming the sheet metal was held securely in a fixture while it is welded, it will not contract freely in the lateral direction as the weld solidified and undergoes shrinkage due to the phase change from the liquid to solid state takes place. The depression along the centerline can be seen with the naked eye. If my memory serves me correctly, it seems the problem is exacerbated when the travel speed increases.

Try to radiograph the weld after the face and root reinforcement is removed flush with the adjacent base metal surfaces. The line should not reappear if it is due to shrinkage.

The presence of low melting point constituents (LMPC) can manifest themselves along the centerline of the weld as a result of solute rejection. However, LMPCs usually result in centerline cracks. That being the case, I do not suspect LMPC as being the root cause of your problem.


Best regards - Al

 

[falco]

Well You may see the attached picture, thereis actually a staright line in the center but there are no sign of depressions unless they are very fine and due to film sensitivity it can be shown on the radiograph.
Will do the micro as well as the X rays inspection when removing the reinforcement! I will let you know thanks that really helps