Weld P91 to P22 2

[PMCap]

Does anybody have any experience welding P91 material to P22. I have heavy wall P22 nozzles that are to be welded to P91 pipe. Should an interface buttering material be used? Any other recommendations? thanks

 

[metengr]

PMCap;
What kind of service? Normally, for this type of dissimilar metal weld (DMW) you want to locate it in a low stress region to avoid bending stress in the P91 base metal heat affected zone (Type 4 cracking).

The DMW can be deposited using either 9Cr-1Mo filler metal or 2.25 Cr-1Mo filler metal. No buttering is required because PWHT will be required for Grade 91 and P22. So, you need to carefully control the PWHT temperature to ensure adequate tempering for the Grade 91, and not overshoot the lower critical transformation temperature for the P22 material. I would suggest using a 1400-1425 deg F for PWHT. Monitoring of this PWHT is critical. This material is unlike other Cr-Mo materials.

Also, after welding the Grade 91 make sure you allow this weld to cool to below 200 deg F before PWHT. Use low hydrogen consumables.

 

[PMCap]

Service if steam -4000 psi 1050 F (fossil power main steam piping). Question was generic and I did not provide you the exact configuration at the weld. The weld configuration would be a full penetration full circumferential corner joint reinforced with an appropriately tapered fillet weld. I would be welding a P91 material to an existing P22 pipe. This would result in some local secondary bending (Q) stresses at the weld joint location due to transfer of the P22 pipe bending moment loadings into the P91 component through the attachment weld joint at the discontinuity.

Design and analysis of the weld joint configuration would be by finite element methods (to the satisfaction of B31.1 - 2010 edition, paragraph 104.7) to assure that all code primary and secondary stress limits were satisfied.

Provided, Code stress criteria are satisfied and the appropriate precautions are taken regarding welding and PWHT, do you see any Code or long term service issues that need be considered.

Thanks

 

[metengr]

Yes, I do see a real material issue. At 1050 deg F the P22 is marginal material in terms of allowable stresses for creep strength. As a metallurgist I would not recommend welding a creep strength enhanced ferritic steel (much stronger in creep strength), to an existing lower creep strength material using a corner joint design. This mismatch in creep strength will result in local creep damage or accelerated creep damage to an already marginal P22 pipe material.

Why is Grade 91 being considered when the weak link is the existing P22 pipe?

 

[PMCap]

Actual temperature is 1035 and pressure is 3690 psig. Existing pipe is P22 and was designed per B31.1. Operating has been about 200,000 hrs. Pipe remote from welds is sound and in good condition. However, due to differential creep of the pipe and the pipe tot pipe Girth welds, cracks have been found at the weld joint areas.

As the pipe remote from the weld joints is sound and only the weld joints are the problem we are investigating encapsulation of the weld joints using P91 material encapsulation components. We would remove any existing cracks and drill through the P22 encapsulated pipe - This would remove the encapsulated portion of the pipe from the pressure boundary. By equalizing the pressure on both sides of the encapsulated pipe it would totally remove circumferential pressure stress loading. The encapsulation component would be designed to take all pressure loading and system moment loadings. As the encapsulated portion of the pipe would remain, some load would still be transferred through it (although all analysis of the piping system with the encapsulation components and the encapsulation component itself would be performed considered that it did not exist). The low stress levels is the existing encapsulated P22 pipe would not provide for a condition that new cracks would form. - However, if this were a concern - a full 360 cut of the encapsulated pipe could be made to totally remove any loading on the encapsulated pipe.

Any thoughts/concerns/ideas on this would be greatly appreciated.

 

[metengr]

PMCap;
I gave this some serious thought, and if it we my steam line I would not use this repair method. Why? If you have creep damage at existing pipe to pipe girth welds, I would remove the entire girth weld and re-weld, PWHT and be done with it. Especially if the P22 pipe spool is in acceptable condition. I know that at some point all of my main steam line girth welds will fail because of the Type 4 , fine grain cracking along the base metal heat affected zone. However, my current plan is to eventually replace each entire girth weld with new material (as it becomes necessary) and leave the P22 remaining pipe spool.

Using creep strength enhanced ferritic steel to encapsulate creep damaged P22 girth welds sounds costly to me, and more importantly, one is setting up conditions for accelerated Type 4 cracking in the P91 attachment weld fine grained region. I would not allow this ECrepair method on any of my steam lines.

 

[MJCronin]

This is an excellent thread...much to be learned here.

I am sure that this P22 to P91 interface issue has come up many times in the past considering the steam line retrofits that have occured.

Metengr, should there be changes in the ASME code rules regarding the special metalurgy in these kinds of joints ?

 

[metengr]

MJCronin;
There is a fine line between writing Code to provide safe design and writing an engineering guideline that provides step by step guidance (aka consulting). The Code continues to evolve based on experience and examples of this includes Code Cases related to T23 material and later versions of Grade 91 where post forming has shown to significantly effect creep life. As we get more experience, Code Cases will be revised and eventually code revisions will happen.

At this point, I am unsure what we would change other than if you are working with creep strength enhanced ferritic steels that are enough cautionary statements in the code to raise alarms, and seek technical guidance.

 

[davefitz]

I agree with the comments by Meteng- the repair proposed by PMCap does not seem to be optimum solution in any pipeline case I can imagine, and it doesn't appear that the Code practice on creep strength enhanced steels is going to change anytime soon.

Given that the Code will not provide an all-encompassing guide for the design and use of these alloys, and not more than 1% of the practicing designers are likely to obtain a graduate degree in metallurgy, it would seem that ther is a need for a central database of (a) correct design and fabrication procedures for a range of standard piping applications, including typical specifications and (b) an anonymized listing of past errors.

Also of Note- ASME will be holding 2 sessions of a 3-day course on use of these alloys this year.

 

[metengr]

One document that has been recently made public is

EPRI Document 1023199 Guidelines and Specifications for High Reliability Fossil Power Plants, Best Practice Guideline for Manufacturing and Construction of Grade 91 Steel Components.

 

[stanweld]

metengr,
Is the referenced EPRI publication available for purchase by non EPRI members?

 

[metengr]

Yes. Go to the EPRI web site and I was informed at the recent Code meeting that the public can download it. It was decided to allow the general public access because of a safety concern.

 

[PMCap]

Gentlemen - Thanks to all of you who offered your experience on this issue.

I'm still investigating a way to remove/minimize loading on the existing degraded P22 to P22 girth welds by encapsulation as previous mentioned. There were concerns noted in making the dissimilar welds and additional concerns regarding cost.

Regarding cost, I believe the encapsulation method would be less expensive as the need to remove existing pipe (most of which is in good shape) and procure and install new pipe (which would be quite expensive.) would not be required. Additionally, encapsulation of the welds would save downtime as this method of restoration could be done in significantly less time than a wholesale pipe replacement.

However, I note your concerns and value your opinion on the issues regarding the welding of these 2 dissimilar metals and the potential for Type IV cracking.

I found a recent research paper dealing with using Inconel 625 to weld P91 to P22 which indicates favorable results.

http://www.material.chula.ac.th/Journal/v21-1/93-99 Tammasophon. N..pdf

Disregarding cost considerations (which I believe can be managed) would use of Inconel weld wire with appropriate PWHT result in a sound weld and alleviate your concerns?

Would other type material for the encapsulation component be preferable or perhaps some other combination of material and weld type that you may recommend?

Thanks in advance for looking into this issue for me.

 

[metengr]

PMCap;
I believe it would be far better to locally remove the creep damaged girth weld and re-weld this joint versus encapsulation. 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.

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.

 

[PMCap]

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?

 

[metengr]

If encapsulation and conventionl repair are compared and similar in costs, I would use P22 on P22 ecapsulation.

 

[PMCap]

Metengr -

Thanks so much for sharing your expertise and insight

 

[davefitz]

PMCap:
The use of inconel 625 weld filler on P91 ( buttered) has recently been a cause of concern due to its tendency to form an embrittled layer in the P91 parent metal adjacent to the HAZ after 6-10yrs service. If bending moments are applied to that weld, it may fail ; the total weld interface fails at once. It had been used for dissimilar metal welds ( P91 to 304 or 316), and failures of such a DMW in piping in the workspace has developed into a significant concern- for tubing inside the boiler such failures lead to outages and loss of availibility.

Granted, a T22 to T91 weld is not exposed to the high, alternating shear stress that a T91 to austenitic tube woud experience, but the high chrome leveles of the inconel 625 weld filler will yet remove carbon from the P91 parent metal and lead to embrittlement. Other alternative fillers could be considered.

 

[PMCap]

davefitz - thanks to you an al who have responded to this issue - looks like metengr's suggestion to use P22 all around is currently on the top of the list.

 

[Guest102023]

Excellent thread, gentlemen - thanks.