Metengr – Thanks for all the insight.
You said - I believe it would be far better to locally remove the creep damaged girth weld and re-weld this joint versus encapsulation.
Question: Done properly, I agree that this would be a sound restoration method. However, in removing the creep damaged girth weld and adjacent HAZ, there would be a resultant large gap between the remaining pipe sections that cannot be easily welded up without using a filler pup piece requiring 2 girth welds (yes – same as would be required with an encapsulation restoration). Also, as we are all aware – when you cut out the weld the pipe will most likely want to spring – thus requiring that the pipe be restrained locally to prevent movement and maintain alignment after cutting (no easy task as we are generally talking about large diameter main steam piping that is in the size range of say 20”OD with >3” wall thickness.
You said - Second, using Inconel filler metal does not remove the concerns related to Type IV cracking because this location is into the fine grained region of the Grade 91 base material.
Question: Your response seems appropriate. I assume that you are saying that Type IV cracking in the P91 or P22 HAZ would not be a concern when using P22 type filler metal or that using P22 weld metal would be less of a concern?
Question: The report I cited seemed to infer that use of Inconel 625 weld metal and with appropriate PWHT may result in less of a cracking concern than in using P22 weld metal. Did I read the report correctly or is this just not true?
You said - I really see no advantage to using Grade 91 because of the Type IV cracking concerns. I even thought of pre-fabrication of encapsulated weld joints in the shop and using P22 to P22 girth welds for installation in the field.
During shop fabrication you could weld the DMW by first buttering the P91, PWHT and complete the weld joint to Grade 22 using 2.25% Cr- 1%Mo filler metal. However, this shop cost and staging would exceed conventional weld repair using field machining, prepping and welding P22 to P22 weld joints.
Question: The shop prep method is a good idea that I have been giving consideration. I could make the encapsulation component from P22 material (I was thinking that a P91 encapsulation component would be lighter due to the increase strength of P91 material – only that location where the P91 were to be welded to the P22 would I need a greater thickness) – Your thoughts on using P22 for the encapsulation component or perhaps some other suitable material?
Question: Regarding your comment on cost consideration I believe that having to deal with the gaps resulting from cutting out the bad welds and HAZs (see above), the shop cost and staging + installation would be less or as a minimum a tradeoff (shop work is generally less costly than field work) and using the encapsulation method does not have the associated risk of not being able to maintain pipe alignment in the field. Again – if the cost considerations can be reconciled – your suggestion of buttering and PWHT in the shop would seem a good option to consider. Do you see any other concerns that I should be aware of or investigate?