SStl Pipe, SStl (SW + Bell End) Fittings, Magnetic Weld Material

[racookpe1978]

Doesn't seem right.

I've field-checked several 6" and smaller lines that have been pre-fabricated and staged (but not installed yet), and all the pipe material is non-magnetic. All the fittings that I've checked are non-magnetic. Spec calls for Stainless steel.

But the filler material is magnetic. Doesn't seem right that the filler material should attract a magnet that way.

 

[unclesyd]

If you mean that a magnet will be lightly attracted to the deposit what you are finding isn't unusual when welding Austenitic SS even if using the proper electrode. The magnetic characteristics of the weld metal are determined by the amount of Ferrite in the deposit which is determined primarily by the chemistry of the electrode. Ferrite can and is controlled in some piping specs.
If this is a very strong attraction before you go any further if possible you need to check the electrodes used in the fabrication of the pipe spools in case of a mixup.

Can you come back with the materials and welding process as each can contribute to the ammount of Ferrite present in a weld deposit.

I would also check with the client to check whether Ferrite is permissible in the inteneded service.

Also the intended application with details.

 

[racookpe1978]

The (most recent) filler material used by recent welders (GTAW wire) is stamped 316L. Earlier? Don't know for certain.

Fittings are mostly socket weld and RFSO. Reading the stamps and etchings, they are a mix of both 316 and 304 SStl.

 

[unclesyd]

316/316L have tendency to form Ferrite when welded. i would be surprised if a 316/316L weld wasn't slightly magnetic if there were no controls on the amount allowed by the piping spec. If the chemistry of the pipe is such you can get magnetic weld metal with an autogenous weld.
Don't do this on your pipe but if you heat magnetic material to rosy red and cool the magnetic properties go away.
You will also see magnetic properties on cold worked Austenitic SS.

If you can get the MTR report on the welding electrodes it might have a FN number listed. If there is no FN anything goes. There is also a relatively inexpensive Ferrite meter that will give you a ballpark number, generally acceptable, for any weld.

 

[EdStainless]

The welds should be somewhat magnetic. You need some ferrite in the welds to help prevent hot cracking.

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Plymouth Tube

 

[racookpe1978]

That's what they "feel" like - the magnetic tip on my scribe is drawn to the weld - never the pipe or fitting - but it is not "strongly" attracted to it like I see when I touch conventional black iron fittings, or regular CS bolts.

Service is demineralized cooling water to the generator, moderate temps (well under 120 degrees) at modest pressures.

 

[unclesyd]

You should have no problems. Our piping has bee in service for more that 50 years with no problem. The only difference is that we don't have any socket welds.

We have literally miles of SS piping carrying Demin and pure water that run all over the site. It was all welded and I don't think there was any concern about the welding other than meeting the piping specs.

 

[moltenmetal]

I'll add to the chorus: ferrite in these welds is a normal occurrence. Shocked me the first time I detected it too, but it's never a problem.

 

[EdStainless]

I am more concerned about the use of socket welds. If this system ever becomes contaminated (biological) there is no way to clean these areas out. The crevices also are a great place for corrosion to start. Butt welds are preferred in clean service.

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Plymouth Tube

 

[racookpe1978]

Generator cooling water: Filtered, demineralized pure water at around 65 - 70 psig, 85 - 115 F.

While biologics are not exactly irrelevant, they are not a concern so long as pure water flows when the generator is at power.