Dear all,
I hope someone could help me about the following:

I have SS321 tubes with OD 25.4 mm (1") and thk 2,11mm (0.083") which shall be bent as "U" tubes.

The requirement of NACE Standard RP0170-2004 and Client (Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Acid Stress Corrosion Cracking During Shutdown of Refinery Equipment ) claims the additional stabilizing heat treatment @900°C for 2-4 hours.

This would be an additional requirement to the ANNEALING HEAT TREATMENT usually perfomed by the mills @1050°C approx..

My questions / doubts / concerns are:

1) Since we are talking about thin tubes, 2-4 hours time seems to be much. Could half an hour be sufficient for the involved thickness??

2) As said, the tubes will be bent to hairpins by cold working. After bending the tubes should usually be solution annealing treated by resistors at 1050°C for few seconds (15 sec. approx.) .
Could this cold working and solution annealing affect the stabilizing treatment??

3) Someone says that, after bending, the solution annealing at 1050°C for few seconds only and that this does not negatively affect the stabilizing and does not compromise the performed stabilizing.
Other source think that, after bending, the "U" tubes shall receive the stabilizing heat treatment for approx. 20/30 minutes.
Which one is wright??

4) the tubes will be welded to the tubesheet in SS321. There is NO concerns during the welding of the tubes since the ends of tubes do not receive any treatment or cold working.
The concern is about the U bends / HAZ after the cold working and solution annealing.

Have you faced this kind of problem in the past??

What do you suggest to do??

awaiting for your precious help I thank you everybody in advance.

Regards

GENGI