SS 321H Material Welding Joints PWHT Necessity

[sahsanb]

Dear Experts,

I am reaching out to you to seek your expert advice regarding our 14” OD & 12.7 mm thk. Process Air Line (SS 321 H metallurgy) operating at 550 oC (1022oF) which we intend to replace due to multiple Stress Relaxation related cracking related leakages in the past. Root cause of Stress Relaxation Cracking was identified to be non-compliance to WPS (high heat input) at the time of original construction ( i.e. year 2010).

We are planning to replace our Process Air Line in our upcoming Plant Turnaround and are proceeding with making below mentioned improvements as per Licensor guidelines.

• Use 16-8-2 welding consumable for welding. Previously ER347 was used at the time of original construction.
• Use piping material of grain size of ASTM 3.5 or finer
• Use piping material with Ti/C> 8 and Ti /(C+N) > 7.3
• Ensure Ferrite number is below 5 at welding joint
• Ensure hardness reading is below 200HV at welding joint
• Minimize welding heat input and use stringer bead welding technique
• Weld joint weld capping flushing with pipe base metal to avoid any stress raisers

Additionally, licensor has also recommended carrying out PWHT at 850 o C (1562oF). While we are making arrangements to comply with all aforementioned precautions, we are struggling with necessity and potential drawbacks of carrying out PWHT of filed joints. New SS 321H piping material complies with aforementioned relevant requirements and has also undergone solution annealing and stabilization heat treatment at the foundry.

We would be grateful if you can share your expert view of weather there is any merit in carrying out PWHT and do the intended benefits outweigh the potential drawbacks as we fear that PWHT may result in thermal cracks in the welding joint during PWHT activity.

 

[EdStainless]

If you were to see cracks after PWHT (actually just stabilization) then they were there before and you just didn't see them.
The concern is that the weld and HAZ will not be stabilized and at your temp there is a chance that they will sensitize first.

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P.E. Metallurgy, consulting work welcomed

 

[sahsanb]

Dear EdStainless,

Thank you very much for your feedback. While we agree with intended benefits of PWHT, we are struggling with the fact that owing to field constraints implementing PWHT as per intent will remain a challenge and there is a possibility that we may end up with multiple other issues. My humble submission is, considering the material is Titanium stabilized, is sensitization still a big enough concern?

Regards,

 

[kingnero]

The above mentioned list of precautions seems rather odd and random to me.
I would put most effort in ensuring you have a good grip on the chem. composition of the base metals, and qualify a welding procedure with, in addition to the standard tests, the necessary microscopic analysis to ensure the quality of the weld metal.

 

[EdStainless]

When you are at the lower end of the sensitization range for these alloys the Cr carbide formation in the grain boundaries is likely to progress faster than the Ti carbide formation.
Sensitization isn't the formation of Cr carbides in the grain boundaries, but the local depletion of Cr along the carbides that have formed.
And at the lower temperatures the diffusion rate of Cr will be low.
The result is that if any carbides form you will have significant local Cr depletion.
If you were operating 100-150C hotter I wouldn't worry about this, but your temp is at the lower end of the range.

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P.E. Metallurgy, consulting work welcomed

 

[sahsanb]

Thank you very much gentleman for your feedback. We will take this into account while finalizing our execution strategy. Appreciate if you can recommend some literature to study the subject in more detail.

 

[Rubje]

@sahsanb

"Root cause of Stress Relaxation Cracking was identified to be non-compliance to WPS (high heat input) at the time of original construction ( i.e. year 2010)"

Did the damage mechanism start shortly after the equipment was installed?

 

[sahsanb]

Rubje,

First leakage was observed after 06 years of operation in 2016.