P91 joints left as-welded for a long period: how to proceed? 4

[afocuf]

I am involved with a project where the plant owner 'lost sight' of the site welding contractor for a few months (a wise man used to say: "you get what you inspect, not what you expect!).

As a result, hundreds of P91 welds (1.2 to 2in thick) have been left as-welded for months in an uncontrolled environment and I guess exposed to the elements. I am aware of the risks associated with as-welded P91 (SCC, hydrogen cracking). The plant owner asked how they should inspect the joints to detect any damage occurred, check the microstructure and decide whether they can proceed to PWHT or reject the joints.

In my opinion, if any SCC has developed this should be detectable by surface inspection (PT or MT). Hydrogen cracks may be embedded, so volumetric inspection should also be applied.

The owner asked if they should also do replicas, to verify that the microstructure is fully martensitic, but I do not see how it would not be 100% martensitic, as the joints have been cooled to ambient temperature after welding.

Any suggestions will be appreciated.

 

[Mr168]

I think you're on the right track with your approach, with phased array UT being the preferred volumetric exam. Your limiting factor will likely be what the owner wants to spend. With any luck, a bakeout was performed prior to dropping preheat altogether.

Early in my career, I had a similar situation with a fabricator inexperienced with CSEF's. Not only did the welds sit for months outdoors before PWHT, but they were even lifted from the prefab area and rigged into place before the PWHT was performed in-situ (including 30+ foot spools). NDE confirmed the absence of cracking, and it became a non-issue with no problems during service.

 

[metengr]

Absolutely agree with the surface NDT as a minimum, especially using wet fluorescent MT. Replica would be a waste of time. Plus, the microstructure for Grade 91 is not the easiest to view. If anything the risk of SCC will be driven by hardness of the microstructure (greater than 35 HRC).

 

[weldstan]

Also had similar experience to that of Mr168. Performed surface WFMT and RT prior to PWHT. Did not have access to competent PAUT provider at the time. Fortunately all welds were subjected to dehydrogenation treatment and welds proved sound after subsequent RT after PWHT.

Did not have the same luck with some P22 welds. 11 of 12 welds revealed delayed cracking with RT performed prior to final PWHT after being welded 9 m0s prior. Original RT of welds performed immediately after welding proved sound.

 

[davefitz]

Your NDT plan looks correct. It appears that the risk of SCC is more problematic if the joints are exposed to condensation of water in environments that have sulphides or chorides in the air, such as indoor shop air that is heated by natural gas with odorants, or outdoors near industrial sites with sulphides or chlorides in the air. I recall one site in central Florida that had field welded all P91 mainsteam piping outdoors and left it unprotected in the elements for 6 months prior to final PWHT ( wih many rainstorms) and there were no SCC issues.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[EdStainless]

In this case you will want to re-inspect after PWHT also (at least a sample).
As has been said, if they cooled gently from the weld you are fairly safe.
Are you really worried about the creep properties? If so then some micros are in order.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

EdStainless
Metallography can give useful information, but CSEF alloys can fail prematurely even when all composition, NDE and mechanical property indicators are normal and within spec. There may be damage mechanisms going on that cannot be detected by any kind of physical examination or testing (short of fully destroying the part). With CSEF steels it is imperative to know the full history of every raw material input and of how every weld was made, and be able to apply metallurgical knowledge as well as Code rules, to be able to do a meaningful risk assessment. For example, PWHT hold temperature must be tailored to composition.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."