PWHT to Reverse Sensitization?

[tc7]

We have several weld procedure for austenitic stainlesses and they do not require either a high preheat or any PWHT. We do however take measures to monitor interpass at 300 deg F max and control welders heat input by reminding them to stay within listed amps and travel speeds (monitoring from afar).

We have a piping job coming up for a seawater application, I don't know what operating temps will be. But my question here is with regard to recommendations for PWHT. I am aware that austenitics are vulnerable to intergranular attack and that carbide precipitation as a result of welding can be a cause.

Apparently any application for my other weld procedures did not cause sufficient IGA or corrosion to be brought to my attention. But I am worried that this harsher environment (sea water) may need some new procedures to ammend sensitization issues.

Will be GTAW welding the following combinations:
316L to 321 using 316L or 347 wire;
316L to 316L using 316L wire;
316 to 316 using 304, 308 or 316 wire;
321 to 321 using 347 wire;


Any recommendations on PWHT for this application?
Thanks

 

[metengr]

NONE - no PWHT.

 

[EdStainless]

Any PWHT short of full solution anneal will only make matters worse.

I hope that you are handling cold seawater, because the alloys that you have listed will not tolerate seawater exposure. Biological activity, scaling, silt, or fouling can lead to rapid crevice corrosion in these alloys.

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Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[SJones]

If you are worried about any liability for subsequent 'defects', you may like to flag up that the alloy combinations you are being asked to weld are not the most ideal for the service!

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[tc7]

I believe service temps may fluctuate between 75-90 deg F on the low side to probably around 175-200 deg F on the high side. The process which generates the heat is of short duration, typically lasting only ~90 seconds but perhaps holding at 150 deg F for longer periods of 5-10 minutes.

With regrets, I have to say I didn't specify these materials. If I were asked for recommendations I may have chosen a Cu-Ni alloy. Perhaps initial cost is what drove the designers to 316L or 324. Given the situation, I wan't to be sure our shop doesn't cause or exacerbate other service problems.


Ed-
To your point that anything short of full solution anneal may make matters worse, when I asked for advice on PWHT, the real question on my mind was if a full solution anneal was warrented even though the low carbon "L" version of 316 and the niobium & titanium additions to 321 and 347 alloys are intended to resist interganular attack or other stress corrosion possibilities.

 

[EdStainless]

Don't worry. Full solution anneal and pickle will not help service life. It will be short.
At those temps they might be able to get away with 2205, but for extended service you need either a superaustenitic, superferritic or 70/30 Cu/Ni.

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Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net