Welding Nonconformance-Preheat 1

[vesselrajj]

Guys, We are fabricating a carbon steel vessel of 38mm thickness(shell). As per WPS, the base metal has to be preheated to 100C minimum. Unfortunately the external pad to Shell welding has been carried without Preheating. This is the Non conformance. What is the recommended resolution for this NC? Is there any alternative other than removing the welded pads. Nearly 30 pads has been welded. I also want to add that the vessel will be subjected to full PWHT later.Code of construction is ASME SEC VIII DIV 1 but no stamping.

 

[metengr]

If the vessel will be subjected to PWHT, I would recommend as resolution to the nonconformance surface NDT of the affected (no preheat) locations on the shell before PWHT. If there are no cracks, the regions will be subjected to PWHT. Since preheat is used to control the rate of cooling to avoid harmful thermal gradients and to reduce the formation of harmful phase transformation products, which can result in cracks, if no cracks are detected the end result will be a PWHT weld and surrounding base material.

 

[DekDee]

vesselrajj,
As usual metengr has given good advice but you need to be very careful here.
The resolution to your NCR will be whatever your client is willing to accept - I think you need to submit a bit more technical back-up than what you can gain from an internet forum.
Regards,
DD

 

[vesselrajj]

Metengr
Thanks for your valuable advice.
Dekdee
Just for the case, let me assume you are my client. My proposal for this NCR is "Carrying out 100% LPT on the defected weld metal+HAZ before and after PWHT in presence of client to assure the conformity of weld. If any linear indication(crack) identified on anyone of the weld, then the defected pad will be removed and will be rewelded. In that case, Method statement will be prepared and will be submitted for client approval before any rework".Will you accept this?.
Regards,
Rajj

 

[DekDee]

Rajj,
I am sorry to say but if I was your client I would reject the whole vessel.
If you have external pads welded on how can you be sure there are no cracks emanating from the weld metal underneath the pads - conducting LPT will only confirm sound metal in areas you can visually see - what about areas hidden under the pads ?
I spent 4 months crawling around inside some brand new vertical pressure vessels (manufactured offshore) on an LNG project in NW Australia supervising crack removal.
100 mm (4") thick shell, all the shell welds were great. Then they welded brackets on for internal screens without preheat and there were literally thousands of cracks.
Good luck,
Cheers,
DD

 

[gtaw]

Removing the offending pads will not provide any assurance the undesirable microstructures that may have resulted from the absence of preheat will be removed.

If the PWHT is carried out above the transformation temperature, the microstructure can be controlled with appropriate cooling. However, a PWHT conducted below the transformation temperature may not be sufficient to eliminate the undesirable microstructures. At best, a PWHT conducted below the critical is essentially a tempering operation. It is better than a sharp stick in the eye, but the end result may be less than what the customer bargained for.

As DekDee mentioned, subsurface cracks will not be detected by penetrant testing. The subsurface cracks, should they exist, will not be magically healed by PWHT. They may become an issue after the vessel is placed into service.

An engineered solution may be possible. Perhaps the contractor responsible for fabrication could qualify a WPS without preheat to demonstrate or to validate the welding conducted without preheat is capable of producing an HAZ with hardness values low enough that underbead cracking is not an issue.

Since the vessel is not code stamped, the absence of preheat is a contractual violation and not a code violation. The intent of the preheat is to mitigate the probability of underbead cracking. The use of preheat provides everyone with a warm fuzzy feeling that everything is good. It is no warrantee delayed cracking will not happen. Likewise, the absence of preheat is no assurance delayed cracking will occur. A demonstration that the welding procedure used, i.e., no preheat, produces an HAZ with a low hardness value may be all that is necessary to ease the concern that underbead cracking is a potential problem and the vessel can be completed and placed into service.

If there is an lingering concern there may be underbead cracking, the contractor may further ease the Owner's concerns by using UT with a straight beam transduced to detect or to verify the absence of underbead cracks. An underbead crack would most likely lie in a plain parallel to the surface of the rolled plate. As long as the shell thickness is thicker than the near field of the transducer, cracks should be detectable from the inside of the vessel as a reflected signal that shifts to the left of the back reflector thereby indicating a difference in "thickness." That difference in thickness would be indicative of an underbead crack. It could also be indicative of a lamination, but that would be ruled out if the reflectors are relatively small areas limited to the HAZ of the weld.





Best regards - Al

 

[vesselrajj]

DekDee,
Thanks for sharing your experience.
Al,
Thank you very much for your explanation.So informative. we added UT in our corrective action and client accepted our proposal.

Cheers
Rajj

 

[gtaw]

That's good news for everyone involved. Excellent.


Best regards - Al