This post is in reply to CRAIGB's comment in a thread "Extra Calculation" initiated by SAM on November 11, 2006. I am initiating a new thread rather than continuing on with a reply to "extra Calculation" thread because I feel that discussion on P91 piping is a worthwwhile topic that needs to be explored.

From CRAIGB's post, "There is always something new to learn about piping. Materials improve, or turn out to be useless (and, in the case of A335 P91, both)" . This suggests (no, it doesn't suggest, it states) that A335 P91 piping is an improved material and it is also useless !! With the resurgence of critical and supercritical boilers operating over 4000 psig and at 1050 degrees F, A335 P91 piping components are almost a necessity. At 1050 F, A335 P22 pipe has an allowable of 5700 psi. P91 pipe has an allowable of 12900 psi or 14000 psi, depending on thickness. At 4000 psig (or higher), pipe wall thickness could easily be in excess of 5.5 to 6" !! I understand and appreciate the complexities of designing and building with P91 pipe ... it is highly susceptible to proper pre-heat and PWHT temperatures. I would like the forum's comments and experiences in working with P91 pipe.

Pat LaPointe

Hi Pat,

I don't really think there is any question that the material is "good stuff". I think the only issues being open to discussion are the care needed to assure the quality of the fabrication welding and the PWHT. This is a material that demands attention to detail and close attention to the PWHT data recorder. It must not be overheated during the treatment.

http://www.steamforum.com/steamforum2005/_discstm/00001042.htm

I think we are also looking at how the material should be treated after hot or cold bending but there doesn't seem to be a consensus on that yet.

A very sophisticated material which gets its amazing qualities by tiny amounts of what to me is pixie dust and then made in an alchemists forge!!!!

A presentation I attended on fabrication indicated that even machine cutting the stuff can be problematic along with how much time a cut end is exposed to the shop floor!!!!


Useless is rather harsh term, but complex, delicate and prone to misfabrication are a given.... EPRI even has a paper out on how to fabricate because of the difficulties that people have had with it....

One mans opinion... btw I recomend staying away from newly started up systems for a couple days.... if there was mis-fabrication with any luck it should manifest itself rapidly....

The creep strength of the welds has been shown to be signficantly less than the base material. A Japanese paper at the last PVP Conference shows the welds joint strength reduction factors falling below the default values now provided in ASME B31.3.

Chuck,

Thanks for this update it is a lump of coal for us just before the Holidays... but better forewarned. Was the filler used in the paper a common filler used in P91 fab??

Does this mean we have to look at decreasing the current weld joint strength reduction factor in B31.3??

And finally what did B31.1 ever do on this nasty bit of stuff???

Go Browns!!!!

J.C.Luf

Regarding P91 material, many problems crop up due to our business as usual (BAU) approach we use for materials like P22 in general. If one takes proper care in design itself like use of P91 transition pieces for joining with high thickness P22 valve ends, introduce enough flexibilty in piping to have comfortable thermal expansion stress instead of usual thinking of fast creep relaxation and minimise the number of field welds by fabricating large piping spools, one can ensure safety.

When working with gold or watch, we need to be careful! With P91, we need to test the hardness of any weld whenever in doubt! Then only we can achieve the benefit of P91. This is why we still use P22 pipes of high thickness where we can't maintain high quality work.

regards,

sam

As the originator of this somewhat flippant comment (no surprise to those who know me), I would like to clarify that "useless" and "improved" are not necessarily in any sort of time sequence. The material can be both at once if you handle it imprecisely. P91, when used properly, is indeed a "magic bullet." But magic can be notoriously slippery stuff. And much of the currently installed P91 material in the world has been installed incorrectly.

Before we fully understood the enormous care that must be taken when handling P91, we just blindly went ahead, installed, and used the material with a less-than-necessary understanding of what we were doing. Some of the early P91 installations were handled with less care than we now consider prudent, resulting in accelerated creep damage and other nasty results.

See, for example, an article published in the Combined Cycle Journal, 1Q 2005, "Growing Experience with P91/T91 Forcing Essential Code Changes," by Jeffrey F. Henry at Alstom Power in conjunction with an ASME Task Group. (I will send a pdf of the article to Richard Ay so that COADE can post it for you all. See link below {R. Ay}. )

Some significant quotes from this paper:

"It is imperative for all industry participants to understand and comply with the new code requirements [defining proper heat treatment, chemical composition, contamination limits, cold working, and joint design] as they are published."

"Many in the industry are rightfully impressed with these mechanical properties [of P91], but they fail to grasp one fundamental principle: The superior properties of Grade 91 depend entirely on the creation, by heat treatment, of a precise condition of microstructure, and the maintenance of this microstructure throughout its service life."

"Failure to obtain this precise microstructure during original steel production, or any subsequent action that alters the microstructure of the steel - such as the hot bending, forging, and welding that regularly occurs during component fabrication, plant construction, and steam plant repairs - will seriously degrade the material's high-temperature properties."

"The sensitivity of the Grade 9 microstructure recently was demonstrated at two combined-cycle plants in the Southeast. ... Symptomatic of the industry's 'business as usual' nonchalance toward Grade 91, the heat-treratment procedure used by the plants' [installation] contractor was similar to the one they had used for decades with P22."

"Throughout virtually all of its history, the sole purpose of ASME Codes was to ensure safety - to prevent catastrophic failures and the resultant loss of life, injury, and property. Only in recent years have users come to view the codes as some sort of 'design and construction handbook,'..."

In summary:

1. P91, used correctly, can solve a lot of very difficult problems very economically.

2. P91, handled without extreme care, exhibits accelerated creep rates at the elevated temeperatures we like to use it at. Superheater tube failures have been observed in as little as 4 years of service.

3. The level of care required to handle this material properly is way beyond that of any other material that went before it. Even now, we may not have reached a full understanding of how to fabricate and install it properly. I am comfortable that the understanding we have today is sufficient for general use, but I doubt that anyone but the boldest could say that the gap between "sufficient" and "full" has been completely closed.

If you are going to use P91 for a project, please grill the prospective fabricators and installation contractors with all the TLC that Torquemada used on heretics in the Spanish inquisition. Make absolutely sure that (a) you know how to handle the stuff, (b) your owner is aware of the material's special installation and in-service inspection requirements, and (c) the fabricator and installer have convinced you that they are aware of them too. Joe's Welding is most likely not going to be able to do the job, and your owner is very likely to want to use Joe because his hourly rate is so low.

Yep and based on Chucks post more and other interesting details are forthcoming....

Most designers unwisely ignore weldments, the insidious part of this is that as Chuck has pointed out the long term creep properties of the weldments in this material are far less hence weaker than the base metal as it concerns creep strength.

Previously it was demonstrated by test that its fatigue strength is no greater than perhaps carbon steel... (That is why B31.3 placed a limit on Sc and Sh of 20KSI max) so we are left with a metal which has significant strength for only a portion of the loading a piping system undergoes... and more importantly each and every weldment must be considered as being significantly lower in strength than the base metal when the system is operated above ~ 800F or so (creep range or time dependent range).

So while P91 is not useless it is very special and when I think about some of the people in this forum using this material in designs, versus the simple types of questions they pose that illustrate a lack of knowledge on their part. As well as in some cases no desire on their part to increase their knowledge through self-learning…. I shudder at the end results that may occur!

I view the B31.3 weld joint factors in the creep regime as a reasonable approximation of very complex behavior and not by any means a lower bound of the creep performance of all possible combinations of weld material and base material. As such, I don't think adjustment to the factors now in the code is necessary. If you are working with P91 seam welded pipe, it would probably be prudent to review the Japanese paper and determine if you should do something different as part of your engineering design. The paper has extensive creep test results, based on which they developed P91 specific weld joint strength reduction factors.

Chuck - which paper is that? I have searched the PVP 2006 brochure and can find several candidates. Since the program for the conference is 96 pages, it's a needle in a haystack.

The paper is PVP2006-ICPVT-11-93350

its conclusion statement is...

"COCNCLUSION
Creep rupture data of 370 points for welded joint of Gr.91
steel offered from seven Japanese companies .... Weld strength
reduction factors for design for 100,000 hours were determined
as 0.75 at 600℃ and 0.70 at 650℃ for Gr.91 steel. The master
curve for life evaluation of welded joints of Gr.91 steel was
developed."

Thanks, John. I believe the paper recommends weld joint efficiencies of 1.0 up to 550 C (1022 F) and then joint efficiences as you cite for temperatures in degrees C.

Yes....

However the designer needs to read the paper fully and become involved with the actual welding processes to make sure that whatever creep weld joint strength factor they use is apropriate otherwise one must use the code soley.....

I have heard that in case of P91 it is very difficult to control hardness in H2S environment and there were reported problems in use of P91 in H2S environments and that is why use of P91 is typically confined to power plants and not to refinery.

Any comments on that?

Regards

Please Check in the following:

NACE Standard MR0103-2003, “Materials Resistant to Sulfide Stress Cracking in Corrosive Petroleum Refining Environments,” Houston, Texas: NACE.
2 NACE Standard MR0175-2003, “Metals for Sulfide Stress Cracking and Stress Corrosion Cracking Resistance in Sour Oilfield Environments,” Houston, Texas: NACE.

But, the best source could be our material suppliers, although sometimes they provide biased information like pharma companies do to doctors.

regards,

sam

Is there any corresponding weld joint strength reduction factor in B 31.1 like one in B 31.3:2004 ? I don't have access to latest B 31.1 code at this moment, so I seek this help / clarification from our mentors here.

regards,

sam

No... not yet....

Sir,

Thank you for reply. Now, I expect a 'Yes/No' type answer from our mentors on the following qiestion -


In most of the process plants nowadays captive power plants are provided which are governed by ASME B 31.1. If a pipe is having B31.3/B31.1 interface with a design temperature above 550C, what will the designer do ? Will he voluntarily use weld joint strength reduction factor or wait for B 31.1 to get revized?

regards,

sam

Ahh Sam well that’s the heart of the question now isn't it??? Legally the interface is covered by both codes and the more stringent of the two should be used.

But the real questions are....

Q1: Why does B31.3 and B31.1 differ???

Q2: Should a weld joint strength reduction factor be used despite the lack of requirement by B31.1?

Q3: Will B31.1 ever address this issue?

A1: Both B31 design task groups were presented the same information by the Mechanical Design Committee at the same time. The codification process requires a broad consensus by the committee members; one or two individuals can stalemate a proposed revision. Also the two committees perhaps operate on different set of priorities. So two sets of different people operate differently, time will tell on this. But I find it a bitter piece of Irony that the piping system failure and deaths that created the need for the factor occurred not in a process plant but rather in a Power House.

A2: This is an issue that must be addressed by the owner and the designer prior to the start of work. The B31 codes do not prohibit any one from going above and beyond the written measures. But these decisions will have a real affect on the construction costs borne in the end by the owner.

A3: Beats me I am so busy between work, and tending to B31.3 I do not attend B31.1 meetings, so as to what the status of this issue is unknown to me.

P91 piping .... you've got to wonder about this material.

In a November 15th post, Chuck notes that the creep strength of welds in P91 pipe is significantly less than in the base metal. This is supported in a November 2002 EPRI report ("Performance Review of P/T91 Steels).

CraigB's November 16th post has quotes from a Combined Cycle Journal 1Q, 2005 that raises the hair on the back of my neck (..... excellent properties of P91 pipe .... depend on creating and maintaining a precise microstructure ... and failure to maintain this microstructure will degrade the material's high temperature properties ...) WOW

I gather from John's post of November 16th that a piping system is only as good as its weakest link; in this case the weldments. I wholeheartedly agree.

What I find disturbing in all of this is that there is little long term data (information) of the behaviour of P91 weldments. There is very little information beyond 150,000 hours and the EPRI report that I mention above is full of examples of P91 weldments that have failed at less than 100,000 hours !! The report emphasizes over and over the importance of weld pre-heat and weld PWHT. It is also suggested (however impractical that this may be) to maintain this pre-heat until the weld has been completed and heat treated. If this can't be done, the weld has to be kept dry, clean and warm (winter construction on the Canadian plains !!).

There is also mention about Type IV cracking in circumferential welds. Type IV cracks occur at the outer edge of the weld HAZ. Cracks are more prevalent in this area and there is less than 100,000 hours of data about this phenomena.. The report goes on mention weld strength reduction factors to safeguard against Type IV cracking.

So with all of this as background and with the many excellent replies in this thread, what is a designer suppose to do ?

I personally will be specifying seamless P91 pipe with tight specification requirements about welding QA, weld pre-heat, PWHT, handling of pipe spools etc ... I will apply the weld strength reduction factor to axial stresses at circumferential welds and locate these welds in the pipe layout in areas where the bending moment is lower rather than higher. I also plan on insisting that the contractor have a full time professional metallurgist on staff that intuitively understands all of the complexities and nuances of handlilng and working with P91 pipe.

Thanks for your replies,

Pat LaPointe

Patrick...

You have made an excellent start... minimize welded attachments such as lugs, trunnions etc. Make sure that any welded attachment material is very close to the base metal.... make sure that your design stresses in the local shell of welded attachments are no greater than Sh*strength reduction factor....

The fabricator who put on the presentation I attended basically tries to have the entire spool constantly in production such that it has all welds taken care of in order and no weld is left untended nor is any prepped end left untended

Its not a useless material, BUT woe unto anyone that thinks they have plain ordinary A106 GrB...

Space age materials require space age fabrication!

Here is a related thread ... "Pipe stamped as P91, when it isn't."

http://www.eng-tips.com/viewthread.cfm?qid=176859&page=1