Stainless sensitisation 2

[dbsmith]

Can anyone explain the circumstances in which PWHT of 321 SS is appropriate? A customer has recently asked my shop to prepare a WPS for welding SA-312 TP321 pipe to itself and apply 1100 deg F PWHT for 1 hour. This seems like a recipie for carbide precipitation to me. The base metal thickness is 0.432", welded with GTA root and SMA fill and cap, using 321 rod. My stainless weld prcoedures are usually designed to keep the heat input as low as possible and keep the interpass temp are 375 F. The customers primary concern seems to be ductility in the as-welded condition, but I believe the proposed "cure" is going to make things worse.

 

[unclesyd]

A PWHT of 321 SS will do nothing for the weld ductility. 321 SS is not hardenable by thermal treatment. In fact the weld area is probably a little more ductile. It would not be detrimental for a one time excursion, only an added cost.
We made thousands of butt welds in 321 SS piping fabricating serpentine heat exchangers that operate at 600°F. We had a number of 321 SS storage tanks and associated piping.

There was never any thought of PWHT.

 

[metengr]

I have come across 321H ss in boiler SH tubing applications and for some reason I have also seen others specify a stress relief heat treatment on welded attachments. The concern over sensitization is mute because Grade 321ss(a lower carbon version of 321H) is stablizied by titanium or niobium additions. Therefore, it should not be sensitive to carbide precipitation by heating in the 800 deg F to 1500 deg F temperature range.

I would agree with unclesyd. I see no real benefit for the cost and time related to stress relieve unless you have specific concerns over residual stresses from welding related to the performance of the component under ambient temperature service. Keep in mind that at elevated temperature service any residual stresses will diminish.

 

[dbsmith]

Thank you both for the quick replies. I'll use this to try to help prevent the customer from throwing his money away.

 

[GRoberts]

Do you think the part will see any sort of fatigue or SCC (stress corrosion cracking) service? Those are types of service where residual stress can do damage.

 

[dbsmith]

The application is above ground piping carrying SO2 vapor. It is a relatively low pressure system, and not subject to cycling, so it is not likely to be affected by fatigue.

 

[PAN]

dbsmith & GRoberts,
Is this for prevention of SCC by polythionic acid?

 

[dbsmith]

Attack by polythionic acid is a consideration, and I believe the 321 was specified due to its resistence to SCC. The operating temperature will be 820 F, which is in the range that service related sensitization can occur. Since placing this post originally, I have obtained further information from the pipe manufacturer, Allegheny Ludlum Steel. They recommend applying a stabilizing anneal process for maximum corrosion resistence in 321. The anneal is applied at 1550 f to 1650 F for up to 5 hours depending on thickness (the thickness I am using is 1-3/8"). This restores the chromium to solid solution, and combines the carbon with the titanium to form carbides. A descaling treatment is necessary after applying the anneal.