×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

New ASME Code case for P91/T91 ???

New ASME Code case for P91/T91 ???

New ASME Code case for P91/T91 ???

(OP)
to all...

Can someone describe the new code case I have heard so much about for heat treatment of ASME P91/T91 components ?

How will this affect the market for new high temperature boilers/HRSGs and how might it affect possible future litigation that I sense will occur in the near future ?

Any "war stories", tall tales, secrets or even rumors that can be shared ?

-My opinion only....

-MJC

  

RE: New ASME Code case for P91/T91 ???

Yes. The new ASME Code Case deals with an upper limit for PWHT of Grade 91 material to avoid over tempering or forming new martensite by exceeding the lower critical transformation temperature. The upper temperature limit is 1470 deg F.

In the event of exceeding the maximum temperature, the component would require either re-heat treatment or if it is a localized problem, the affected material is removed and replaced with material that is properly heat treated. I listed the Code Case number a few weeks ago, but I don’t have access to it at the moment.


I see no impact on market conditions.

No affect on the market. The reputable manufactures that know the pitfalls of Grade 91 will continue to treat this material with respect and buyers that want to cut corners by using in-experienced fabricators will continue to do so. Litigation? Go ask your friendly lawyer.

RE: New ASME Code case for P91/T91 ???

metengr,
Have they taken into account the %Ni + %Mn content of base metal and weld filler metal with regard to the 1470F max PWHT Temp? Will they be addressing heat treatment after bending? If not they ought to; just one man's opinion.
 

RE: New ASME Code case for P91/T91 ???

stanweld;
I didn't have a copy of the Code Case in front of me at the time I posted. I have a copy, here are the specifics related to revisions to Table PW-39;

Note 3). If % Ni + Mn content is less than 1.5%, but greater than or equal to 1.0%, the maximum PWHT temperature is 1450 deg F

If the % Ni + Mn is less than 1%, the maximum PWHT temp is 1470 deg F

Note 4). If a portion of the component is heated above the PWHT temperature stated above, one of the following actions shall be performed;

a). the component in its entirety must be renormalized and tempered

b). If the maximum holding temperature above is exceeded, but does not exceed 1470 deg F, the weld metal shall be removed and replaced.

c). the portion of the component heated above 1470 deg F and at least 3" on the either side of the overheated zone must be removed and be renormalized and tempered or replaced.

d). the allowable stress shall be that for Grade 9 at the design temperature provided that the portion of the component heated to a temperature greater than that allowed above is reheat treated within the temperature range specified above.

No specific mention of heat treatment after bending/forming.

RE: New ASME Code case for P91/T91 ???

metengr,
Just reviewed the 05 Addenda to ASME Section I. All has been defined therein as you stated.  It permits raising the PWHT from 1425F max. to a maximum of 1470F provided the %Ni +%Mn <1.0% or 1450F if < 1.5%. Min. PWHT is 1350F.

RE: New ASME Code case for P91/T91 ???

metengr,
Further to my review of the 05 Addenda, I noticed that Table PW-11 had deleted the > 850F temperature limits for RT. Was this a misprint?

RE: New ASME Code case for P91/T91 ???

stanweld;
Yes, I believe you’re correct, this is a misprint. I don't recall seeing any proposed changes regarding this item.

RE: New ASME Code case for P91/T91 ???

The case can be read at <cstools.asme.org> , case BC05-1483 , section I paragraph PW-39 , page 96.1 ( rev A 05).

See note 4 for the revisions. However, as I read table PW-39, it may only reference P91 sections  that  are overheated in the welding process, and might not strictly apply to the hot bending overheats and associated material property degradation.

To close that loophole, it might be neccesary to transfer the same notes in note 4 to the ASTM spec SA-335.

RE: New ASME Code case for P91/T91 ???

I doubt that SA-335 is the place, since it is incumbent on the manufacturer to meet the material specification's requirements.  Section I needs to address hot and cold bending similarly to B31.1, which in turn should be amended to address the specific parameters required for both hot and cold bending of P/T91.  

RE: New ASME Code case for P91/T91 ???

Stanweld;

It seems the manufacturer of the base pipe does supply the SA-335 pipe, in straight sections , with proper material properties as per SA-335. The hot bending fabrication error occurs at the downstream foundry that uses a hot bending machine to fabricate the bends, and many of these foundries appear to be ignorant of the need to N+T the entire  section of piping to correct the sections that were overtempered ( ie, zones adjacent to the local 1900 F induction heated zone).

Generally, what seems to happen is the EPC vendor obtains competitive quotes from several foundries for the bent els, and selects the lowest bidder. The bent els are then purchased , under the generic spec of SA-335, but without addending to the purchase spec a QC requirement to perform a hardness traverse of the bent piping or mandating a N+T. It often happens that the lowest cost foundry doen't have a staff metallurgist, and being unaware of the P91 material degradation during overtempereing will merely follows code stated requirements  and would only add the neccesary N+T if stated in the purchase spec from the EPC vendor. As I read B31.1, the responsibility for correct fabrication procedures is the "designer's", not the foundry.

Until adequate corrections are made to the code and ASTM specs, it will be neccesary for the EPC specification, and the ultimate customer's specification, to explicitly require the N+T for hot bent P91 els and/or addend the same wording as Section I is proposing in note 4 of case BC05-1483.

Other equally common and equally damaging  errors are (a) insufficient rate of cooling following N+T- it must be cooled faster than -9 F/min to avoid ferrite formation  ,and (b) the max permitted thickness transition angle may not exceed 30 degrees at the OD in the vicinity of butt welds- this is explicitly defined in the codes, but lately been  ignored by reputable designers, to the detrement of the end user.

RE: New ASME Code case for P91/T91 ???

(OP)
davefitz....

Thank you for putting your finger on what I believe to be an important EPC responsibility in the design and fabrication process...

You stated:

"It often happens that the lowest cost foundry doen't have a staff metallurgist, and being unaware of the P91 material degradation during overtempereing (sic).. merely follows code stated requirements  and would only add the neccesary N+T if stated in the purchase spec from the EPC vendor. As I read B31.1, the responsibility for correct fabrication procedures is the "designer's", not the foundry."

I agree....and this exact point will be examined in a courtroom in the near future. The impending slew of Type IV cracking failures and the resultant disruption of revenue will cause a lot of heartburn in the owners suite.

With today's frequent churning of EPC power plant personnel and a gutless management structure that tries to "pin" any PE that was associated with the design, fabrication or testing of a failed component, it should be no surprise to anyone what will happen.

My initial question was meant to solict details and opinions about the extent of the problem and to establish if it was even reasonable to conclude that the EPC vendor's would have known about this heat treatment and Code issue as it was developing.

My opinion only...

-MJC

  

RE: New ASME Code case for P91/T91 ???

MJC:

If by PE you mean professional engineer, then the legal issues with the EPC's can become even more convoluted than you indicated.

It appears it has also happend that EPC vendors have claimed that some of their staff PE's had reviewed the design documents, when in fact the staff engineer had never seen the documents. The lack of any requirement to add the PE stamp or signature on contract documents based on the EPC's filing with the state PE board  that a PE was on their staff is a huge loophole that allows unreviewed work to enter the marketplace.

RE: New ASME Code case for P91/T91 ???

MJC,
    You seem to be suggesting that their is a pending legal situation regarding Type IV cracking found in some equipment. Can you expand as I'm interested in anything to do with P91 failures.


Regards

RE: New ASME Code case for P91/T91 ???

davefitz,
From my experience, the OEM and the Engineer-designer/Procurer for the EPC company bear greatest responsibility for most of the problems encountered with P91. They seldom addressed the PWHT, dissimilar materials welding and hot/cold bending post heat treatment requirements for the alloy.  Futhermore, the attention payed to shop surveillance was often non-existent or when in place, the inspector had little or no guidance.   

RE: New ASME Code case for P91/T91 ???

stanweld:
B31.1 is clear on the responsibility. For power piping  ,  Paragragh 129.3"heat treatment of bends and formed components" states:

"If hot bending or forming is performed , the material shall recieve a full anneal, normalize and temper, or tempering heat treatment as specified by the designer"

"If a postbending or postforming heat treatment is to be perfromed, the designer shall fully describe the procedure to be used".

For section I boiler components, the problems with P91 have been generally well addressed by the boiler OEM's, with the possible exception of the max slope of 30 degrees at thickness transitions and use of transition pieces.

For B31.1 components, the bubble of activity of building  combined cycle plants  in the period 1998-2003 led to assigning the design responsibilities for the P91 piping to unqualified ,newly graduated engineers and designers that had no exposure to the metallurgical quirks of P91. They didn't know what they didn't know, and had no clue that they needed to even question the adequacy of code requirements. ( I venture to add that most did not even read the code).

RE: New ASME Code case for P91/T91 ???

As a side note to this entire discussion; Type IV creep failures in fabricated Grade 91 components are not entirely attributed to improper heat treatment during fabrication. In certain cases, Type IV cracking is a final outcome of Grade 91 components that are sub-critically post weld heat treated AND are subjected to thermal/mechanical bending stresses in service. The European Creep Collaboration Committee has performed extensive research on the threshold thermal/mechanical bending stresses that will trigger Type IV creep damage. I would bet most designers are not even aware of this information.

So, let the buyer, designers, etc., beware AND understand that Codes and Standards used in design provide minimum requires for safe operation. For boiler external piping under ASME Section I administration, ASME B31.1 technical requirements prevail, which is good.

Regarding boiler headers and appurtances not covered under ASME B31.1, yes, Section I does not specifically describe fabrication process detail as B31.1. I would venture to guess that headers which have developed incipient Type IV creep damage over the years pose less of a safety risk in comparison to external piping. I would agree that Section I should have a comment or two, similar to B31.1.

It is unfortunate that the ASME Boiler and Pressure Vessel Code committees over the years have become stacked with fabricators and consultants with small fry users like myself as a minority. Until more users get involved and express concerns related to changing the Code, nothing more will happen.

RE: New ASME Code case for P91/T91 ???

I just realized that the code case referred to above might be  impossible to implement during field construction of P91 components.

The type IV creep damage is caused by overtempering ( weld reheat aging) of the HAZ- some narrow band of parent metal adjacent to the weld line will be overheated above 1425 F during the welding process. If we literally follow the code case note 4, the weld plus a 3" band of metal on either side of the weld must be removed , but the process of replacing the section via welding simply repeats the same defect. The alternative of N+T the entire piece cannot be accomplished when the entire piece is the entire main steam gtrnasfer pipe.

If it is true that the weld process overtempers the HAZ at a temp above 1425 F, then that particular overheat would need to be excluded in the case where the weld is a butt weld done in the field.

RE: New ASME Code case for P91/T91 ???

davefitz,
         I know its a while since you posted but I've just read your post regarding the cooling rate as being 9 Deg F/min as a minimum. This has got me a little worried as the pipe spec provided by a Contractor stated the cooling rate after PWHT as 100 Deg C/hour. I wonder if you could give your thoughts on this cooling rate (This rate is identified as being cooling down to 400 Deg C with no rate below that)

Regards

DSB123

RE: New ASME Code case for P91/T91 ???

DSB123:

The required cooling rate is at least as fast as -9F/hr in the temperature range of 1900F to 1400F, to avoid formation of ferrite. This is known from TTT temperature-time-transformation curves for this alloy. They were originally developed in the late 1970's ( I am looking for my copy of the original and will advise later) and a similar copy is presented in the Mannesman Vallourec booklet "the P91 book".

As different crystal structures are formed, they change the volume of the melt. By monitoring the volume or dilatation of the melt, they can infer which crystal has formed. This is confirmed by later metalurgi8cal analysis and hardness testing.

For this reason, the ASTM specs for P91/F91 alloys requires forced air cooling for thinner walled parts  and oil quench for thicker walled parts. We discovered this problem when the Mom and Pop foundry used on our project had erroneously covered the part with inisulation to provide a slow cooldown, and it was all ferrite and as soft as a fresh baked doughnut.

RE: New ASME Code case for P91/T91 ???

Correction- -9F/minute.

RE: New ASME Code case for P91/T91 ???

The original TTT curves were as per T. Wada "The continuous cooling transformation diagram and tempering response of 9Cr-1Mo-V-Nb steels", J-4672, Climax Molybdenum  Company of Michigan, Ann Arbor, Mi ,1981. They are probably also available by the Oak Ridge folks ( V Sikka et al) and from ASME associated with the original code case .

RE: New ASME Code case for P91/T91 ???

Correction:

The Ferrite forms in the temp range 820-650C ( 1500-1200F ), see the Mannesman "The T91/P91 book" fig 6.6 CCT continuous cooling transformation diagram zone "F+C". However, this figure did not include the naming of each cooling curve with its cooldown rate. As per the Wada curves, the cooldown curve that borders the left of zone F+C is at -9F/minute.

RE: New ASME Code case for P91/T91 ???

The original Wada paper can be obtained from the new owner of Climax Molybdenum, Phelps dodge, at <climax@phelpsdodge.com>.

To confirm the -9F/minute, all recent papers I could find  by the Oak Ridge folks ( Swinderman, Sikka, etc)  use a cooling rate of -6C/minute on lab samples that are being tested via Gleeb dilation machine.

The Mannesman CCT curve figure 6.6 indicates the resulting hardness at the end of each cooling curve, so a rough check of the hardness ( prior to tempering)  can give you an idea of whether ferrite was formed.

RE: New ASME Code case for P91/T91 ???

davefitz,
         Thanks for your swift responses. The problem I have is that there are those in senior positions who do not appreciate the problems associated with P91 and the Contractor whom is respected is thought to be right all of the time even though they have no experience with P91. Their spec as I said previously states cooling at 100 Deg C/hr (3 Deg F/min) and its taken that this "must be correct".

I have just found the following on another forum which highlights the situation with P91



"Welding of P91

Our program also has 400 minimum preheat and we use combined GTA/SMA process for the wps. Once the weld is completed or if the welding process is interrupted, we do an intermediate bake out at preheat temp for 2 hours or elevate to 600 f for 1 hour ,insulate and slow cool. The welds are allowed to cool to less than 200f prior to wrapping for any pwht. PWHT must be accomplished within 7 days during which time welds must be kept dry. PWHT cycle is 400f per hour to 800f then 200 f per hr to soak temp of 1375 to 1425 soak time of 2hrs min for thickness up to 2"( 4hrs for thickness over 2") Cooling rate is 200F per hour max. Once PWHT is complete any final nde is done after 48 hours waiting period. "


This states the max rate of cooling to be 200F per hour. Any comments?

Regards

DSB123

PS I'm with you on the cooling aspect.

RE: New ASME Code case for P91/T91 ???

DSB123;
Do not confuse subcritical PWHT/tempering heating rates and cool down rates used for field welding with a normalize and temper heat treatment. The normalization heat treatment requires complete transformation of original base material (mixed structure) to austenite followed by either rapid cooling or quenching at or above 9 deg F/min to achieve 100% martensite. The aforementioned cooling rate applies during a normalization heat treatment for Grade 91 material, not for welding. Your stated conditions above are fine for subcritical PWHT of field welds.

RE: New ASME Code case for P91/T91 ???

I agree with Meteng- the -9F/minute is only applicable to those parts that were heated to 1900F in the normalization process and not to weld processes. The slower cooldown following welding will help avoid cracking associated with transient thermal stresses.

RE: New ASME Code case for P91/T91 ???

(OP)
To all...

I think that this has developed into an interesting thread and ,in many ways, become instrcutive and informative to many.

However, it seems to be going into a direction that I did not intend.

I am still curious..... does anyone out there feel that:

1)   There will be a large group of HRSG 9Cr materials failures (type IV or otherwise) occuring in the near future due to a failure in either the Code rules or those who are supposed to understand the rules.......and

2)   This may turn into some kind of interesting class action or "group type" of litigatation activity involving Texas and New York law firms, Mexican fabricators, metalugical consultants and,.....hopefully.......CNN

Oh....and with regard to the "DSB123" comment of 4 January 06, ......no, I have nothing specific to reveal, but I do like to watch busy traffic intersections where one simple accident can cause a whole chain of destructive events....

Again, these are my thoughts and opinions only....


-MJC

  

RE: New ASME Code case for P91/T91 ???

Failures in P91 are becoming more numerous as time progresses. Manufacturers have supplied SA-234 WP91 fittings and induction bends with improper heat treatments as late as 7 mos age that I am aware. In some cases the materials exhibit through thickness ferrite grain structures. Improper PWHT, over tempering and undertempering, has been observed and reported at welds. One must assume that more failures will occur as time progresses. Operating time at temperature for a number of plants is approaching 30,000 to 50,000 hours, which based on P9 or P22 design allowables, should be the life expectancy of inappropriately heat treated P91 materials. Failures should be expected to increase.

Will there be law suits? You can bet the bank!!

RE: New ASME Code case for P91/T91 ???

The more interesting development is that ,in the position paper released last year by the relevant ASME committee, they recommended AGAINST increased NDT and surveilance of installed P91 components. Must be Ostrich DNA in their genes.

RE: New ASME Code case for P91/T91 ???

davefitz,metengr,
                  Thanks for the clarity. It was just the note regarding ferrite formation between 820-650 Deg C that had me confused as the normal PWHT is performed at around 760 Deg C which is right in the middle of this band. Also appreciate the sense of humour "Must be Ostrich DNA in their genes". Strange but we have a HUGE sandpit here also.


MJCronin,
         Thanks for the response. It's just from your earlier post I thought you were aware of some litigation going on.

Stanweld,
         Our P91 system has been operating for about 15 months now so we are some way off a failure (hope so). My thoughts are that it will be around the 2 year mark. Hope I'm totally wrong!!!

Regards

DSB123

RE: New ASME Code case for P91/T91 ???

stanweld;
Have any of the clients that you deal with been warned after the fact especially for external piping components? The peer groups I deal with in the Power Generation (more utilities) have in-house metallurgical engineers that have enough knowledge to avoid these pitfalls. If I was in your clients position, I would begin targeted replacement of these improperly heat treated Grade 91 fittings on external piping to avoid premature failure in service for the sake of personnel safety.

If these improperly heat treated components are internal to the boiler setting, it has more to do with boiler reliability versus personnel safety and could be replaced at a later time.

RE: New ASME Code case for P91/T91 ???

MJCronin:

My own guess is that no significant efort would be made to audit the installed components until a fatality is registered as being caused by this issue. In the meantime, culpable entities will be busy cleaning out files to prevent or limit discovery of their errors.

A more pro-active approach would be to have the National Board issue a notice that all P91 external piping failures be reported to them on a confidential basis, and if their review indicates a significant hazard exists, then an industry wide survey of hot bent els  or other high hazard items be scheduled for NDT inspection, as was done in 1989 for P11 type IV longitudinal weld failures following the Mojave accident.

RE: New ASME Code case for P91/T91 ???

(OP)
davefitz...

Thank you for your response and perspective....I have always found your posts to be well written and informative.

While I realize that many corporate project files have probably already been purged, I wonder if this will turn into some kind of "class action" suit against a particular tube vendor or boiler fabricator. (something like the asbestos litigation that has been with us for the past 25 years)

Do you think that the National Board is adequately "on top" of this topic like it was in 1989 and that they would attempt to assist in inspections/retrofits like they have in the past...?

I completely agree that only a major catastrophe will bring attention to this matter.......I only hope that a major Republican Senator is somehow attop a steam drum someday when it happens.....

My opinion only

-MJC

  

RE: New ASME Code case for P91/T91 ???

metengr,
The problem is finding the fittings. Those that have been found have been replaced but how many are out there in use; that's the big question?

In the main, pipe fabricators purchase fittings from warehouse suppliers not directly from the mill. On one job, I saw the same size fitting from 4 different manufacturers; all spools fabricated by the same spool fabricator.

RE: New ASME Code case for P91/T91 ???

The advantage of having a single entity like the National Board organize the response is that they may find a statistical clue that only a certain foundry or certain EPC vendor has a high number of failures associated with it, and this may limit the need to audit every single P91 component down to perhaps 5% of the components. But I am not informed of the legal ramifications of this approach.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources