New ASME Code case for P91/T91 ??? 1

[MJCronin]

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

 

[davefitz]

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 .

 

[davefitz]

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.

 

[davefitz]

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.

 

[DSB123]

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.

 

[metengr]

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.

 

[davefitz]

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.

 

[MJCronin]

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

 

[stanweld]

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!!

 

[davefitz]

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.

 

[DSB123]

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

 

[metengr]

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.

 

[davefitz]

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.

 

[MJCronin]

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

 

[stanweld]

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.

 

[davefitz]

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.